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Green Building Bible, Fourth Edition
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    • CommentAuthorstones
    • CommentTimeDec 16th 2010
    We have built a house with the Supawall system - see thread


    and fitted a Nibe F470 Exhaust Air heat Pump to meet or ventilation, heating and hot water requirements.

    Nibe decribes the Heat pump as follows:-


    'The NIBE F470 is a complete heat pump unit for recovering
    thermal energy from exhaust air. Warm, oxygen-poor air is
    channelled through a ventilation unit which is built into the
    system via a heat exchanger located in the heat pump circuit.
    The recovered heat is transferred, via a heat exchanger to
    a double-jacket tank. The recovered heat is simultaneously
    channelled, via another heat exchanger, to both the supply
    air and the heating circuit. The hot water tank’s double-jacket
    system not only helps to heat the radiator water and domestic
    hot water, but also heats the fresh, incoming air. The unit comes
    complete with a central heating pump, heating pressure vessel
    and filling loop'.

    Our thoughts in installing this unit was that it catered for all our needs in what is a highly insulated and hopefully reasonably air tight house, gaining as much benefit as possible from the heat contained within the outgoing stale air on the ventilation side.

    First thing to talk about is the unit itself. It measures 600 x 600 x 2100mm or thereabouts. It is also very heavy, and is a four man lift. It is therefore very difficult to manouvere if you are siting it in a confined space / inside as we did.

    We had a few issues getting the unit as it was the first one in the UK, due to the previous model being discontinued. When we did get it it was delivered by one man and thus we really struggled to get it inside. The installation took a day and a half to complete. Because a grant can be claimed for having the unit intsalled, the cost of installation was sky high and certainly bore no relation to the limited number of (simple) connections required. The unit was then commissioned by the supplying company which involved both setting up the machine and checking ventilation flow rates.

    With the machine on we have a very pleasant internal atmosphere - fresh and never stuffy, constant temperature and plenty of hot water.

    It also seems to be working cost effectively. Over the past two weeks our electricity usage with heat pump on has been 856 Kwh, of which 252 accounts for day to day electricity and running the ventilation fans. Heat pump and back up immersion came to 604 Kwh, for a constant 18 - 19 degrees inside 24 hours a day, and all our hot water - 2 showers plus a full bath and misc sink fulls daily. Immersion back up accounted for 347 kwh with the rest being the heat pump.

    £52 for two weeks heating and hot water when the temp has been dropping to -10 most nights and not above -2 through the day doesn't seem too bad, especially when compared to to last house we were in - in a similair fortnight of extreme cold weather last year in a new rented house (140mm glass wool walls) we used £250 in gas over a fortnight and it was not even that warm. I accept that this is a crude comparison, but I think we can conclude that the Heat Pump perfoms reasonably well in a highly insulated envelope.

    There is a good control panel and there are various changes you can make to the units operation to suit your circumstances / specific electricity tarriffs - e.g. timing hot water operation, blocking operation of back up immersion, adjust fan speed for ventilation flow rates.

    We have yet to experiment with the settings to see how much difference there will be. We have meantime decided to stay on a normal tarrif rather than Economy 10, having done a comparison based on anticipated consumption. Our conclusion was that E10 would be slightly more expensive. I am continuing to monitor electricity consumption on a daily basis and will try and update on a regular basis.

    If anyone has any questions...
    Is the fresh air supplied via trickle vents? Do you notice a draught in windy weather?

    • CommentAuthorstones
    • CommentTimeDec 16th 2010
    No - all fresh air via mechanical ventilation. No trickle vents in windows. No draughts in windy weather.
    Does the unit prioritise hot water delivery? If so, does it supply cold air when heating the hot water?

    If you're using a heat pump to extract heat from the exhaust air then the only "new energy" the heat pump provides to the house is the electricity it consumes. In other words, you have a COP of around 3 for energy recycled within the house, but a COP of 1 for new energy introduced to the house.

    Given that the compressor's rated input power is around 725W & the immersion heater is 10.25kW, isn't most of your energy coming from the immersion heater & the rest coming from the heat pump with a COP of 1?

    Posted By: davidfreeboroughIf you're using a heat pump to extract heat from the exhaust air then the only "new energy" the heat pump provides to the house is the electricity it consumes. In other words, you have a COP of around 3 for energy recycled within the house, but a COP of 1 for new energy introduced to the house.

    This is a fallacy. The exhaust air temperature will be below ambient outdoor temperature so the COP will most certainly be greater than 1. Air has heat energy in it all the way down to when it turns to liquid. The heatpump will extract this - the amount extracted, of course, depends on the temperature drop and flow rate of the exhaust air but it will definitely extract energy.

    Paul in Montreal
    • CommentAuthorstones
    • CommentTimeDec 16th 2010
    You can prioritise either heating or hot water, depending on your preference. I can breakdown the usage for the two week period I quoted as the unit records immersion use, compressor use and hoiw much of compressor time devoted to DHW. The Immersion is variable and a maximum level can bet set. Ours is currently set to provide a maximum of 5.3kw

    Compressor on for 248 hours of which DHW 46 hours
    Immersion on 65.6 hours

    The unit cycles between heating / hot water depending on what is required, however, the air supply remains heated at all times (unless of course you specifically deselect it)

    The unit runs a heating loop for radiators / UFH. This loop as I understand it, passes through a 'supply air battery' which is used to heat the incoming fresh air. Clearly the colder the outside air temp, the more energy required to heat it. Over the past 2 weeks of cold weather 57% of the load has been shouldered by the immersion. At higher outdoor temperatures, the immersion requirement drops. Unfortunately I only have one example day for that, as we have only had one mild day since it was installed.

    Temp outside 7.5 c
    24 hour electricity consumption 35kwh - less 18 kwh for household plus ventilation fans = 17kwh
    Of that 1.4 hours immersion activated = 7 kwh, Compressor 10kwh (I do not have the compressor / DHW breakdown for this day as I did not start recording it until a week later) This means the immersion accounted for 41% of the demand on this milder day.

    When commissioned, the immersion was programmed to cease functioning when the exterior temperature is at 12 or above (averaged over 2 hours).

    I can if I wish change this, or indeed program the unit to block immersion / hot water heating to times of my choosing. What appears to happen is that after showers or a bath has been drawn, the unit heats the water back up straight away, and in doing so uses the immersion.

    This will be the first area I will experiment with - programming the water to heat on ly at specific times. The average compressor use over the two weeks was 17 or so hours a day (albeit there were 2 powercuts due to the snow which knocked the system out for 30 hours in total) Even taking that into account there would appear to be 5 hours a day that the compressor could be used to recycle heat with the higher COP, rather than using the quick fix immersion.

    I take your point, I was over-simplfying my point.

    I was trying to get my head around the energy balance. Its relatively straightforward if you have a separate MVHR unit of x % efficiency & a heat pump which takes all of its energy from outside the house. But in this case the energy taken from the air extracted from the house is being used to heat an equal amount of air supplied to the house, provide heating & hot water.

    As you point out, the exhaust air temperature can be below the outside air temperature. So you can extract more energy than that required to heat the incoming air to the extract air temperature. I can see how in this part of the energy flow the heat pump can have a COP of 3.

    But isn't a significant part of the heat pump capacity being utilised to recycle heat within the house? What proportion of the capacity is left for introducing new energy to the house?

    • CommentAuthorstones
    • CommentTimeDec 16th 2010
    Reference the temp of exhaust air, the unit is currently extracting from the 'wet' rooms in the house at 18.1 c and expelling to the outside at -5.4 c. I have not yet seen the temperature of the expelled air rising above -1 c except when the defrost cycle is activated.
    • CommentAuthorDantenz
    • CommentTimeDec 16th 2010 edited
    Then the element has been on for 26% of the compressor run time and accounts for 57% of your purchased primary energy. So am I correct in thinking that the heat pump only has a COP >1 for 74% of the run time. If so, that doesn't sound that good to me for a heat pump.
    Posted By: stonesReference the temp of exhaust air, the unit is currently extracting from the 'wet' rooms in the house at 18.1 c and expelling to the outside at -5.4 c. I have not yet seen the temperature of the expelled air rising above -1 c except when the defrost cycle is activated.
    Thanks, that's interesting. It would help me understand how well it works if you could record the extract air temperature, exhaust air temperature, outside air temperature & supply air temperature when the heat pump is running & the supply air is being heated while the tank/hot water is not being heated.

    • CommentAuthorstones
    • CommentTimeDec 20th 2010
    Got the following figures taken at 0830 each morning over the past 2 days. I cannot unfortunately break down the figures for when temperature when the heat pump is running & the supply air is being heated while the tank/hot water is not being heated because it is not currently set up to allow me to do so. Additional heating (immersion) comes on to supplement the heat pump when it is required for either hot water or heating.

    Sat 18/12 - Sun 19/12

    80 Kwh used
    Internal temp 17.9
    External temp -4.3
    Back up Immersion 6.8 hours ( immersion set at 5.3kw)
    Compressor run time 17 hours of which 3 hours water. Specified comp output 0.65kw
    Extract air temp 16.2
    Exhaust air temp -4
    Supply air temp 25
    Flow temp 44 to air supply battery

    Sun 19/12 - Mon 20/12

    83 Kwh used
    Internal temp 18.4
    External temp -4.2
    Back up Immersion 7.1 hours ( immersion set at 5.3kw)
    Compressor run time 18 hours of which 5 hours water. Specified comp output 0.65kw
    Extract air temp 17.2
    Exhaust air temp -8.8
    Supply air temp 32.3
    Flow temp 45.1 to air supply battery

    Mon 20/12 temp readings

    Internal temp 18.7
    External temp -4
    Extract air temp 18
    Exhaust air temp -7.4
    Supply air temp 28
    Flow temp 44 to air supply battery

    We were averaging 11 to 12 kwh per day in domestic consumption prior to the unit being fitted. The ventilation fans are rated at 25 - 140 watts. We have the ventilation flow set at 55%. Circulation pump rated 100w. A crude calculation gives around a 4kwh daily consuption for the ventilation fans (2 of) and 2.4kwh daily consumption for the circulation pump. We therefore assume a daily consumption of 18kwh. This figure replicates our consumption in previous homes where we have run a MVHR. This figure has not been deducted from the electricity consumption listed.

    Assuming a conversion factor of 10.35 KwH to a litre of fuel oil, we have used the equivalent of just under 6 litres of oil to heat our home / DHW. At current oil prices 72p per litre, that comes to £4.32. Electricity use average of 64 Kwh used at 8.66p = £5.36. On face value, the heat pump would appear more expensive than oil to heat our home. However this excludes the benefit of the COP on the compressor side of the heat pump, i.e. what multiple we would have to apply to the amount of oil that would need to be used to achieve the same level of heating / DHW.

    Unfortunately, I do not have a similair sized house to compare these figures / anticipated consumption to. I am aware of a couple of SIP houses locally which use wood pellets for heat/DHW, which although insulated to a similair standard, use more fuel in KwH terms (albeit at a similair cost to my anticipated use).

    Hopefully David can make something of all the figures above. I have not read the relevant standard to which COP is calculated. I expect that it will be worked out as an average over the year / based on certain temperatures. In the real world, COP will vary throughout the year as the outside temperature varies. It is however clear that the heat pump is extracting heat energy from the extract air in excess of the air temperature itself.

    Nibe provides a chart in the instruction manual giving % electrical consumption of the machine each month of the year. Based on this and the normal November temperatures in the UK, I anticipate a total household consumption of 13500Kwh over 12 months. 6500 of that is for domestic electricty and the ventilation fans (which we would have via an MVHR if we did not have this machine) That leaves us with an expected annual consumption of 7000Kwh for the heat pump / Immersion providing DHW and Heating. At current electricity prices DHW and heating would therefore cost me £606 per annum, which stacks up with the anticiapted costs in the EPC (however flawed the calculation is - it was an accurate indicator of both consumption and cost in our previous 2 homes).
    • CommentAuthorDantenz
    • CommentTimeDec 20th 2010
    Posted By: stonesAssuming a conversion factor of 10.35 KwH to a litre of fuel oil, we have used the equivalent of just under 6 litres of oil to heat our home / DHW.
    That is the energy content in a litre of oil but you have to buy more than this as max boiler efficiency is 90%. So you actually get 9.35 kWhr/litre = 6.8 litres = £4.92
    Standard for COP calculation is BSEN 14511
    Posted By: stonesGot the following figures taken at 0830 each morning over the past 2 days.
    Thanks for the data; some interesting numbers.

    The first thing to strike me is that even under current conditions the compressor is not running 24 hours. I would have thought it simplest to run the compressor 24 hours & only use the immersion to "top up".

    Do you know what determines whether the compressor is running? How is the incoming air heated if the compressor isn't running, does the outer tank act as a thermal store for the duct heater/air supply battery?

    What determines whether the immersion heater comes on? Does it normally only operate during the Economy 7/off-peak tariff period?

    I'll take another look & let you know what I can deduce. In the meantime, do you know what the air flow rates are for extract & supply in terms of litres/minute or air changes per hour? If the latter, what's the approximate internal volume of the building?

    • CommentAuthorstones
    • CommentTimeDec 22nd 2010
    The running of the compressor / immersion top up is determined at present automatically by what NIbe call the Heating curve - it heats water for the heating circuit to specific temperatures based on the temperature of the outdoor air. The colder it gets outside, the higher the temperature it heats the water to, to maintain the desired indoor temperature. As I understand it the energy retrieved via the evaporator and compressor is transferred to the boiler water - the outer tank which would seem to act as a thermal store, for the supply air battery, heating circuit and DHW. As the outdoor temperature has fallen, so the immersion back up consumption has increased.

    Interestingly, when we stayed in our last rented house the DHW immersion had been left on by the previous occupants in addition to the normal Gas boiler. The energy use for that top up was 25kwh per day!

    The operating times of the immersion, and indeed the timing of DHW heating can be scheduled / blocked to be run on off peak tarriffs. I have meantime not gone down this road, having done a cost comparison with my current supply costs. I have not as yet experimented with blocking the immersion. The biggest demand would appear to be after we have drawn DHW so I will probably try blocking the immersion for a couple of hours in the morning after showers and in the evening after bathtime.

    The venitlation rates have been set to 33litres per minute extraction, supply to 80% of that.

    Some additional figures

    Mon 20/12 - Tues 21/12

    84 Kwh used
    Internal temp 18.7
    External temp -4
    Back up Immersion 7.1 hours ( immersion set at 5.3kw)
    Compressor run time 21 hours of which 4 hours water. Specified comp output 0.65kw
    Extract air temp 18
    Exhaust air temp -7.4
    Supply air temp 28
    Flow temp 44 to air supply battery

    Tues 21/12 - Wed 22/12

    84 Kwh used
    Internal temp 18.2
    External temp -6.2
    Back up Immersion 6.4 hours ( immersion set at 5.3kw)
    Compressor run time 16 hours of which 2 hours water. Specified comp output 0.65kw
    Extract air temp 17
    Exhaust air temp -7.2
    Supply air temp 31.4
    Flow temp 47.1 to air supply battery

    Wed 22/12 temp readings

    Internal temp 18.5
    External temp -8
    Extract air temp 17
    Exhaust air temp -9
    Supply air temp 29.4
    Flow temp 47.4 to air supply battery

    One last point on the heat pump. It does have automated defrost cycles which will account for a small percentage of the run time.
    • CommentAuthorRobinB
    • CommentTimeDec 22nd 2010
    Just wondering why the difference between supply and extraction rates?
    • CommentAuthorstones
    • CommentTimeDec 22nd 2010
    As I understand it, it is to prevent the house becoming over pressured
    I have fitted the third NIBE470 to be imported.

    I am far from satisfied. What the 470 should be described as is a socking great immersion heater with a little heat pump attached. NIBEs sales literature (for the 410, which is essentially the same) says is that the 410 is designed for single and two family house with a heat demand of 4-6kW, and that the immersion heater switches on only when required. It says that at a heat demand of 6.1kW you will get an annual COP of 2.65. It also quotes "savings/year" of 6-8500 Kwh, but since I haven't got a copy of EN255 which this is based on, I don't know whether this is against all electric, gas or oil.

    I have recently completed a barn conversion - stone walled barn, dutch barn roof. Internal footprint of about 100sq m. We built a house within the barn, having blockwork cavity walls with 100mm kingspan in the cavity,(avge 0.19W/mK) 125mm below floors (avge 0.14W/mK), whilst the roof is 200mm Warmcel filled within a Paneline box and intello membranes.(avge 0.17) In essence we have a well insulated house sheltered by 600mm of stone wall all round. We have paid great attention to airtightness, although we have not been required to have the building tested. We have installed a woodburner in the living space,(Hwam Monet - expensive but brilliant) which can feed a Chelmer Ecocat tank, which will also be fed by solar when I get it fitted.Whilst this tank can supply loads of DHW, we have built a 600mm deep insulated sand bed into the centre of the house (about 25sq m), and there are underfloor heating pipes set in the sand such that the sand bed acts as a storage medium. All the rest of the house has conventional UFH fed by the 470. The idea is that the sand bed will be charged up using free solar in late summer, and topped up using the by product heat from the woodburner which we light up on winter evenings. We were advised to install the 410/470 in order that the stray, disorganised heat from the sand bed could be captured from the house air, and organised into the rest of the UFH.

    The house has been designed to have a high thermal mass, so we've used concrete block walls and beam and block floors. The 470 is in a room outside the barn, which also contains the EcoCat.

    Our SAP calcs indicate an annual heating cost of £434pa, and £216pa for DHW, based on an air source heat pump. We rate C75 efficiency, and B83 CO2. A heat loss calculation gives 7.5kw at -3; 6kw allowing for the sand bed being in operation.

    We moved in in the middle of the cold spell, but the house had been built slowly, and had dried out over the autumn. However, the sand bed had not been charged up, so that area was a cold spot. I didn't expect the heating to work perfectly from the start, until the house had fully dried, and the sand bed had been at least brought up to the internal temperature of the house.

    We have moved to the barn from a 15th century black and white cottage which is adjacent. That has oil fired central heating, and I was used to stumping up about £2500 a year for oil. Our electricity bills were about £250 a year, which covered washing machine, dishwasher, summer cooking (Aga in winter) and the usual lighting, but no immersion heater. We now have all the same appliances, efficient lighting. and induction cooking.

    My electricity bill for February (a relatively warm month) was £290. taking £30 off for our normal useage means that the 470 had cost me £260. I thought this excessive and demanded attention from NIBE. Their technician thought that some of the settings were not as good as they could be, and in particular we should not stop the machine using the immersion heater at the 8kw setting - we had limited it to 5.3kw. Particularly he said that the output was 4kw (for £5k!) and that the immersion heater was brought in largely by the outside temperature sensor. We could expect to see it in operation any time the outside temp was below 12 degrees.

    We have just been away for a week. That exposes a weakness of the 470. We are used to leaving the house lightly heated when we go away, (10 degrees)to keep out frost, save the houseplants etc, and this makes it all the easier to warm the house up when we return. The 470 has a nervous breakdown if it is fed with air below 16degrees, which in our case means 18 internally to be sure. Since fully heated is 20 in living rooms and 18 in bedrooms, we can't turn the thing down. There is a "holiday" setting, but this is accompanied by a cautionary note, which says if the exhaust air temperature falls below 16, the compressor is blocked (stops working), and the immersion heater takes over. So, turn it down, and you get 100% CO2 rich electrical heating whilst you're basking in the Carribbean. So we left the 470 on nearly full heating (20/18) and with no other use apart from a couple of 12w lights we used 430kwh in a week with mild days and a couple of light frosts. No DHW was used

    It looks to me as though this will be about our average use, if we're lucky, and if we use the woodburner on cold nights. Our electricity bill for heating alone will be between 2 and 3 times the estimate in our SAP.

    In the defence of the machine, we were also very badly advised over our ventilation ducting. A design was done by a supplier of Polypipe Supertube 125. What this entirely omitted to include was duct insulation. A large proportion of our ducting runs in two service ducts which run outside the internal envelope of the house. Although this duct has 100mm of kingspan above it, and is stuffed with Rockwool, the ducting lies on its floor which spans the wall cavity, so heat is lost there. To get from ground floor to first floor the ducting runs up the cavity for about 1.5m, where it lies against the outer leaf, with a small air gap. The insulation is sculpted around the other two sides. I think we lose a lot of heat there. The total heat loss between the house and 470 is about 2 degrees; more alarmingly, the 470 produces return air at around 37 degrees, and much of this appears to be lost in the ducting. The furthest inlet to the house has had to be closed because in really cold weather it delivers air to the room below the general temperature of the house interior. The NIBE trained guy who commissioned the 470 said it was normal practice to run ducts in cavities, but it's not helped our system. Comfortingly, perhaps, he took the view that the production of warm air was simply a by product, although I'm not convinced.

    NIBE say in the instructions that the machine should be connected with flexible ducting with the extract air and outdoor ducts being insulated. Where the machine isn't inside the envelope of the building it's vital to insulate everything.

    I'm pleased for Stones that he has cracked some of the control systems. I'm a 68 year old engineer. It's vital that I have a system my wife can run if I keel over. I cannot say I understand the way this thing works, and she never will. It is the most complex piece of domestic machinery I have ever come across, by a long way.

    When I embarked on designing and building my house I wanted a heating system with a degree of resilience, in the light of peak oil and under capacity in generating systems. I also wanted economy and least possible CO2 production. The system also had to be automatic for the reasons I give above. Whilst fitting the 470 my plumber was telling me of a customer who was over the moon having fitted an air source heat pump in a 5 bedroom dwelling built just to current building regs, whose total electricity bills were coming out to £450 a year.

    I have built a house which surpasses building regs requirements. I feel cheated by a company whose sales literature is so poor that it is impossible to tell what you can expect - in particular, if they said that running consumption was anything like the figures quoted by Stones or myself, they wouldn't sell any of these machines. My next investment was to have been £10-12K on solar PV. I'll get a better return by ditching the 470 and investing in a conventional air source heat pump and a heat recovery ventilation unit.

    Cheerio Guys - feel free to tell me where I've gone wrong!
    • CommentAuthorqeipl
    • CommentTimeApr 4th 2011

    I've heard similar tales locally about the Nibe units. As someone else says, they sound like a socking great immersion heater with an auxiliary air source heat pump.

    I opted for an Ecocent ASHP/DHW cylinder. The heat pump runs at a maximum input of 810W and has a nominal heating capacity of 2.6kW.
    The unit includes a 1.5kW immersion heater but it only comes on once a week to boost the water temp to 60C to kill any bugs.
    It's almost fit and forget. The only setting I've changed since I installed it is the max water temp, which I reduced from 55C to 50C.

    The Ecocent does DHW and 70m2 UFH on the ground floor and I've set it up to do whole house ventilation heat recovery, drawing all of its air from inside the house.
    A wood boiler in the adjacent workshop heats a radiator upstairs and one in the utility room where the main Ecocent intake is situated. This warms the air for the heat pump in mid winter when solar gain is scarce.

    My house is 100m2. It has only slightly better u-values than yours and similar SAP/CO2 ratings.

    However, the difference in energy consumption is stark.
    My heat loss calculation for deltaT of 23C (same as your 20C inside and -3C out?) shows 1.7kW.
    The measured loss at dT20C has confirmed that my calcs are reasonably accurate.

    Apart from two radiators the house is all electric and the meter also covers the attached workshop which has lots of woodworking machinery.
    In February I used 515kWh of electricity.
    I'm on course for an annual electricity consumption (June - May) of 6000kWh.

    Without seeing your house it's very hard to tell what's gone wrong but it would be worth getting to the root of the problem before you ditch the Nibe so that you can properly size whatever replaces it.

    Your calculation of 7.5kW heat loss seems very high.
    Did you do the heat loss calcs yourself and are they the same as was used in the SAP calculation?

    Did you do the insulation yourself? If not did anyone check that the builders were diligent? A poor insulation job will do horrible things to your heat loss.

    Posted By: DaveHempyardsThe total heat loss between the house and 470 is about 2 degrees; more alarmingly, the 470 produces return air at around 37 degrees, and much of this appears to be lost in the ducting. The furthest inlet to the house has had to be closed because in really cold weather it delivers air to the room below the general temperature of the house interior.The NIBE trained guy who commissioned the 470 said it was normal practice to run ducts in cavities, but it's not helped our system.
    It may be standard practice in centralised extract systems, but its a disaster in a system designed to recover heat. Its critical that the extract & supply ducts remain within the thermal envelope & that the inlet & exhaust ducts are insulated. Even within the thermal envelope, insulated supply ducts may also be required to ensure warm air reaches the furthest corners. Air supply below room temperature points to either a loss of air supply heating at the unit or serious heat loss from the supply ducts. So this would be top of my list of things to try.

    Posted By: DaveHempyardsI have built a house which surpasses building regs requirements. I feel cheated by a company whose sales literature is so poor that it is impossible to tell what you can expect - in particular, if they said that running consumption was anything like the figures quoted by Stones or myself, they wouldn't sell any of these machines.
    The basic problem is that an exhaust air heat pump is seriously limited by the amount of heat available in the extract air. If it reduces the exhaust air to the same temperature as the inlet air then it only provides enough energy to heat the inlet air to the extract air temperature. This only serves to eliminate ventilation losses, provides no net heating & can be done far more efficiently (with an effective COP of >20) by a counter flow heat exchanger.

    Additional energy for space heating & DHW can only be gained by reducing the exhaust temperature below inlet air temperature. This is the same for any air source heat pump, but an exhaust heat pump is limited by the volume of air available. Increasing the extract air volume increases the supply air volume which needs to be heated & with low winter temperatures you quickly get into diminishing returns & defrost problems.

    Taken together this makes the inlet air temperature critical. If you're looking for a minimum change fix, you have the space outside & you've not finished the garden yet then you could try feeding the inlet air through an underground pre-heater duct.

    Posted By: DaveHempyardsMy next investment was to have been £10-12K on solar PV. I'll get a better return by ditching the 470 and investing in a conventional air source heat pump and a heat recovery ventilation unit.
    There are some units which use a MVHR type counter flow heat exchanger with an integrated air source heat pump to provide top up heating. This would require a separate solution for DHW, but you could use the boiler stove & solar thermal for that. The downside is that the air source heat pump would typically need to run 24x7. A separate (larger) air source heat pump would allow use of Economy 7 electricity.

    One last thought; is the boiler stove room sealed with a separate external air supply? We have an open flue in our living room & even gas fired central heating struggles to make up for the heat lost up the chimney on a windy day.

    • CommentAuthorSlowmo45
    • CommentTimeApr 6th 2011
    Mr Hempyards

    I hope the Nibe sales person made clear that exhaust air heat pumps will not qualify for the Renewable Heat Incentive because they do not use primary air.

    Sadly, you are left with an inefficient, expensive to run heating system which merits no Government subsidy.

    Typically, companies are keen to copy a good idea which might trigger a question:

    Why is Nibe the only company which has focussed on selling this technology?

    The answer might be that nobody else is quite so dumb or, more likely, quite so deceitful.
    • CommentAuthortony
    • CommentTimeApr 6th 2011
    I have one made by a different company and it is the only thing that I have that keeps my house warm. I have very low heat losses (100W for the whole house in October) and there is enough heat in the air to cover the heat losses at least until outside temperatures drop below minus two. It draws 350W of power and seems very efficient. My heat losses in Jan/Feb are only 10% of the OPs.

    Trying to get 7.5Kw from a system like this would be unrealistic in my view. the 7.5 would seem to be the problem, are there big areas of glass or major uninsulated areas or breaches of airtightness. I also like anything that gets warm to be inside my thermal envelope.

    As I understand it, your system is a high efficiency counterflow type heat exchanger with a small air source heat pump wrapped around it, like the Genvex Premium series.


    This is a far better solution because the inlet air is brought up to extract air temperature by the heat exchanger which has ~90% efficiency & an effective COP of >20. The air source heat pump is only used to provide space heating & all of this energy comes from reducing the exhaust air temperature below the inlet air temperature.

    Even so, using this as the sole means of space heating requires that the house is to Passivhaus levels of insulation & airtightness. The house described by DaveHempyards doesn't meet that standard.

    The NIBE in this case can instead be used to supply underfloor heating, but the heat pump can only deliver energy that would otherwise have gone to the supply air. So it helps to reduce the peak supply air temperature, but it doesn't increase the energy available for space heating. The gap can only be made up by the immersion heater.

    I suspect that most of the NIBE's heat pump capacity is used doing the job that a MVHR unit could do with a lot less electrical input & the space heating & DHW is mainly provided by the immersion heater.

    • CommentAuthorstones
    • CommentTimeApr 7th 2011
    Just been reading through DaveHempyards experience. I can of course only talk about my own house and how the unit has performed. I have the following figures since installation in Mid November. Household consumption which tallies with our electricity consumption in previous homes (where we had a basic MVHR installed) - 475 kwh per month. Although a high figure this incudes the constant operation of the ventilation fans.

    The following figures are therefore exclusively the Nibe 470 consumption - cost of electricity per unit currently being paid 8.66p per kwh

    Nov - Dec 1075 kwh used (1550kwh inc domestic consumption) £93
    Dec - Jan 1535 kwh used (2010kwh inc domestic consumption) £132
    Jan - Feb 2025 kwh used (2500kwh inc domestic consumption) £ 175
    Feb - Mar 1525 kwh used (2000kwh inc domestic consumption) £132
    Mar - Apr 525 kwh used (1550kwh inc domestic consumption) £ 45

    As the weather has warmed up so consumption has diminished. Immersion consumption which is included in the above figures has been as follows

    Nov - Dec 604 kwh
    Dec - Jan 848 kwh
    Jan - Feb 583 kwh
    Feb - Mar 800 kwh
    Mar - Apr 254 kwh

    As previously described our house is very well insulated with an internal footprint 134 sq metres. The Nibe is within the thermal envelop and all the ducting is insulated.

    It is disappointing that DaveHempyard is having the problems he is. I suspect that the big issue is the one highlighted - uninsulated ducting. I would agree with qeipl that you need to get to the bottom of why the consumption is so big and the ducting issue would seem to me to be the first thing to investigate.

    I am still however experimenting with my unit. In terms of our lifestyle the Nibe suits us as a fit and forget technology, albeit I am trying to tweak things to get it operating that bit more efficiently. As a technology it suits our house and location as we are off the gas grid, and is certainly preferable to me when compared to burning logs/ biomass / oil / LPG. Using SAP figures and fuel efficiencies, I have done paper comparisons against oil / LPG and biomass / logs and have found that for my house there is very little difference in total fuel cost. If anything, the NIBE was working out slightly cheaper to run, without the hassle of burning logs / having to worry about oil / LPG supply as many in my locality did in the cold snap.

    The Nibe is not a unit I would fit into a big house and I think the only way it becomes viable is in a highly insulated house.

    I therefore agree with davidfreeborough's comment 'Even so, using this as the sole means of space heating requires that the house is to Passivhaus levels of insulation & airtightness. The house described by DaveHempyards doesn't meet that standard'

    It seems to me that the unit can either heat or provide DHW in winter but not both without (immersion) back up. It is my understanding that the unit can be coupled with Solar or indeed mated with a boiler or wood burning stove which had been my initial thought ( i didn't hook up my wood burner in the end having been advised against it). This past months figures would suggest to me that as the temp rises, and certainly with outdoor temps above 11 degrees (when immersion use is blocked)the unit can both heat and provide DHW without immersion back up.

    The Nibe was only one of two units that did the job I wanted, the other being a Genvex £2000 or so more expensive. Whilst I could have fitted a Genvex as described by David (which I considered) to provide ventilation and space heating, I would still have had to provide a means of heating DHW. It always seemed to come to a choice between installing a secondary boiler (oil / LPG / biomass) which I didn't want to do, solar with immersion back up, night rate electric with immersion back up or go for all instant hot water and no bath (electric showers and instant hot water taps). Lifestyle meant we discarded the last option and the other options seemed to me to raise the overall cost of installing a dual system when compared to the all in one solution that the Nibe offered.

    Compared to previous houses the heating / DHW costsa are significantly lower. I would however attribute this to the insulation rather than the Nibe. All the Nibe has provided is a ventilation/space heating/DHW solution at a comparable capital and ongoing running cost when compared to alternatives that would meet our demands and our off grid location.
    • CommentAuthorstones
    • CommentTimeJun 21st 2011
    Just a further update on running costs over the past 2 months.

    Electricity use for the past 68 days has been 1122 kwh, which works out to 16.5 kwh daily. This figure includes our household electricity consumption which is generally around 11 kwh daily.

    5.5 kwh per day has therefore given us all our hot water, ventilation plus some heating, Internal house temp maintained 18.5 - 19 degrees. All via heat pump / compressor - no Immersion.

    Over the period the water accounted for 695 hours operating time out of a total of 757 hours (heat)

    Water demand would therefore appear to take 10 hours of running time to heat up sufficient water for our use (2 showers in the morning, bath in the evening, plus handwashing / dishwashing etc).

    Operation through the winter showed that the compressor ran for 18 hours (never more) daily after defrost cycles.

    That gives us up to 8 hours of spare capacity in the winter for heating. Based on operation to date and changes that can be put in place to prevent immersion use at specific times, it would appear that we can run the Nibe withoput Immersion March through to the end of November.

    Given our previous electricity consumption in a house with MVHR, I am more than happy to be able to run the house for the same energy consumption and get my hot water and heating thrown in for 9 months of the year.

    Come this winter I plan the following - temperature step down overnight, to say 17 degrees. Due to the time lag in the temperature being actually changing the timings would probably be from 1900 / 2000 to 0400 / 0500 the following morning. I have already 'blocked' use of the immersion in both the morning / evening and have lowered the outside temperature threshold so that immersion can only be activated below 8 degrees.

    I am also continually monitoring electricity tarriffs as at some point it may be that an econmy 10 tarriff becomes viable.

    The other area which could be changed is the flow rate / rate of air exchange. Currently the NIbe is operating at 55% to give an airchange every 2 hours, the thinking being that if I reduce the amount of cold air coming inot the house in winter, I'll need less heat to bring it back up to temperature, and thus reduce overall energy consumption.
    • CommentAuthorGreenPaddy
    • CommentTimeJun 21st 2011
    Hi Jason,

    (David J here). Useful info included above. I was interested to see your note about your electricity costs of 8.66p per kWh. Who supplies you at that price? Taking either a straight rate, or with a standing charge, and averaged out, I can't get better than about 20p per unit (allowing for economy 7). Have you not included for the standing charge?

    Also, our annual total elect usage will be about 3500kWh. That of course does not allow for solar panels, and log burner with back boiler (both mainly for DHW). Your numbers look like you might be up at 8000kWh or 9000kWh per year?

    Making lots of assumptions about same room temps/DHW usage, which is not unreasonable as have similar size families.....

    ....assuming my 20p per kWh (just for rough numbers, until I change to your 9p supplier), that's £1000 per year contibuted by solar and log boiler ((8500 - 3500)*£0.2).

    I don't believe the solar and log boiler contibute that, so there is some way that your set up is consuming a lot of electricity. Looking briefly at your numbers, you use about 3000kWh annually for the Nibe, and 6000kWh for other elect. The 6000kWh for other looks really high - makes the 3000 for the Nibe less of a concern.

    As discussed previously, I would definitely reduce your air change rate, but the first area to look at, for me, would be the 'other' elec - maybe a couple of clip-on meters (about £30 each), to see where that 6000kWh is going?

    Cheers, David.
    • CommentAuthorstones
    • CommentTimeAug 12th 2011
    Hi David

    Deal with the electricity tarrif first. Currently with EDF. The 8.66p excluded the stadning charge of 29.07p per day. However, when I signed up (in December last year) they were offering a £100 credit, so the standing charge was effectively paid for. Prices rises have however come into effect mid July this year. Up to 10.97p per kwh, 23.97 standing charge. Even assuming that you didn't get an introductory credit, for you, the SC would be £87.49 + 3500 x 10.97p = £383.95. £471 total. Even rounding up to £500 if your usage increase a bit more than anticipated. Best advice would be ditch economy 7. I looked at economy 7 and economy 10 and worked out that I would need to be up around 15000 kwh before real cost saving came in. The standard rate for E7 and E10 is ruinous. The half rate when I looked at it was only 1p or so cheaper than the flat rate I could get. I keep checking the comparison sites for deals / offers etc, bearing in mind that even being locked in to a supplier, the highest 'penalty' I have seen for leaving the contract early is £30. Even if you have meter change costs I think it would be weel worth it given your low usage.

    Just checked our figures for the past 134 days (since end March) Have used a total of 2027 kwh, which equates to 15.12 kwh per day. This period corresponds to the start of when I made changes to the way the immersion could kick in. Normal usage for cooking, fridge, TV etc is 10 - 11 kwh daily. The 5.5 kwh figure for ventilation and DHW would therefore seem to hold, albeit this may increase slightly over the winter months when the incoming water temp drops slightly.

    Although still available the Immersion has not been used since the end of March when I made the changes to the system programming specifically blocking its use in the morning and evening after DHW has been drawn for showers , baths. From what I can see the unit, like all thermostat operated devices wants to heat the DHW water back up as quickly as possible, and will use Immersion to do so if it were allowed.

    I think my year one energy use will end up at 11500kwh, of which I think I can confidently say 4000 is used for daily consumption, 2000 for ventilation and DHW, certainly 6000 kwh combined, the remainder being heating - 5500kwh, 3200kwh of which was Immersion.

    We can obviously tinker with our living habits to hopefully reduce daily consumption. DHW and ventilation are in my view reasonable. The main area in which I could get savings would therefore be heating. We know form the several very long power cuts we had during the winter artic cold snap, that our house retains temperature very well, remaining comfortable within when the ventilation and heating was off.

    Reducing air flow change rates is the first thing we will try, but given that the exhaust air provides the heat for my DHW I need to ensure sufficient extraction to fuel the compressor. As per my last post, 10 hours compressor running time for DHW water (and a little heat). It would therefore seem possible to reduce airflow by 40% or so and still ensure sufficient extract air heat for the compressor to extract to heat the DHW.

    Alternatively, or in combination with the reduction in airflow, I could completely block use of the immersion, and direct that water be heated first, with anything remaining being directed to heating. The unit has (in my view a quite questionable) quirk whereby if the internal temp drops below 16 degrees, compressor use is blocked and the immsersion kicks in, so I have to ensure that this does not happen. I have the wood burner which would fill the gap. Almost a reverse of your set up, where instead of heating water, I heat the house by means of the wood burner, leaving the water to the heat pump.

    The system allows me to switch off the heating function. I can therefore ensure that all theenergy can be directed to the DHW using the unit on a reduced air flow, and rely completely on the wood burner for heat.

    Because of the way the system works, lighting our wood burner does nothing other than raise the internal temp over and above what the unit provides. It does not impact on what energy is used by the unit as this is determined by the external air temp, which has to be heated as it enters the house. In general we found the wood burner would lift the whole house by 2.5 to 3 degrees - easy to overheat. We could therefore lower internal temps supplied by the unit and use the stove to top up. This of course is more lifestyle dependant, i.e. actually being in to light and keep the thing going.

    This of course then brings me back full circle to the original rationale for us choosing this system. We wanted a fit and forget technology that could deal with all or DHW and heating requirements. I have the option of retrofitting Solar DHW, albeit that would not really solve the issue of the heavy winter electricity consumption.

    My SAP calcs showed the following energy requirements 2191kwh for DHW, 2101kwh for ventilation / pump, 1245kwh for heating + 532kwh wood stove heating - 3458kwh when respective 250% heat pump COP and 65% of stove factored in. As above, actual consumption for heating 5500kwh, 3200kwh of which was Immersion.

    We therefore find ourselves at least matching and probably exceeding performance on DHW and ventilation / pump costs. Heating on the other hand is poorer than anticipated, albeit we maintained higher internal temps. Immersion use in particular can be targeted for significant reduction by simply maintaing the block I have put on to prevent rapid DHW re heat.

    With firewood locally at £180 (and rising) for a 4 cubic metre load, it is not a cheap alternative. Without trying it is difficult to judge, but given the known volume we could burn in a day, I suspect that we would need between two and three loads for the winter months if using as our main form of heat.

    Reducing the air flow would therefore seem to be key to lowering energy consumption.

    I will also get a meter so I can confirm exactly what the Nibe is actually using.


    • CommentAuthorGreenPaddy
    • CommentTimeAug 13th 2011
    Hi Jason,

    when I get a little more time, I'll go through you're notes in more detail, cause I really feel that 11500kWhr/yr is a lot (maybe it's not). The 5500kWh/yr for heating is 1000kWh/month (heating months) which is 35kWh each day, or a 1.5kW elect fire running non-stop. I'll dream about it some more...

    As for the elect costs - I'll e-mail you a little cost calculator I made up last night (excel), which plots graphs of total annual costs v's energy consumed (for any unit rate values you want), and compares two sets of rates.(eg. standard against Economy7)

    It shows that my Eco7 rates versus your EDF rates breaks even for me at 3500 kWh/yr, assuming 20% night usage (which it currently is - intend to move more to night time)

    I reckon that you should be able to have 25% of your power usage at night (easily)...

    Using your figures from above...

    2000 vent&DHW - 75% at night by heating water at night - so that's 1500
    5500 heating - 30% at night - so that's 1800
    4000 other - some will be at night, but let's ignore that

    So of 11500, you could use 3300 units at night (with very little lifestyle change) - that's nearly 30%.

    By putting on dish/clothes washes when you get up in the morning, etc, you could drive that further.

    So you'd actually be better off on Eco7 after 2500kWh/yr

    You can pump in the numbers yourself to the spreadsheet (when you get it), and try different ratios of day/night usage - it takes seconds, honestly!!

    Regards, David.
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