Posted By: roseramblerIf a simply solution is what you are looking for, try reducing the load on the heating system. Your woodburner seems to have more KWs output than needed, have you thought about a back boiler on the woodburner (and / or another woodburner) and link in a few rads to it to one side of the house. That means you could focus the heat output to a smaller area, thus more efficent, less noise and more heat.
Only a thought.

About 18 months ago I happened upon a small fully automatic pellet boiler that put about 2 kw to air and 4kw ish to rads. There is no obvious place for it though at the north end of the house where we could get the chimney to an external wall. And it doesn't solve the DHW issue. From there we've drifted towards a log gasification boiler with a large TS. Providing DHW and space heating through rads. Problem is the cost.

I would say also, 10,000kw per annum for a 220 sq m house - is that just space heating, space heating and water heating or all energy.
Most normal houses usually need to throw 15,000 - 20,000kw for energy costs to a family home
RR

That's total energy use for family of 5, all electric from a zero carbon (and thus high cost!) energy company.

We have a Nibe fighter 470 exhaust air heat pump fitted to our house (150 sq m, wall u 0.11, ceiling u 0.1, windows u 1.2) which provides ventilation, space heating and DHW.

http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6696&page=1#Item_0

We have used 11500 kwh over the last year (total consumption) Of that assume 4000 kwh household consumption and 2000kwh DHW, which leaves 5500 kwh for space heating. Of that 5500kwh, 3000kwh was immersion back up, virtually all used in the three coldest months last year/earlier this year.

Echoing roseramblers comments, I do not think energy consumption levels are bad when you compare to a normal build using (in my experience) 20000 - 25000kwh of energy to space heat and DHW. The cost of the energy you are using is of course a different matter.

We keep our house at 19 degrees, which is achieved by the warming of supply air to rooms, and a wet radiator system also run of the heat pump unit. The only issues we have condensation wise are in the bathrooms after showers, and in the utility room where we dry clothes (no tumble dryer), where we can see some condensation, perhaps 1 - 2 com at the bottom of the window. We are going to try a condensation trap (like you buy for caravans) in the utility to see if that will deal with the problem this year.

It may therefore be as Paul suggests, that getting your internal temperature up may help. We also have a 5kw wood burner (external airsource). We have ours in a big open plan room (60 sq m) and it can very quickly over heat. Some of that heat is circulated by the movement of air caused by the MVHR side of things, but the far end of the house is cooler than the main living area, remaining at the 19 degree level.

We let the system manage itself last year, but this year we are going to change settings such as ventilation speed, as well as timing DHW etc to E7 rates to see if we can reduce our energy consumption.

In terms of what you should do, I would echo Davids comments, and try the E7 route with your U/F heating / investigate hooking up the woodburner / get some strategically placed radiators, as it seems that heat delivery is the issue.

At the very least with E7, given your units will be operating 24/7, you should be able to get 25 - 30% of your energy at the lower rate. Have you considered solar PV. Putting aside recent drop in FIT rates, having a higher level of energy consumption may mean you could match a lot of your consumption to what could be produced by a 4kw system orientated east and west to get a better spread of energy through the day.

Before ripping out, you could of course cost out what a gas/ pellet / oil system would cost to run. Obviously there would be the capital investment which may of course in itself be enough to put up some solar PV. When I last looked at it, it was break even at the 9000 kwh mark for space heating / DHW when compared to the quantity of oil, propane and pellets that would be required to provide the heat needed (as we cannot get mains gas). This was calculated on the basis of assumed CoP for the heat pump / assumed % efficiency of other systems as per SAP 2009. Recent hikes to electricty prices will no doubt have altered this calculation, but you have to bear in mind, whatever fuel source you use is going to cost, either cash or in time if for example you scavenge wood.

Posted By: daysleeperSo unless our AHSPs have a COP over 3 (and bluntly I suspect they struggle to hit 1 in cold weather) then we are being sorely let down.

ASHP's are available which maintain their capacity down to -20oC, but you need to read the specifications carefully & you pay for what you get. However, I don't think this is the main issue. Even if the GE215's heat pump was maintaining its 1365W capacity, this would not be enough to keep your house warm. Whoever designed the heating system did not do their sums correctly & they've specified the wrong equipment.

I'm sure a cost effective ASHP based system could have been specified. My starting point would have been a standard MVHR unit & a thermal store to combine solar thermal & a decent 5kW CO2 ASHP run mainly during the Economy 7 period. This would have avoided the expensive Genvex & Vanvex units, the need for a log burner & the need to distribute heat via the air supply. Even with underfloor heating &/or radiators throughout the house this would have been a lot cheaper to install, a lot cheaper to run & a lot more dependable.

David

Think you could do a bit better electricity price wise - N power deal I've just gone to 13.18p per kwh / 6.87p per kwh night rate (was cheaper but they hiked the price just after the switch - so much for shopping around to get the best deal Mr Huhne!).

I would agree in many ways that these technologies are limited. As has been said before, exhaust air heat pumps are limited in they can only extract whatever heat is in the outgoing air, and that of course is limited by the flow you set. That said, it would seem logical that the CoP of an exhaust air heatpump / what heat energy the heat pump can produce will remain constant as its supply (air extracted from the house) temperature remains constant. However, the heatpump can only ever produce a fixed amount of heat, and in our case, is unable to meet both DHW and space heating in the three coldest months. Something has to fill the gap, in our case electric immersion.

Is this better than a direct air source heat pump ? The CoP of air source heat pumps reduce as the ambient air temp reduces. Eventually you get the point where the CoP is 1, and at that point you are as well using immersion. That is the reason Iavoided a direct air source heat pump.

We certainly spent a great deal of time trying to decide what system would best suit us, and in the end cam to the conclusion that we needed an automated 'fit and forget' system because of the way we live. Greenpaddy, who comments on various other threads has a house spec similair in many ways to mine, has an electrical energy consumption just below 4000kwh total. He uses a wood burner to heat a big thermal store for his DHW and some space heating. He has made various adjustments to his MVHR flow rates, uses E7 to keep costs down etc, but he is more actively managing his house temperature and energy use, using his fire and an coil heater in his MVHR to maintain an even temperature. Such an approach is great if you are able to manage things in this way, but will not necessarily suit everyone - work, social lives/kids etc.

I suppose it comes down to convenience. We chose our system as we wanted fit and forget. My neighbour, has just installed a large wood burner for his DHW and heating, and doesn't buy in any timber, yet he spends all his spare time scouring the locality for fallen wood, and is forever chainsawing and chopping wood.

Hopefully, your PV and if you go for it, E7 will dramatically bring your energy costs down. Can you program your heatpumps to operate heating / DHW at specific times (to coincide with energy production PV / night rate E7)?


A thought for moving around the heat from your fire - you could try a low energy fan / passive fan that sits on top of the stove ? On occasion I've used a fan heater (coil off) to blow hot air from our living area down the hall into the bedrooms.

Posted By: stones As has been said before, exhaust air heat pumps are limited in they can only extract whatever heat is in the outgoing air

I had to think about this issue a lot before I could make sense of it. My first reaction was say...

Hold on a moment, how can this possibly work? How can it possibly put heat into a house if it's only source of heat is the heat already in the house?

I think I figured i out...

a) If the flow rate going in and coming out are the same then the ASHP must be cooling the outgoing air below that of the incoming air. Thats the only way the air coming out can have less energy than the air going in (meaning some has been extracted and left in the house to raise it's temperature).

b) Some outside air could be mixed into the exhaust air to increase the flow rate through the ASHP. It could be that having a higher volume of cooler air is better than having a lower volume of warmer air. What matters is how much energy there is going through the ASP collector in total and how efficiently the heat pump can extract it.

Posted By: davidfreeboroughIn an Exhaust Heat Pump system the ASHP is being used to move heat from the house to the incoming air or from the house the DHW. In either case you have already paid to put that heat there, so you're paying once to put it into the house & then again to move it to somewhere else in the house.

Better than throwing the heat away though.

Just to confirm (my unit anyway) extracting air from wet rooms in house at 18 degrees, getting expelled to the outside having passed through heatpump at -7 or -8 degrees. Air temp at moment here 8 degrees and has been most of the day. Has heated up DHW after morning showers and hs heating supply air into house to maintain 19 degree temp inside. No immersion used today

Posted By: RobLDo I understand this system correctly - is the MVHR unit exhaust being used as the energy source for the ASHP heating for the house ?

Sort off.

Right - House air from "wet" rooms is extracted, on its way outside it passes through a heat exchanger and gives up some heat to incoming fresh air increasing its temp. That fresh air then supplies the house. When the heat pump is running then the extract air goes through the pump.

So on a "warm" day , Extract temp of 22c, fresh air of 9c gives supply air of 20c (no heat pump running)
On a cold day Extract temp of 19c fresh air of 1c supply air 30c (ish)

Posted By: daysleepero that is 0.28 to 0.33 Air changes an hour across the house. (seems low to me?)

No, that's perfect actually.

Posted By: RobLSurely this exhaust gas, which will be a few degrees above external ambient at a slow flow rate, will be chilled massively below external ambient by the ASHP? Won't this make the ASHP COP very low ?

No! Not at all - the ASHP removes the heat from the internal ambient air, it is after that heat has been removed that the exhaust is cold. The COP will be quite high actually. The problem here isn't COP, it's capacity.

Earlier there was mention of insulating the ducts - this won't make any difference as they're all inside the heated envelope anyway (as far as I could tell - if not, then it will help obviously).

Paul in Montreal.

Posted By: davidfreeboroughBut a lot worse than an MVHR unit with an effective COP of 20 to 30.

No, an HRV system has a COP of less than 1 as the temperature of the air supply back into the house is lower than the ambient temperature. The most it could be is 1.0 - which would mean all the heat from the exhausted air is recovered plus all the waste heat from the motor to operate it.

An exhaust air heatpump, as far as I understand, still has the cross-flow heat exchanger to perform the pre-heat of the incoming air, plus the heatpump itself can extract whatever heat there is between the incoming air temperature and the exhaust air temperature - this is where the heat gain comes from, not from the ambient air of the house itself (except for any latent recovery that takes place).

Paul in Montreal.

Posted By: stonesI assume in saying an MVHR with a CoP of 20-30 refers to a passive crossflow type MVHR unit with a 90 to 95% efficency? It may be that this would be the most effective method (with an inline heater to boost temp when needed) of providing fresh warm air into the house, but at some point, the temperature differential between outside air and heat that can be recovered passively will become so much that additional heat is needed to make up the difference (-10 degrees outside, inside 18 degrees for example). Additional energy would be required to lift the temp back up presumably??

I'm assuming a 90 to 95% efficient counterflow type heat exchanger. The type used in Passivhaus designs.

An MVHR needs protection from frost & the simplest/cheapest way of doing this is an electrical resistance pre-heater which keeps the air intake at around -4oC or above. This is estimated to consume around 250kWh per annum for a 200m2 house in the UK. For a 90% efficient MVHR heat exchanger, this limits the drop in air supply temperature to around 10% of 21oC minus -4oC, or 18.5oC.

Posted By: stonesYou mentioned in an earlier post that a suitable size ASHP for daysleeper would be 5kw, run mainly on E7. On the coldest days(-16 degrees last year), my immersion (5.3kw) was on for 5 hours. I assume an ASHP would need to run for a similair period to heat a tank of DHW?

Yes it would need to run for the same time as an equivalent immersion heater but it would consume a third of the electrical power, assuming a CoP of 3. In such extreme conditions the ASHP would need to run for longer than 7 hours &, if the CoP/capacity was to be maintained, it would need to use CO2 refrigerant.

David

If an MVHR unit has a 95% efficiency, then it has a COP of less than 1, surely? (ignoring the tiny amount of power used by the fan).

I don't see how on earth an MVHR can ever have a COP of greater than unity, in fact I doubt it could ever reach unity.

Posted By: davidfreeboroughFor the design I've been looking at a 90% efficient heat exchanger will recover around 10,000kWh per annum, while the fans consume around 500kWh.

So that design will lose 1000kWh per year while consuming 500kWh. That's not a very good COP! The heat is already inside and has been payed for. Your HRV is still cooling your building at a rate of 500kWh a year. An exhaust air heatpump will always be more efficient than this, even without a pre heat of the incoming air as the exhaust temperature is always lower than the inlet temperature. In the HRV case, the exhaust temperature is always higher than the inlet.

Paul in Montreal.

The problem with low airflow is this, right now with fans at 100% and the ASHP running the supply air is leaving the heat pump at 42c (its +12c outside now). It has to heat a volume of house of 418M^3 but you can barely feel air coming out of the vents. If we were using rads a room would have a 1m^2 emitter at about 60c. Under floor heating would have a huge emitter at about 40c. But we have a tiny emitter at 42c and, as best performance is about 30c above Outside temp at 0 the supply air is 30 and at -15 it's 15c!

I can't see how a small volume of air at 40c can heat a large volume of air at 15c (heating of room temp) up to 20c in a reasonable timeframe if ever.

<blockquote><cite>Posted By: daysleeper</cite> If we were using rads a room would have a 1m^2 emitter at about 60c. Under floor heating would have a huge emitter at about 40c.
</blockquote>

Those temps seem a bit high to me, for emitter temperatures. Flow temperature at the heat source might be around 40 deg C for UFH, but emitter temperature will be closer to 25 deg C, or thereabouts. 40 deg C at the floor is uncomfortable to walk on, even in a bathroom. Similarly, flow temperature (for a pretty high temp rad system) might be as high as 60 deg C, but radiator temperature will be a bit lower, probably no higher than 50 deg C or so (I run mine at 55 deg C flow temp in very cold weather, with rad temps of typically 48 deg C). I have electric UFH (yes, I know it isn't great in terms of cost) in my bathroom at the moment and find that anything over about 28 deg C feels a bit unnatural on a tiled floor.