Home  5  Books  5  GBEzine  5  News  5  HelpDesk  5  Register  5  GreenBuilding.co.uk
Not signed in (Sign In)


Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
These two books are the perfect starting place to help you get to grips with one of the most vitally important aspects of our society - our homes and living environment.

PLEASE NOTE: A download link for Volume 1 will be sent to you by email and Volume 2 will be sent to you by post as a book.

Buy individually or both books together. Delivery is free!

powered by Surfing Waves

Vanilla 1.0.3 is a product of Lussumo. More Information: Documentation, Community Support.

Welcome to new Forum Visitors
Join the forum now and benefit from discussions with thousands of other green building fans and discounts on Green Building Press publications: Apply now.

    • CommentAuthortopher
    • CommentTimeOct 17th 2020
    WillinAberdeen, Thanks. I have a GSHP in France and the maintenance is child's play - just clean some filters. Tweaking the settings is perhaps more important.

    I think your advice about warranty is important. Some of the Nibe parts are very expensive. I will check on that.
    • CommentAuthortopher
    • CommentTimeJan 17th 2021 edited
    I used to think that ASHPs were a problem because the max water temp is only 57 degrees, so users will need to install bigger radiators or fan assisted radiators.

    WillinAberdeen had a Dakin installation that worked up to 70 deg, but it only lasted 7 years.

    Anyone got any comments or experience of these high temperature ASHPs? Do they last > 7 years?
    • CommentAuthorGarethC
    • CommentTimeJan 18th 2021
    The running cost economics of heat pumps have worsened as electricity prices have risen relative to natural gas recently, which is unfortunate as the environmental benefits have improved as grid CO2 intensity has fallen.

    But if you can use overnight electricity on an economy 7 or other tarrif as much as possible, it occurs to me that it might change things, because the electricity is much cheaper and even greener on average. Am I right? And if so, how would you do it?

    And does that argue for a high temp ASHP, filling a hot water tank and hot water store at a lower COP during the night, as that's OK because the electricity cost and grid emissions are so low?
    • CommentAuthortopher
    • CommentTimeJan 29th 2021
    Gareth, There are some things you could do to offset the high cost of electricity

    1, Have solar panels and a big battery. Charge the battery at night and in day time. Use the battery to run the ASHP. Hope that it all balances out.

    2. If the house is big enough, have a large amount of hot water stored. Heat this at night with off peak.

    3. Live near an underground stream, this will give a higher temperature to the heat pump input, reducing the amount of electricity.

    Anyone got other ideas?
    • CommentAuthorMike1
    • CommentTimeJan 31st 2021
    Posted By: topher
    Anyone got other ideas [to offset the high cost of electricity]?

    Potentially move to a tariff with dynamic charging; interesting thread below, if you missed it:

    • CommentAuthorbarney
    • CommentTimeFeb 1st 2021
    Or instead of the complexity of electric battery storage, you can run the ASHP from PV and put that output into storage (ie big, well insulated tank). Run the UFH at ultra low temps with a small flow to return delta T and again put that into storage in the building slab

    That approach tends to couple heat generation when the energy source is most effective and a method of usage that both reduces energy input and gives a smooth well buffered output.

    Probably better suited to full refurb or new buid


    • CommentAuthorowlman
    • CommentTimeFeb 1st 2021 edited
    Barney, what sort of interface, if not battery, do you envisage between the variable Solar supply and the variable ASHP demand?
    • CommentAuthorbarney
    • CommentTimeFeb 2nd 2021
    Big tank of water basically - say a day or more worth of kWh

    Coupled with a thermally massive floor running at say 21C - if the room wars from solar gain, then the slab stops emitting heat and that's also the second mode of storage


    • CommentAuthorEd Davies
    • CommentTimeFeb 2nd 2021
    Right, but you'd still want to modulate the ASHP to follow the output of the solar. In principle that ought to be easy but how you would do it practically with most off-the-shelf heat pumps?
    • CommentAuthorbarney
    • CommentTimeFeb 2nd 2021
    Or enable the heat pump to only respond to the cylinder temperature - so the ASHP is only seeking to maintain the store temperature set point and give it a "start" from a minimum invertor output ?


    • CommentAuthorEd Davies
    • CommentTimeFeb 2nd 2021
    Say you have a 4 kW PV array and a 4 kW ASHP running at a CoP of 3 so taking 1333 W input when running at full tilt. A large proportion of the winter daytime, when you want the HP most, when the PV is generating useful power it'll still be less than 1 kW. It'd be sad to not to run the HP getting 1 to 3 kW of heat out of it when it's perfectly capable of modulating down to do that. Or to import extra electricity, of course.

    BTW, I fully support the notion of storing the energy as heat rather than in a electrical battery. It's vastly cheaper per kWh. The nuttiest thing is to ever take electricity out of a battery and use it for heating. If you do it short term (less than a day) you're paying the cost of cycling the battery (comparable to grid electricity prices [¹]), if you do it longer term (many days) then you're tying up capital which would probably have been better spent on more PV to generate the power closer to the time of use.

    [¹] https://shop.gwl.eu/Winston-40Ah-200Ah/WB-LYP100AHA-LiFeYPO4-3-2V-100Ah-WIDE.html

    In very round numbers: £100 for 330 Wh so £300/kWh. Say good for 3000 cycles so £0.10p per kWh cycled.
    • CommentAuthorowlman
    • CommentTimeFeb 2nd 2021
    So assuming grid tied PV, the ASHP would possibly use Grid energy too, plus any bits of surplus that were not used up by house baseload; quite possibly not much in winter, no?
    Doing it direct, non grid tied isn't possible without some sot of fancy non standard circuitry AFAIK.
    • CommentAuthorbarney
    • CommentTimeFeb 2nd 2021
    Indeed Ed

    So if you set the Invertor start signal to say 900W of PV output, you take everything from the PV and the rest from the grid - as the HP cycles down then the house takes any tiny excess.

    As we move into spring, the PV gives more, and the heat pump takes all of that and any excess is also used or exported

    And so on until the PV output swamps the HP requirements and you are back to a summer condition.

    I was just suggesting that we can easily decouple the day to day demand from a steady burn into the buffer cylinder (and the floor)

    Then deal with HWS independently

    Primarily this was in response to running heat pumps at high temperatures - really low and slow is the way to go, using as much site generation (or off peak) as possible, rather than tying to do everything from PV

    In that case, the grid also becomes the battery, in conjunction with the buffer vessel and the floor slab

    It's easier for me, because I also have a summer cooling demand, so I get a decent amount of cooling by holding the buffer at 20C (to avoid condensation) and running that through the floor in summer, driven by the HP powered by the PV


    • CommentAuthorowlman
    • CommentTimeFeb 2nd 2021
    <blockquote><cite>Posted By: barney</cite>Indeed Ed

    So if you set the Invertor start signal to say 900W of PV output, you take everything from the PV and the rest from the grid - as the HP cycles down then the house takes any tiny excess.

    You can't run the ASHP from PV direct 'cos it's an inductive load and needs a steady supply more especially as it's modulating too, therefore not conducive to the variable PV supply.
    So your ASHP power is coming as leftover power, after the house daytime baseload is taken into account, unless of course you're prepared to shut the rest of the house down, whilst you charge your HW tank.
    In winter time UK there is unlikely to be much of that PV surplus left to run the an ASHP on a reliable enough basis and your HP is in the main going to be using grid energy.

    BTW I have 10kW of grid tied Solar PV I also have an ASHP A2A, but essentially the same kit as A2W. Many, no most, days in winter the PV doesn't supply enough for the daytime house baseload and the ASHP running continuously.
    • CommentAuthorbarney
    • CommentTimeFeb 3rd 2021
    The PV is just simply "pushing" power - the HP doesn't know or care where that power comes from, and I wasn't suggesting that the PV is solely for the heat pump - just a conventional grid tied invertor pushing kW into the system

    I was simply suggesting that to avoid using grid "now" that the HP is only enabled when there is a minimum PV output reported - that's the whole point of having the buffer vessel and the floor as storage, so you can sensibly make use of the minimal PV winter output and make up the balance by grid import. If you have a lot of PV output, then happy days, if not then the grid "takes over".

    Think of it as just simply a summing of energy flows - the house needs some energy(which is variable) and you provide it from mains or PV based on the availability of each - all that changes is the ratio of the energy source used to meet the output demand


    • CommentAuthorEd Davies
    • CommentTimeFeb 3rd 2021
    What Barney suggests seems quite sensible to me. I just think it'd be a pity not to use the typical inverter-driven heat pump's ability to modulate to match the PV (minus other house loads) a bit more closely so I'm wondering if there's a practical way to do that.
    • CommentAuthorbarney
    • CommentTimeFeb 3rd 2021
    I guess it could be done with power point tracking, Ed - but I'm not certain that much of the small scale commercial kit has that kind of functionality enabled

    As an example, I have an air source heat pump domestic water heater (this extracts heat from the bathroom ventilation system via a HP, direct to stored water - and/or via the UFH return)

    The unit has a number of enable functions, one of which is a start command from PV - so the invertor output determines when the unit operates (and can be further enabled disabled when the UFH return set point is satisfied)

    It does not have the kind of functionality that would enable the heat pump to vary output based on PV output - but I guess with a "black box" of tricks it could be achieved - first question is "would it be worth it" I guess


    • CommentAuthorowlman
    • CommentTimeFeb 3rd 2021 edited
    "Would it be worth it?"

    That's what I was getting at, complex relays, risk of Heat pump damage, the system fighting itself, and all with off the shelf heat pumps, trying to use fractions of Wintertime kW, the answer's no, uneconomic, IMO.
    If you want to glean excess/surplus PV( almost to the last drop), the simplest and cheapest way is via an immersion and a power diverter. The resistive load, and others too, lend themselves to that type of use.
    • CommentAuthorbarney
    • CommentTimeFeb 3rd 2021
    Once the PV output is on the busbar of the consumer unit, it can feed anything - it's synchronised with the mains and cant exist without the mains (generally) - what you don't use is simply exported (regardless of if you get paid for that)

    If the Heat pump is calling for input power and the PV has output power than that's all that is required. I was simply suggesting that by putting a credible PV output threshold as a heat pump start signal you can force the heat pump to operate when you have PV output available despite the buffer vessel calling for heat (you are still supplying the heating demand from the storage)


    • CommentAuthorowlman
    • CommentTimeFeb 3rd 2021
    Even with a soft start heat pump the start up current required could be in excess of 110% of full load. So lets assume your PV trigger point is set at that amount, but 30 seconds later it's nothing, and the pump has to shut down, and then 30 seconds later the PV supply is OK again, and so on, ad infinitum, and that's not unusual, especially in the winter months.
    In this scenario you've got variable demand from the buffer tank, you've got variable supply from the PV, and a heat pump with a greater start up current, which eventually settles down to modulate, so variable.
    Trying to balance that lot is going to need some very fancy electronics and possibly software, plus a very robust heat pump to cope with the constant on/off cycling.
    IMO it's not worth considering, instead just connect the "in tank" immersion with a control stat, to a diverter and have done with it, cheap simple and effective.
    • CommentAuthorbarney
    • CommentTimeFeb 5th 2021
    OK - you seem to be suggesting that unless your PV array is massive, then it's not possible to run a HP in conjunction with the mains

    Which is clearly not true - I used to run a 12kW heat pump with a 1 kW PV array which took all of my PV output and more supplied by the mains.

    The buffer tends to force the heat pump into a pretty constant run cycle - it won't cycle that much once the PV threshold is exceeded - all that varies is the portion of intake energy to the heat pump supplied by the mains and the PV.

    Once the PV is synchronised on the house busbar the whole of the available PV output is consumed, either by the house load or by the export if it exceeds the house load - as I said, it's just pushing power, up to it's limit


    • CommentAuthorowlman
    • CommentTimeFeb 6th 2021 edited
    No, what I'm suggesting is that it's not possible to run ASHP with standalone PV, it's an unstable supply and the inductive load can't cope, as you say it has to be as part of the home supply.
    In a grid tied array the mains is buffering the supply to the ASHP and in Winter that's almost invariably all of it, what with the fickle nature of winter Solar.

    The house base load is usually consuming all of what's available, at least it is in mine in the depths of winter. Therefore if all that base load consumption occurs at a time when your tank is calling out to be replenished you are going to have to call on something else. That something else is either the grid, or a battery storage which is using the low output in between bits of leftover PV to trickle charge so it's there on call with a stable supply when the tank demands a re-heat. That way all the leftover PV is used.

    However if you wait for the excess PV to exceed your set "trigger point" before switching on the ASHP, you'll be waiting a long time in winter, and possibly all the time your tank is calling for a re-plen.
    In your case, the 12kW heat pump was only using the 1kW PV if your house base load was at or near zero. As a household in general you're only using leftover PV to its limit, when the heat pump is running.

    I have a couple of PV/export monitors in the house and I manually switch on immersions if the sky looks clear and I think production is good. I'm in the process of automating that in the coming months, but to immersions not heat pump. That runs continuously and is therefore part of the house base load from 0630 to 2300.
    • CommentAuthorEd Davies
    • CommentTimeFeb 6th 2021
    Posted By: owlmanNo, what I'm suggesting is that it's not possible to run ASHP with standalone PV, it's an unstable supply and the inductive load can't cope, as you say it has to be as part of the home supply.
    Indeed, but I'm not talking about standalone and I don't think barney is either. My and barney's conversation is about practicality and worth of making the HP load follow the PV output to minimise (not necessarily eliminate) import.
    From an environment perspective it makes no difference - if you export your peaks and troughs into the grid, then someone else a few miles away will have peaks and troughs at slightly different times, and it will average out to everyone's benefit. Then you can run the heatpump when the grid intensity is lowest, often this is overnight.

    From an individual financial perspective, if someone is looking at the price difference between export tariff for a kWh versus avoiding paying import tariff, they could consider the tariffs that pay 5.5p to export during the daytime, and offer a few hours of import at 5p in the early morning, so they make 0.5p overall.
    • CommentAuthorDiarmid
    • CommentTimeFeb 8th 2021
    Been lurking here and considering my own options for an ASHP. Currently using LPG powered UFH, use about 2400l per annum at current prices £1200.

    Would it pay to covert to ASHP?
    • CommentAuthorEd Davies
    • CommentTimeFeb 8th 2021
    According to Wikipedia LPG has a volumetric energy content of 26 MJ/l. It doesn't make it clear, though, if that's the lower or the higher heating value (without or with condensing). Let's assume it's the higher heating value and the boiler, etc, is overall 90% efficient so you'd be getting 26 MJ/l x 2400l/a x 0.9 / 3.6 kWh/MJ = 15600 kWh/a. Assuming a heat pump with a seasonal COP of 3 you'd need to buy 5200 kWh/a of electricity.

    For the ASHP to win purely on running costs electricity would therefore have to cost less than £1200 / 5200 kWh = 23.08p/kWh.

    That seems plausible. Obviously there's the capital cost issue which might swing one way or the other depending on if the boiler needs replacing anyway. Also, the thermal performance of your house matters - if it's reasonably well insulated and has a has significant capacity (decent sized slab) then you've a bit more flexibility in when you heat so can make use of variable tariffs (“agile” or just E7/E10) better. (I used a relatively low CoP above on the assumption of predominantly overnight heating).

    Then you need to think about the future costs of the two energy sources. My guess would be for both to go up but LPG a bit more than electricity.

    Also to be considered is the amount of hot water needed which may need some adaption from the quick heating of LPG. What about cooking, is that LPG now?

    • CommentAuthorowlman
    • CommentTimeFeb 8th 2021
    I already have ASHP, A2A; I too have looked at an additional A2W unit to to power my radiators, no UFH, via a large accumulator/buffer tank.
    I've more or less ruled it out, the upfront costs are too high. I looked at both monobloc and split units with the same
    outcome. Admittedly I didn't look at any Government initiatives, there always seems to be a catch.
    For me, the gut feeling was that some sort of electric immersion/s set up on E10 or E7 or Agile type tariff would offer better economics. Cheap, simple, very controllable, and easily maintained.
    £1200 buys you a lot of grid power on cheap rate. All the better if you can make savings elsewhere in the home, insulation, draughtproofing, supplementary PV, etc., even the odd A2A unit, ( fast response and much cheaper than A2W to install ).
    Ed, I think you want the LHV, which is almost always used by boiler manufacturers when they talk about "90% efficiency" because it makes them look better.

    Price comparison here suggests that LPG and ASHP running costs are very similar https://nottenergy.com/resources/energy-cost-comparison/
    But beware that all petroleum fuels have become very cheap last year since Covid, but the oil price is rising again now so it might not last.

    The intent of RHI is to cover the extra capital cost of ashp compared to installing a new boiler.

    IME Diarmid's question is quite dependent on situation
    - does the current boiler need replacing or not
    - how much energy do you expect to use in future (planning to insulate?)
    - how do you do hot water?
    - how much difference do 'green' issues make - switching LPG to ASHP would remove most of your CO2 at a stroke

    Not sure but I seem to recall Owlman uses not very much heating and already has heatpump so the answer for Owlman maybe different to Diarmid.
    • CommentAuthorEd Davies
    • CommentTimeFeb 8th 2021
    Posted By: WillInAberdeenEd, I think you want the LHV, which is almost always used by boiler manufacturers when they talk about "90% efficiency" because it makes them look better.
    Ta, I didn't know that. Bit of a cheat for a condensing boiler to use the LHV, it could (should?) in theory have an efficiency greater than 100%.

    Still, assuming the Wikipedia value is the HHV is conservative in that it biases the calculation towards the status quo, Diarmid's existing LPG setup.
    This one condenses against really cold mains water and is 108% efficient !

    Diarmid should consider that LPG costs 7p/kWh according to Nottenergy (link above) and also by Eds figures (£1200/15600KWh), but is starting to rise again

    A heat pump with CoP of 3 running on off-peak electricity costs 3p/kWh. Or 7p/kWh if running on peak electricity per Nottenergy.

    RHI pays you 10.9p/kWh subsidy for heat pumps.

    So you need to work out if the savings on running costs, plus the subsidy from RHI, are enough to pay for the extra installation cost of a heat pump compared to a new boiler, considering the other factors I mentioned above.
The Ecobuilding Buzz
Site Map    |   Home    |   View Cart    |   Pressroom   |   Business   |   Links   

© Green Building Press