Interesting, and if some form of electricity storage becomes economic, for later additiion, a very, very green system.

Another thought, and making this up as I go along, I wonder whether it would be worth having A2A for space heating, and a small, completely separate A2W just for DHW? Again, savings from having no rads/UFH etc., and keeping them separate might mean they work better... If the DHW-only A2W were less than £4k, then possibly worth it (since normal A2W costs £6k)?

Posted By: GarethCInteresting, and if some form of electricity storage becomes economic, for later additiion, a very, very green system.

Bit in this weeks comic about storing solar as heat, admitted it is for large scale but they claim high efficiency, though no numbers to back it up.

I am with Ed, PV is relatively cheap, flexible, reliable, silent and can be easily fitted to most sites.

Nice system Monty.

One question, though: do you use enough gas to justify the standing charge, servicing, etc?

For context, I have a Bosch instant water heater in the static caravan I'm staying in. It'd take a long time to fill a bath but gives just about adequate flow for a shower. As it's awkward adjusting the shower temperature by mixing hot and cold I leave the heater set for shower temperatures then boost it with a kettle of water for washing up. With washing and shaving in the morning, one or sometimes two wash ups per day and a shower most nights a £75 bottle of LPG lasts about a year (changed it a few weeks ago, previous time was last winter sometime).

The alternative would be to heat with electricity. Per litre·K it'd be more expensive than gas but take out the fixed costs of gas and it might be cheaper (and sometimes come from partially from wind, particularly at the times of year when the PV is not helping much).

Posted By: GarethC
Another thought, and making this up as I go along, I wonder whether it would be worth having A2A for space heating, and a small, completely separate A2W just for DHW? Again, savings from having no rads/UFH etc., and keeping them separate might mean they work better... If the DHW-only A2W were less than £4k, then possibly worth it (since normal A2W costs £6k)?

Have you seen the Ariston Nuos FS 200/FS 250i? A2W DHW for around £2.5K. COP of real world installs seems to be around 3 from reports I've read online.

Degree Days:
Thanks for the help to jamesingram, SteamyTea and GarethC; pointing me at degree days has been very useful. Three winters of degree day data and gas kWh for this winter seem to have made a sensible breakdown estimate possible.

1: Local data from www.degreedays.net :-
______W11/12___W12/13___W13/14___Deg Day Delta___Pp Monthly kWh___kWh/W13/14
Oct_____90.5_____145________71_______20___74_________41.0___________0.58__
Mar____208.0_____354_______206________2__148________123.7___________0.60__

Feb____316.5_____319_______231_______85___88________167.9___________0.73__
Nov____162.4_____234______242_______-80___-8_________176.1___________0.73__

Dec____284.8_____288______234________51___54________191.4___________0.82__
Jan____301.7_____324_______267_______35___57_________199.0___________0.75__
_
Totals__1364_____1664______1251______113___413________899.1___________0.72__

Notes::
1: The largest monthly degree day delta is for Mar13/14 (148 from a total of 413).
2: The largest degree day number is 354 for Mar13; the largest for W13/14 is 267 for Jan14. The pump has averaged 6.4kWh/day (for a rated maximum of 10.8kWh/d) through W13/14 and has got close to its limit (demand can be limited by closing doors) when morning temperatures have reached freezing. I have not recorded a minus number for ambient temperature this winter.
3: Half of the degree day delta (413) W12/13 to W13/14 is down to colder Oct(74) and Mar(148).
3: The table has been sorted on 'Pp Monthly kWh' and the resulting 'kWh/W13/14' ratio rises with increasing 'Monthly kWh' with the exception of the Jan data. This rising ratio is to be expected because the heat pump power demand rises basically as the square of the temperature difference.
4: My original, back-of-a-fag-packet calculation showed that 1kW of pump rated power (preferably supplied as 2x0.5kW) was needed to do the job properly and this remains the case on power, distribution, noise and redundancy grounds.


2: My rolling annual (on a quarterly basis) gas bill in kWh is shown below :-
Spring14______8444 so far
Winter13/14___11732
Autumn13____16081
Summer13____20441
Spring13_____20535
Winter12/13___19872
Autumn12____18766
Summer12____15291
Spring12_____15165

Notes:
1: The Spring14 data is low because the current quarter does not complete until mid-June. Estimate another 500 for that and call it 9000.


Conclusions:
1: How much gas burn was saved by the 413 or 113 drop in Degree Days between winters :-
1.1: W13/14 and W12/13, and
1.2: W13/14 and W11/12?
Case 1.1: Year-on-year drop in gas burn 20535-9000=11,535kWh.
Case 1.2: Year-on-year drop in gas burn 15165-9000=6165kWh.

2: Average Degree Days over Oct13-Mar14=7.223C on a base of 15.5C, so average temperature is 15.5-7.223=8.277C; this is 1.277C higher than the rating temperature of the pump. Also the pump air inlet runs about 2C warmer than ambient because it is in the loft. The SPF of the pump may therefore be expected to be a bit above 5.7, say 6.0. 6.0*940/0.85=6635kWh gas equivalent at a boiler efficiency of 0.85.

3: Something like half the gas burn drop in Case 1.1: is down to climate and half to the pump/doors.

4: To get the gas burn down further from the 9000 in spring14 would require the lounge gas fire to be taken out of service and replaced by a second pump. Technically do-able but customer resistance is high.

I've been turning this one over for a while now. Undoubtedly some good results from systems that people have fitted and the cost savings if you have the skill and tools to install yourself are obviously significant.

Assuming however, that you would be paying someone else to install, I don't think A2A is quite as attractive as it first seems.

My own experience of them is in an office with a wall mounted unit. Its positioning meant it was either too hot or too cold and we could never get it just ticking over to provide a comfortable environment. Careful positioning such as in a hall, a ducted system or ceiling cassette system would seem ways to mitigate this issue.

So assuming a single ducted system, or two smaller individual systems suitably placed as some comments seem to suggest as a preferred strategy, you are looking at circa £4K to have installed.

Taking my own house where heating demand is the same as DHW demand at 5000kwh each, and assuming a CoP of 4 for A2A, that means you would be using 1250kwh/yr for heating.

That just leaves DHW to sort out. The suggestion of a solar PV system seems good as FiTs would pay for the PV leaving you to fork out for the cylinder, lets say £1000. Assuming 50% of your DHW is provided by your PV, that leaves 2500kwh/yr to supply from the grid - this of course could be done at night.

Cost wise then for £5K and annual running costs £325 ( E7 rates, all DHW during night ) you have all your heating and hot water.

I have been quoted £8K for an ASHP inc cylinder. Again, assuming heating demand is the same as DHW demand at 5000kwh each, and assuming a CoP of 3 for A2W (heating) and 2.5(DHW), that means you would be using 2000kwh/yr for DHW and 1666 kwh/yr for heating.

RHI of £5K brings the cost down to £3K for kit leaving £2K to fit a distribution system of UFH pipes or radiators. Running costs of £332 (E7 rates, all DHW at night )

You still then have the option of adding Solar PV (which as we have previously accepted pays for itself through FiTs) and using it for DHW (50% ? so reducing grid import to 1000kwh/yr for DHW - another £70 saving)

As we all know, RHI has distorted the market and significantly inflated the price of A2W heat pumps. The A2A market shows where prices should be.

Is A2A the answer, for some yes, and I think for smaller dwellings and buildings where DHW demand is very low then it would win. As a retrofit in an older house against direct electric, yes as well. However, as dwelling size increases and DHW demand increases the financials change, being different for each building and every occupant.

Having read through the Carbon Commentary blog, I understand the argument being made in respect of A2A as retrofits to replace heating systems using 60C plus temps through radiators, and indeed believe it to be a sensible proposal and solution (rather than A2W) as no heat pump can deliver at 60C without its performance being impaired.

My comments were aimed at well insulated and airtight houses where heating demand is the same as DHW demand.

It is perfectly possible to heat a home using a water based system (properly designed) at low temps. We run a low temp radiator based system. Flow temp at 0C ambient is 33C to maintain 19C 24/7. Had we gone for UFH instead of radiators, the flow temp could have been even lower. Both A2A and A2W would work for us. GSHP would not as the running costs due to extra pumps would actually outweigh the additional marginal performance benefits of the heat pump. As I said, A2A will be the right solution for some buildings and lifestyles but not all.

I would agree that the powers that be need to be even handed in their approach to such technologies and actually provide test data and unbiased information as to the benefits or otherwise of all heat pumps, including A2A. The fact A2A has been excluded from RHI is not necessarily a bad thing as it stops the over inflation of prices we have seen for A2W and GSHP.