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With ASHP its all about the system design. 55°c is the highest heating temperature most want to be designed to, however that's not the gold standard by any stretch, you want the system designed to be able to produce the required heat output at the lowest possible temperature (bigger radiators/UFH) if you can hit 45°c that system will run a better COP than at 55°c, if you can design it to 35°c that will be significantly more efficient again.
The units use different refrigerant gasses and as such have different outputs at different outside temperatures, ie: R410a units are an older type that can generally only scrape just over 60°c and will require an immersion heater to boost the cylinder temperature during legionella cycles or in winter when the ASHP may struggle to reach required output. R290 units (newer types) can typically hit 75°c so do not need immersion backup, they work to lower outdoor temperatures although there will still be a drop off when very cold, ie it may only reach 65°c when its -10°c outside.
With regards to only being suitable to heat newbuilds, well that's just nonsense. If your house requires X kilowatts of heat, and you provide x kilowatts of heat, you will have successfully heated it. The difference is in the economy of cost, electricity is much more expensive than gas (at the moment) So even on efficient systems its difficult to have the ASHP running cheaper than a gas boiler. This may well change with energy pricing going nuts, and the likelihood that the government will probably shift the green levies off of electricity and load them onto gas in the near future.
Compared to oil/LPG, many people with good installs find them cheaper to run, with mains gas and a good setup you can reasonably break even and have it not cost any more. Insulation is certainly something that should be looked at first, regardless of your heat source, just because we can heat a building "easier" with gas, doesn't mean its not an unforgivable amount of wastage and CO2 emissions. Getting basic insulation and draught proofing done first should be the priority of any house hold, not just those going to ASHP.
Heat pumps are being pushed to help the environment, not to save you money. But there are factors that can help. Solar panels if you already have them lend themselves nicely to heat pumps, as you dont have a heat pump "off" during the day, it has a "set-back" temperature ideally only a few degrees lower than your "comfort" temperature, so the solar often covers this there for making significant contribution to your heating which otherwise may be wasted during the day or fed back to grid for pittance. Time of use tarrifs, such as those that gave 5p/kwh overnight for electric vehicle charging means the HP would run overnight for very very little cost maintaining the background heat when the outside temperature is lowest and therefore least efficient. Also the cheap tarrfis can be used to preheat the hot water cylinder or heating buffer to store heat when the electricity cost is at its lowest.
I know people who take advantage of the above points and have significantly reduced running costs compared to gas. Plus once gas meter is removed they instantly save from the standing charges applied to a gas supply.
Your posts are great.
So, heat pumps can, in a normal domestic situation, generate enough heat during all seasons to match the load requirements of older houses without significant modification to the building?
Didn't know that tbh and it's not what i hear or read during my research. (I'm doing study on this in advance of replacing my boiler in the future).
Ireland has a grant agency and they have a heat load calculator. I struggle to get my house to a BER rating (B2) which is the ASHP benchmark without some significant changes to the house including reducing air flow, replacing glazing and insulating the cavities fully which I don't want to do for a number of reasons.
I'll revisit my assumptions.