Heat pumps cost more to run when it's very cold. Somehow mine managed to stay on par with gas, but many cost more than gas when it's very cold. This should be offset by being cheaper to run when it is warmer. This happens because ASHPs take heat out of the air, so it follows that when it's very cold there's less heat to extract and they have to work harder. 

What's your gas usage in kWh? It should say on your bill what your provider estimates it to be, or you could compare a bill from a year ago to find out. Knowing your gas usage helps weed out if you're being quoted an oversized heat pump. A good guide is to take your yearly gas usage in kWh, divide that by about 55, then multiply that result by 20. See this video for an explanation of why

You could also take the extreme route and do the heat loss calcs yourself. This takes several hours but in my case was well worth it. 

To make any kind of informed decision you would need a detailed heat loss calculation done by a reputable person and who preferably has no specific links to a certain brand or manufacturer. There will be people on here that have had such a service that can hopefully point you in the right direction. You are right to research and ask many many questions first. These systems need to be optimised to get the best out of them; good design, install and handover is vital. Good luck!

@oswiu that was very useful. Thanks for that. Having taken the details and made the calculation, it gives me an answer of 5.2kWh. 

That's probably on the low side. Yes, it includes cooking and hot water but we have  smart thermostat and are careful to turn the heating off when we go out and pit it back on around half an hour before we get back home. It also has  useful "absence" setting whereby we can turn it "off" when we are away but it will still kick in to stop the house from freezing.

Posted by: @grahamb

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I have two indicative quotes, one suggests a Daikin 9kW Monobloc, the other a 16kW Samsung! These are based on information given online only. To me, that’s already quite a difference.

One company offered a free heat loss survey, no obligation. The engineer visited back in October and I’ve heard nothing more from them despite a number of phone calls and e-mails. They simply stopped replying.

I had almost exactly the same thing, among other quotes, and it made alarm bells ring for me. My actual heat loss is about 12.4kW, based on the better quotes plus my own heat loss calcs. The measuring up is tedious to do, but the calcs themselves are straightforward, so much so that there are a number of spreadsheets that do them for you. If you can use a tape measure and a spreadsheet, and you have the time, it is well worth doing the calcs yourself.

Going AWOL is also common. I think it happens when the installer thinks for whatever reason thinks you and/or your property are 'trouble' and so they just walk away. They want the minimum hassle, and maximum profit. The flip side is the persistent caller. The installer who did the 9kW quote was still calling me even after I had the heat pump installed, 12 months after the original quotes!

Heat pump running costs have been astronomical for many, including me, during the recent cold weather, but as @oswiu rightly points out those high costs should be mitigated by lower running costs in milder weather, not just because less heat is needed, but also because the heat pump becomes more efficient in milder weather. This cuts both ways: in cold weather you need more heat, and it is delivered by a less efficient heat pump, in milder weather, you need less heat, and it can be delivered more efficiently. I will only know whether this is true when I have a heating season's worth of usage data, which I don't yet, as the heat pump only got installed in Feb/Mar this year.      

Posted by: @grahamb

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@oswiu that was very useful. Thanks for that. Having taken the details and made the calculation, it gives me an answer of 5.2kWh. 

That's probably on the low side. Yes, it includes cooking and hot water but we have  smart thermostat and are careful to turn the heating off when we go out and pit it back on around half an hour before we get back home. It also has  useful "absence" setting whereby we can turn it "off" when we are away but it will still kick in to stop the house from freezing.

I get a similar, albeit slightly higher, figure when I do it and we have very similar sized houses so it's a good comparison. My heat loss calcs came out to a little bit higher at 6kW so I went with an 8kW Daikin unit. This wasn't the unit I was originally quoted for, but I went for it after discussion with my installer. If yours is willing to work with you like that it's all the better. 

As I said the size estimate from gas is not a very rigorous approach, but if you're getting 5kW and being quoted for 16kW, that seems to me like too much of a disparity no matter how efficiently you run your gas boiler. 

It's worth posting other parts of what you were quoted such as the heat loss they estimated (and how they got to that figure), the flow temperature they're designing for, and what other equipment they'll install. Unfortunately (but perhaps rationally) installers often design for a system that gives them the least number of call backs, and don't worry too much if your system gets the best performance possible. The forum can probably help weed out the baddies from their quote info. 

Welcome! I have a similar sized house, EPC C, 1992, 4 bed detached. Formerly CH and HW from Kerosene, which was actually cheaper per kWh than mains gas a fuel. I now have a 10kW Grant Aerona ASHP. The oil boiler was 24yrs old and, whilst uber reliable, inefficient, smelly and CO2 polluting. I could not in all conscience replace it with another oil boiler.

If you have Solar PV, get an I-Boost or similar for HW generation if you haven't already. Have you any secondary heat, such as a gas fire, log burner or similar? In the very cold weather I've supplemented my ASHP with the log burner, as it can quickly boost the temperature in the way the ASHP can not. The log burner has reduced what would otherwise be very high and not typical electricity consumption last couple of weeks. I run the ASHP through conventional radiators but larger ones running at a low 42degC max supply temperature. Currently it is running 18hrs a day, keeping the house at 18degC, but it really should be chugging away 24hrs a day. I'm just knocking it off overnight and leaving the log burner on low to stop downstairs losing too much heat too quickly.

Get a full heat loss survey done and up-to-date EPC. It's essential to get some idea of the size of the ASHP you need, and how much it will cost to run based on the heat load requirement.

If you have the space, install bigger emitters designed for a maximum of 45degC flow temperature. Once you get up to 50degC or above, the SCOP of the ASHP will drop off. They are not high temperature heating appliances like a fire boiler. So be prepared to change your mindset about how to heat the house, which is to have the ASHP on more or less constantly, at a low setting. If you're at home most of the day, that's ideal.

Ensure that you don't have the ASHP controlled by a room thermostat that switches the unit on and off several times an hour. Use weather compensation & radiator flow balancing to regulate room temperatures with TRVs set above target room temperature to avoid overheating the room. My installer fitted a totally inappropriate 'HIVE' thermostat which has a very narrow on/off hysteresis of about 0.1degC. It's about the worst thing you can do with a ASHP as it was switching the ASHP on a off continually. The ASHP never got chance to operate efficiently. The HIVE is set to 24degC 24/7 now, continuously calling for heat. It's useful to monitor room temperature and control some programmed TRVs, all of which can be done through a phone and remotely, but as a ASHP main heating controller it's useless. The controllers that come with ASHPs are best at controlling ASHPs, but sparks and plumbers from the heating trade seem obsessed with trying to control ASHPs like a fired boiler.

Looking at the quote, they are working on;

150 sq/mtr floor area,

46 W per sq/mtr heat loss,

19309 kW/hr per annum, (Unsure if this is heat generated or electricity consumed)

-3.2 degrees C outdoor design temp,

10.2 degrees C mean air temp,

2255 degree days,

21 degrees C indoor design temp - I normally set the themostat for 18 degrees C,

The MAT locaction stated is Gatwick airport, around 50 miles away,

6.93 kW estimated total heat load,

The other equipment detailed is simply;

18ltr intermediate vessel,

Daikin 180ltr cyclinder.

I have to admit that, up until now, heating was something I simply turned on and off. Lots of further research and education required I think. 

@allyfish I have a log burner in the lounge. We tend to use it during the shoulder periods instead of the gas boiler in the evening. I accept that, regardless of gas or ASHP, my costs will rise as the external temperature goes down.

My solar panels are only rated at 1.295kW/h. Simply boiling a kettle on a summers day involves importing power. A diverter may be worth investigating though. I was going to consider batteries if I went down the ASHP route, once I knew what my consumption was likely to be.

Having studied the Daikin data sheet, I think I can understand why the company that visited for free has not come back. They would need to run a 32amp feed from the other side of the house. The sheet also states a limit of ten metres for the pipe run. I'd need to measure properly but I think my preferred location is too far away. I think I can work out a new location but that may involve lifting a recently tiled bathroom floor!

@allyfish Thanks for this useful info I'm only just getting to grips with my Aerona, and my installer also attached a hive thermostat.

Does weather compensation override the hive or did you have to disconnect it?

And does your set up turn the heating off when the hot water is timed to come on?

Apologies if this has been discussed elsewhere just finding more and more questions all the time!

@saz

Have you considered e-mailing the BBC's Rip off Britain show about your situation?

Posted by: @bretix

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@allyfish Thanks for this useful info I'm only just getting to grips with my Aerona, and my installer also attached a hive thermostat.

Does weather compensation override the hive or did you have to disconnect it?

And does your set up turn the heating off when the hot water is timed to come on?

Apologies if this has been discussed elsewhere just finding more and more questions all the time!

The HIVE main thermostat overrides steady ASHP control if it's switching on and off. You can tell if it is from the HIVE app temperature graph. The HIVE should be set high enough to be continually calling for heat. Weather compensation is a feature built into the Grant ASHP controller, and can be switched on and off using that controller, but unless the ASHP is chugging steadily away (i.e.: on all the time or for very long periods) it won't get a chance to work properly or deliver sufficient low level heat to keep comfortable temperatures inside. Hot water is always prioritised over central heating on most ASHP systems, including Grant. When hot water is timed to come on, the heating will go off during that time.