Will an A2A heat pump save me money in winter?

[Ducky]

Hi everyone,

I’m looking into installing an air-to-air heat pump (air-conditioning) in my living room and am trying to work out whether it will save me any money on my electricity bill in winter.

I spend about 7 hours in my living room in the evenings, and would need to have 150w electric underfloor heating running for about 9 hours (I turn it on about 2 hours before I enter the room). Additionally, I have a 3kw fan heater which runs for about 3 hours. It’s a fairly large room, and I manage to warm it up to about 18 degrees Celsius using the UFH + fan heater which I just mentioned, assuming an outside temperature of about 0 to 5 degrees Celsius.

An installer came around yesterday and told me that I would need two 5kw air-conditioning units to keep the room warm. Do you reckon that this would save me money on electricity bills in winter? Rough estimates are fine.

Thanks for your help.
Ducky

 

[Rodders53]

How big is this room? Height, Width, Length?

The proposal is for a 10kW fan heater in the room you run a 3.15kW heat source currently.
Is it really 150 Watts of UFH heating mat... what area does it cover? Seems far too little.

Modern 2x 5kW aircon heat pumps might consume around 1/4 of the 10kW output so 2.5 kW if you can get a COP of 4. Likely it will use a bit more. I reckon it would also be noisy IF that 10kW were needed for any length of time due to the fans running fast to push the heat around the room. But you would get 3-4x the heat for the same electric burnt. They are also able to ramp up/down in output to maintain a temperature. Random one was between 1.6 to 6.6 kW when heating.

NB The 'rating' is often for cooling and the 5kW may well do 6kW in heating mode. They will need to stop heating and reverse to defrost the outside unit in cold but humid conditions.

We heat the house with oil, but have aircon for the shoulder months and backup plus occasional topup of heat. We seldom if ever need them on at full pelt. Aircon installers - like CH of old - will often over specify the kW needed.

There will be room heating calculators you should use to work out your needs. Plus get more quotes.

 

[Ducky]

Thank you @Rodders53. The room is about 6m X 7m, and the ceilings are very high - about 5m. We actually have 2 150w UFH matts spread out across the room, but hardly ever use both at the same time to save money. I mentioned that we get it to 18 degrees, but that might be a bit of a stretch. On very cold evenings the best we can get to is about 16 degrees.

We will definitely get more quotes.

So from what I can understand, at present we run 3.15kw to get to about 16 degrees. If we get the A2A heat pumps then it would be 10kw, but we would be paying for only 2.5kw to 3kw (approximately), and probably heating the room more comfortably than 16 degrees. Note that the specific model which the installer recommended is the Mitsubishi SRK50ZS-WF.

 

[MNW67]

It seems at the moment you are needing about 10.35kWh in total of heat over a period of 7 hours, in order to keep the room warm. So about 1.5kWh per hour. I think that 10kW is massively too much. Unless I have misunderstood what you have written?

150w x 9 hours + 3kW x 3 hours = 10.35kWh?

 

[Agile]

My rule of thumb would make it need about 5.3 kW of heat.

 

[Ducky]

Thanks for the replies guys. Sorry for all the confusion. I’ve specified some more accurate figures below –

Room size – 38 square meters. 5 meters height.

Current heating, which managed to get the room temperature to 18 degrees (16 on very cold days)
Fan heater – 3kw running for about 3 hours
Electric UFH – two mats covering 25 square meters (15 and 10 sqm each), 150w per square meter (this is what I failed to mention in my previous post). On a normal winter day we would run one 15 square meter mat for about 9 hours. So that would be 150w X 15sqm = 2.2kw. On a very cold winter day we would run both mats for about 9 hours. So that would be 150w X 25sqm = 3.7kw.

So basically we would be using 5.2kw heating on a normal winter day and 6.7kw on a very cold winter day (outside air temperature less than 5 degrees).

Taking that into account, do you reckon that two 5kw A2A heat pumps (air conditioners) would be sufficient or overkill, and if that would save me money on my electricity bills?

I've attached specs of the recommended unit. Note that it's the 5kw one on the far right of the specs table.

Thank you again and apologies for the confusion.

 

[MNW67]

I am not attempting to answer as I am only learning about heat pumps myself. I have been reading another forum as well which has a dedicated heat pump section.

Air Source Heat Pumps (ASHP)

UK Self Build forum and web community with advice about building, extending or renovating your home along with general DIY & energy efficiency discussion

forum.buildhub.org.uk

One thing I have learnt is that with heat pumps it is important for them to be sized appropriately because their efficiency varies greatly with the % output.

P.S. Thanks for clarifying. Although on reflection it should have been obvious. I was being a doofus!

 

[Rodders53]

A random aircon calculator says 39,300 BTU for cooling that room, 2 people, 1 PC/TV = 11.5 kW of cooling.

A random heating calculator (Dimplex) gave me 9.3 kW of panel heaters to get 20 C with -3 outside... and I made many, many assumptions about walls, insulation and windows areas and even that it is an even 5m over the whole floor area. So do do a 'proper' heat loss calculation for the room to get a more accurate number.

I note from your other thread https://www.diynot.com/diy/threads/energy-efficient-air-conditioning-solution-for-heating.623012/ that you are in a flat.

That will add complications for you:
1 You'll need Landlord/Freeholder permission to install them.
2 Air to air units need Planning Permission (unlike air to water that can be permitted development in some circumstances).
3 Noise of the outdoor units could be an issue in Flats for neighbours. PP will need noise calcs, no doubt.

The same would apply to air to water units to a great extent but one might be eligible for Government grants toward the installation and provide hot water as well as heating at reduced cost cf full rate electricity.

What heats the rest of the flat?? Are you on E7 or similar dual rate electric?

 

[Ducky]

Thanks @Rodders53. Yes, I am in a flat and will need a bunch of permissions, but a few of my friends in the building have managed to get the required permissions so it shouldn't be much of an issue.

I did explore air to water, but it's not feasible as I would need to rip apart most of my flat to lay the pipework etc. so the costs will be in the 10s of thousands even after factoring in the Government grant.

It's only the living room which has high ceilings and is difficult to heat. The rest of the rooms in the flat are a reasonable size and normal height, so wall mounted electric radiators are sufficient.

 

[MNW67]

I forgot about another forum as well:

Air Source Heat Pumps (ASHPs)

renewableheatinghub.co.uk

Just going back to your calculation of your current actual usage. You are only using the 3kW fan heater less than half the time so that would alter that calculation a little bit. It's probably not important but I thought I should mention it.

 

[Agile]

I am amused that my simple rule of thumb was within 2% percent of your consumption test for heating the room for a standard winter's day!