ASHP performance

[Bam Bam]

Hi all,

I mentioned in a post that we've had a heat pump installed and I got a DM from a curious member about how it performs in cold weather.

Since I looked up the data to write them a response I thought I would share it with the group!

The Vaillant Heat Pump app provides us with data. I don't know how reliable it is but it seems plausible.

The efficiency does seem to reduce (as expected) when the temperature drops. It has to work harder and there is less heat available.

Our install was by the Heat Geeks and they are really the gold standard for efficiency calculations so that you use the minimal amount of electricity to achieve your heat requirements.

Monthly data
So far for January it reports 434%
December we got 481% (but we were away for several days over Xmas so had it on a lower temperature output which was very efficient)
November it was 483% (only the second half of the month, since commissioning)

Anything above about 360% should be cheaper than gas, but ours mostly runs off the battery which we fill up at 8.5p.

Daily data
Today where it started very cold (about zero until noon) it is reporting 431%.

Lowest day was yesterday 401%, but we might have lower efficiency this evening when the sun goes down.

Caveats
That may just be the heating efficiency, and the hot water efficiency (if not included as I suspect) will be lower. But, the hot water efficiency will still be much higher than the immersion heater we were using last winter to reduce our gas burn.

Also - During install Pebbles described herself as a 'victim of climate change' because there would be a big box taking up space on our decking.

Comfort improvements
The heat pump runs 24 hours a day (set back to 18 when it isn't on 21 degrees).

Since we are both home (and at least one of us it) most of the time it is much better for us.

We haven't had to use our oodies so far, which we usually have in the past when the temperature drops!

Thoughts?
I don't know if anyone has any other questions about them but we've been very pleasantly surprised.

For me it is all about the carbon savings - which will be substantial - but I've been very impressed with the efficiency and so the cost will be very reasonable too (especially with our battery and solar and EV tariff).

 

[MG Clive]

That all sounds very technical @Bam Bam, but it's obviously working for you.

 

[Bam Bam]

That all sounds very technical @Bam Bam, but it's obviously working for you.

I've put the efficiency in terms of percentage as I thought it would be easier for people but it is commonly expressed in terms of COP (coefficient of performance) or SCOP (seasonally adjusted coefficient of performance).

Essentially heat pumps can exceed 100% efficiency because they don't generate heat (like a boiler/burner/heating element). Instead they use electricity to move heat around.

The more efficient the better as it means more heat for less energy/money.

The best gas/oil boiler will be low 90%s and the best electric heater will be 100%.

Because electricity is much more expensive than gas (in the UK the climate costs get put on to electricity and not gas even though electricity is the answer and gas burning is the problem), you need a good efficiency (over 360%) for the heat pump to match the running cost of gas.

Taxing gas for its climate impact would change the situation massively in favour of heat pumps but it isn't likely because some poor granny will die (or some millionaire won't be able to heat their West Wing and horse stable when no one is in there).

 

[LenEV2016]

That all sounds very technical @Bam Bam, but it's obviously working for you.

Well summed up there Clive

 

[MG Clive]

Well summed up there Clive

To be honest, I wish I understood it, but it just went straight over my head really.

 

[Bam Bam]

Sorry for making things too complicated.

There are loads of great explainers on heat pumps online.

Useful to get your head around since they are basically the only thing that makes any sense to meet our climate targets.

Heat Geeks have made lots of videos...



Loads of other You Tubers talk about Heat pumps (quite a lot of them also EVs, like Andy EVMan)...

 

[MG Clive]

Don't be sorry mate, I'm sure you explained it perfectly.

 

[LenEV2016]

Don't be sorry mate, I'm sure you explained it perfectly.

I thought you explained it better

 

[Susanna]

I’m sure that solar/batteries/heat pumps etc are definitely the way to go for fuel efficiency and the good of the planet, and if I was younger I like to think I would have gone down that route.

 

[Gomev]

I would love a heat pump but we are in a modern build that came with a combi boiler and absolutely nowhere to put a hot water tank.

 

[Bam Bam]

I would love a heat pump but we are in a modern build that came with a combi boiler and absolutely nowhere to put a hot water tank.

Combi boilers really took us in the wrong direction unfortunately.

People are putting hot water cylinders in garages these days since cars rarely live in there any more. Perhaps in lofts as well but then they are large and heavy to get up there.

The Heat Geeks have attempted to come up with a small water tank for those moving away from combi boilers that can fit below a kitchen counter.

Heat Geek Mini Store - Newark Cylinders

Newark Cylinders has launched the super-compact, highly efficient Heat Geek Mini Store, designed by leading heating experts, Heat Geek.

newarkcylinders.co.uk

Hopefully there will be solution that works for you when the time comes.

 

[Bam Bam]

We've just had our energy bill.

For December 2026 we used 200kWh more electricity than December 2025. Average cost of 10p because our home battery couldn't cover every day so we used some expensive electricity as well.

However, we used 700kWh of gas in 2025 and NONE in 2026.

So our energy bill in total is £21.82 less than it was last December. This is largely because we have a home battery which allows us to use the cheap EV tariff for most of the day to run the heat pump.

Annoyingly we are still paying the gas standing charge, though, because our meter has not yet been removed.

With the heat pump we have...

• Been much warmer
• Used a lot less energy in total
• Used energy generated by sun and wind and not just gas
• Greatly reduced our greenhouse gas emissions
• Saved money

Very happy to have done it!

Perhaps with more battery storage (or solar) we could avoid more of that expensive peak rate electricity and reduce the cost of running the heat pump further, but that would be an extra investment in the house.