Understanding heat pumps!

As well as Carbon Coops bank of recorded webinars about heat pumps - I came across this great article - a brilliantly quick takeaway which we found explains heat pumps really well for beginners like my husband and I. It’s called @disadvantages of heat pumps - yet it a surprisingly positive read :raised_hands:



Some questions
We 're having a survey for a heat pump through the Green Grant Scheme and have lots of questions from our understanding so far. Doing a bit of research we’re managing to answer some of our own questions through some general information links we found and are shared here.

What are the implications or effects of having a heat pump that is oversized?
How is a heat pump sized for an individual home?
This link explained and cut out the jargon and gives good analogies that helped us understand .

If we install underfloor heating in to work off an existing ASHP would we need a bigger pump?

How will we know if our radiators are the right size - how is this calculated?

@andyham @Polly_Anna_Steiner @Lewis_Sharman

Just found this handy conversion for us heat pump beginners - it converts BTUs to KWs…

Hi Pottyone,

Quick answers to your Qs.

What are the implications or effects of having a heat pump that is oversized?
I know of installations were the electricity input has been doubled by oversizing. Rapid cycling occurs - very inefficient

If we install underfloor heating in to work off an existing ASHP would we need a bigger pump?
No - having more load for the ASHP will make it more efficient.

How will we know if our radiators are the right size - how is this calculated?
It took me several days to calculate optimum radiator size - but it gives significant increases in efficiency - especially with underfloor heating as well. You can find tables online.

How is a heat pump sized for your home?

For my house heat loss calcs showed 6 or 7kW was needed for 20 degree temp difference. So I installed a 5kW ASHP.

I can add more next week.

Best Wishes



re: Radiator sizing.

If you know the U values of your walls/roof etc you can have a stab at the MCS heat pump calculator spreadsheet.

Heat Engineer website have something similar too.

Alternatively you have someone round (who hopefully knows what they are doing) to do the same thing for you, a room by room heat loss calculation that will tell you the radiator sizes required and various flow temperatures.

By putting all the U values into these tools they spit out the heat losses for each room, which in turn tells you the radiator sizing requirement.

This is hopefully the positive side of heat pump installation, that this sizing process is done to ensure less problems.

This process can (and should) be done for combi boilers too. I’ve been using the MCS spreadsheet to size my rads for lower flow temps using my combi.

Also, MCS have had some good webinars on heat pump sizing and installation recently.

Intro to MCS Heat Pump Calculator (V1.10)

How to correctly specify heat emitters’ here:

Locating a heat pump for building regs

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This comment was on my LinkedIn Feed following the article last week in the Sun by Angela Terry CEO of One Home:
“We have an air to air mini/split heat pump here and they work well until about -5C but below those temperatures the efficiency declines dramatically. At -15C and below they are pretty much at the point of zero heat transfer . It would be best to shut them down at about -8C and use a back source-unless you have a ground to air system. That is what makes the difference.”

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This is really helpful bit of info Andy.
It resonates with our advice so far about our heat loss calculated in our home energy planner survey with
Carbon Coop. It’s currently 9kw and after our planned measures (triple glazing, roof insulation in the lean to kitchen) we should gain 2kw.
Looking forward to your Heat Pump webinar this month.

Best Wishes and many thanks

Hi Zarch,
thankyou for sharing this. I’m going to have a good sit down with my hubby and look at these together.
Re radiators: Ours are already fairly big - so I’m hoping we might not need too many replacing.
As well as being cautious to get this right, I’m so pleased and somewhat excited that we can get a heat pump on the grant scheme and move away from gas.

Best Wishes and Many Thanks

Hi Ian ,
thankyou for sharing this. We’ll be having an air to water heat pump. I have had some questions and concerns about how effective an ASHP will be be in much colder conditions.
We better not go into an ice age :hear_no_evil: :sweat_smile: …that would be just our luck!! Got to say I don’t take anything for granted with the weather these days.
We actually thought to get a wood burner too…for those super cold days where we might need a little back up. As a rule though, I avoid burning anything - I won’t even burn my garden waste. :earth_africa:

All the best and many thanks

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The best analysis I have seen is one done by Mike Parr (https://www.linkedin.com/in/mike-parr-105ba71?lipi=urn%3Ali%3Apage%3Ad_flagship3_profile_view_base_contact_details%3BU1JVnks%2BSOek2GthkKrRZg%3D%3D) based on some research done in Poland. I can send you these if you let me have your email

Sure. That would be really good to see.
This is me…

Thanks :+1:t3:

This video shows COP when -1 and snowy. Might be of some use.

My relations and friends in Norway use ASHP, air to air for space heating. They are fine in the depths of their winter. They also provide cooling during the hot summers. Yes, I did put “hot summers”, Norway can get surprisingly warm. However the models they have are not available in the UK.


We recently had our ASHP installed (on 8th April) and are happy with how it is operating so far.
We chose a NIBE F2040-6 unit which is rated at 6kW output. You could argue that this is slightly over-sized (see my room by room heat loss calc attached) but when it comes to choosing an ASHP there will inevitably be a limited number to choose from at present. You may also need to size the HP somewhat in relation to hot water demand. You will often find that installers take a fairly pessimistic approach to heat loss calcs so that they over-estimate the size you will need.
My heat loss calc attached here is somewhat more realistic and has been “calibrated” by detailed energy use stats and temperature measurements, ie I was able to accurately quantify the heat loss to help my installer avoid over-sizing. You can see from my heat loss calc that at -1C outside temp our house needs ~3.8kW of heat energy to maintain comfortable conditions. The calc also shows that at 38C flow temp to the radiators then the heat output would be ~4.0kW, ie sufficient to cover the heat demand.


Here is the equivalent heat loss / radiator sizing calculation carried out by my installer, together with my annotations in RED to show how their calcs compared to mine in terms of the “adequacy” of the radiator sizing. As you will see most of the rads provide 100% or more of the required output , with just a few being under-rated. In all cases the under-rated rads are able to borrow heat from adjoining spaces, eg the kitchen is not really separate from the dining room so really it might make more sense to add the 2 radiator output together and treat as one. My calcs / comparison is inherently “pessimistic” and this is based on sizing for sustained 24h temps of -3C (NB not just 1hr or so at -3C). All in all I am relatively happy that our radiator circuits will be adequate 99.9% of the time and if necessary I may further increase the size of a couple of rads if this proves not to be enough.

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My presentation on our transition to ASHP Doms transition to ASHP via low temp heat - Google Slides

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Thanks for sharing this Mick. It’s great to see this data and the heat pump in action during the cold spells. I think for us - we’re wondering what heat pumps the UK or more to the point Eon’s installers are invested in.
We haven’t heard from an installer yet.

Well , after starting to have a look at all of the replies and data shared here in this thread. I have come to the conclusion that I need to understand how to work out heat loss calculations and match up a heat pump which is the right size.

Additional complexities for us lie in our situation of having just started to take a fabric first approach on a limited budget , then realizing we are eligible for the green grant part 1b and going after this for a heat pump. We already have heat loss calculations in our home energy planner survey through PPR , but that survey has be updated now to take into consideration the heat pump - we know insulation is a must .

Our home energy plan was to redesign and insulate the large and very leaky lean to kitchen roof, install triple glazing and top up insulation in the loft.
We’re not financially ready for internal wall insulation, we wouldn’t have cavity wall or external wall. Which is why we have gone with windows and good draught proofing measures.

Where underfloor insulation for the suspended floor is concerned - we know we need to take up the wooden floor, inspect the DPC and joists for any moisture and adequate cross ventilation before we even consider it. that itself is a huge job !!! At the moment this leaves us with the option of draught proofing the floor and high performing underlay with carpet for the back room at least.

Lots of thinking and learning for us!
Yet I feel it’s the only way forward for any householder - we need to take an interest in our home energy systems if we are to look after our planet while we save money.

Next stop - school of heat loss calculations!!
Any tutors out there :slightly_smiling_face:

You can undertake this on a room by room basis as you refurbish/retrofit each room. I found that by the time I’d finished I wanted to redo the first “learning” room. Maybe one day.

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Hi Andy,
thanks for all your sharing - I did realize it’s Florence Collier doing the webinar on Wednesday - not you !!
I’m cramming my learning!! Questioning every measure around this heat pump - ventilation - trickle vents in windows - to have or not .
We’re getting there - I love this forum and the advocacy. :raised_hands:

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Wow !! We have actually been accepted for a heat pump on the Green Grant Scheme. We have mixed feelings :smiley: :roll_eyes: :grimacing: and so many questions …
@sianrichards55 @Ann_Kolodziejski @Polly_Anna_Steiner @andyham
It’s a major amount of money to be granted - £10,000!! We couldn’t possibly afford a heat pump without this grant.
The pump will enable us to move away from fossil fuel.
We’re super aware we need the right size pump to be efficient.
We need the pump sizing against our fabric measures - which won’t be fully completed till next year.
Our biggest concern is bills - electricity is said to be 4 x more than gas and we can’t afford solar panels - our roof needs repairs before we can even consider solar - we’d probably need a grant for this too.

Our overarching thought is we need a neutral person who knows how to forecast our bills with the heat pump.
Huge energy bills scare us.
Is there an equation to work out our potential bills based on our current gas usage.

Has anyone else recently changed from gas to ASHP?
I really hope to advocate for our CC members and other householders here - to do this I know need to be on my ‘a’ game and know what I’m talking about while navigating the grant process to procurement and installation. @jonathan I need guidance if I’m going to steer a course for others and feedback to the retrofit taskforce - we just can’t afford mistakes being in that financial bracket of household who could really struggle if our energy bills shot up as a result of this. There must be a way of looking at this - forecasting bills once we know the model of the machine etc…

Thanks in advance for all help.