Correct sizing of a heat pump

Thanks. Our overall heat loss estimate is 11,283 W and we’re lined up for a 14kW Ecodan - does this sound about right? It will also heat a 250l hot water cylinder.

I understand the heat pump should be sized as close as possible to the estimated heat loss to avoid inefficient frequent cycling. But if I’m right in thinking the Ecodan is a newer inverter type ASHP - will it not behave like a 11kW or even 7kW pump as needed? If the output can modulate down from 14kW - is there any disadvantage to having a slightly generously sized unit apart from the initial outlay?

Really appreciate all the advice.

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That’s fairly high. What type of property is it? How does this compare to your daily gas usage on a cold day? (This article explains how to do that).

Most modern heat pumps can modulate down to about a third their rated output, which is fine if the heat loss calculations were accurate. Being slightly oversized helps deal with the shortfall when the unit has to defrost frequently (I thinks MCS adds 6% or something).

While my 11 kW Ecodan can modulate down to about 4.5 kW, my actual measured heat loss is around half what was calculated, so in mild weather when my house only needs a couple kWs, the heat pump has to cycle. This isn’t particularly bad, just sub-optimal.

A perfectly designed heating system would run continuously at the minimum level needed match the heat loss of the building, within the output range of the heat pump.

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Indeed. Really, you want to operate so that it operates between close to full-speed and minimum-speed. Most of the heating season should be in the middle of its speed range. If oversized, it will spend more time between mid-speed and low/ cycling. I often suggest people consider how much they want to spend on electricity. i.e. if the 14kW has COP 3 in mid winter, the average input would be around 4.6kw. @ 30p/unit, that is about £1.40/hr. Will you choose to spend anything approaching £30/day? mid winter? If not, then you might find a samller one gives better COP on the average winter’s day, say 6-9C. There are lots and lots of such days. I undersize because I dont want to use a lot of energy, and want it efficient on average days. We are however happy to wear jumpers in extremely cold snaps.


Hi Simon,

I’d echo Tim and John that your estimate sounds high. My place is a 3-bed detached in Wiltshire, reasonably well insulated. My heat loss calcs at -8degC ambient and living room at 21degC showed 3.5kW, but my installer’s MCS calcs were 7.5kW so we put in an 8kW unit. Max measured heat loss to date (-2degC) is ~2.5kW so it looks like my own calcs were much more realistic. Beware greedy installers!


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Intersting thoughts,
I guess they are not necessarily greedy, but cautious. They want reports of ‘toasty warm’. But running costs should also be a concern to installers


It’s easy to imagine that many installers are keen to avoid any callbacks for “house is too cold” just after installation, so err towards bigger heat pumps and higher flow temps. There will be many other installers that aren’t sufficiently trained in design of low temperature systems.

My parents are about to get a heat pump fitted, and went through several surveys and quotes before they got one that seems to be a realistic fit for their property.


If you have not had it fitted yet get another survey or use third party software like heatpunk (free) or heat Engineer (£12 a survey and a bit more complicated) to get a better idea of your actual heat loss.

You can ask the installers to swap it out after if it is incorrectly sized (may involve a lot of discussion) but it is far easier to do the legwork beforehand.


Indeed and that’s how the incentives are now. “House too cold” gets blamed on the installer. “Heating too expensive” gets blamed on heat pumps, thanks to the PR work by Big Oil, or the power suppliers.


Thanks for all the advice. Our property is old and quite cold! The original structure is a solid limestone wall cottage built circa 1890. It has a 1950’s extension on one side and a 2003 extension on the other. The 1950’s section was poorly built & v. cold so we have internally insulated this part with 50mm insulated plasterboard.

To add slightly to the complexity we’re in the middle of adding a single storey extension to the rear - another 36m2 - but will be to current building regs and with wet underfloor heating.

Thanks for the Heatpunk recommendation @KnightPhoenix - this is a great tool. It estimates our heat loss to be around 12 kW.

Heat-Box using our current EPC certificate with it’s historical 24,000 kWh / yr heating requirement and taking into account the new internal insulation, improved loft insulation and increased floor area estimates our heat loss to be 12.3 kW.

Heat-Box’s tool based on current gas consumption estimates our heat loss to be 10 kW but this doesn’t take into account the enlarged footprint from the extension.

Our installer heat loss survey came up with a figure of 11.2 kW.

Guess it’s a slightly imprecise science but the estimates seem reasonably close around the 11kW mark. Given the margin for error, slight unknown of the new extension and the fact our 14kW Ecodan has already been delivered and is sitting in the garage(!) - do you think it’s reasonable to go with the planned 14kW model?

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It looks like the installer’s survey and selected heat pump is not far off the mark, so probably fine to proceed with the 14 kW. I’d guess that the 11.2 kW might be a little too tight. #NotAHeatingEngineer

The next thing to check is that you have planned to install sufficiently big radiators to output 14 kW at 45°C. That’s a deltaT of 25°, so radiators will be putting out around 44% of their rated output at a deltaT of 50°. The bigger they are, the lower the flow temperature can be.

If your heat loss survey shows 12kW of heat loss and others are saying ~12kW then you would be looking for a unit to match that as closely as possible to avoid cycling and poor performance.

If you have the 14Kw unit you can install that and it will be warm. It all depends on how much you want to perfect and optimize the system.

If you want to truly optimize then can you look at under floor heating in the upcoming extension? Additionally you may want to look at a blower door test to get a good idea of Air changes per hour (will change the calculations) and also some thermal performance testing be it manual HTC calculations ( Cold Weather Measurements of Heat Transfer Coefficient | Protons for Breakfast ( or professional testing - Veritherm Testing - Veritherm

Unfortunately to get the better performance may require a bit of legwork and some further investigation but it will be more efficient over time.

I am quite performance driven so am working to squeeze every last therm from each watt but if you just want to put in an ASHP as it is greener and can run off your own solar generation and not spend weeks or ££ learning and assessing it all that’s also cool.

Here’s the performance data for the R32 14kW Ecodan: Ecodan ATW Databook R32 Vol5.9 - Document Library - Mitsubishi Electric

I would assume that actual performance in terms of capacity available is at least 10% below the figures here (based on my own experience). So at 45C water outlet temperature and somewhere between -7 and 2 the datasheet suggests 15-16kW. Minus 10% = 13.5 - 14.4. Which hopefully gives you an even healthier buffer of 20-30% if your heat loss is correct. My own 5kW R410a Ecodan is technically about 1.3x oversized yet gives good results, you really just want to avoid the 2-2.5x oversizing that is possible if the heat loss is overestimated and then you add a further oversizing factor beyond that.

Heatpunk typically assumes higher air change rates ~1.5 ACH than you may have in reality. 0.5 ACH is quite typical. It might be worth redoing the calc with a lower air change rate for interest.

Annual gas consumption is a good sanity check if you have it. What is your current annual gas consumption?


PS: we sailed too close to the wind with my father in laws 11 kW Daikin, which should have been plenty for his 9kW heat loss (based on accurate gas consumption data). But the Daikin seems to be falling 20-30% short of it’s stated output, delivering <8 kW on average during defrost conditions. The key here is adding a healthy buffer on an accurate heat loss but to avoid oversizing on an overestimated heat loss…


About 21,000kWh/yr in 2000/21 before the internal insulation work and 18,000kWh/yr more recently after the work completed. Not sure how much of the reduction I can attribute to the insulation or whether we’ve had milder winters…


We’re installing K3 radiators in nearly every room and will have wet underfloor heating in the new extension so should be able to run a flow temp of 45oC at design temp of -3.5oC

Been trying to find out how low the 14kW Ecodan can modulate down to without cycling - am I right in interpreting the image you posted as showing it could drop to an output of only 3.7kW with a COP of 3.5 at 7oC outside temperature?

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You may find this useful - Another Heat Pump Spreadsheet: Beyond the Rule of Thumb | Protons for Breakfast

The ‘Rule of Thumb’ is splendidly simple : one just divides the previous year’s gas consumption by 2,900 to give the heat pump size in kilowatts. So if a dwelling used 10,000 kWh of gas the previous year, then one would estimate that it needed a 3.4 kW heat pump.

This is obviously based on the same heating requirements and the same outdoor temperatures as last year (from memory it was a little mild). All of the MCS calculations will rightly assume every room is heated and all to around 18-22°c based on MCS guidance. If you were zoning or only heating part of the house this would be distorted.


Yes at 35C flow temp, the minimum is 4.2 kW and at a cop of 4.45 that means ~ 950W minimum electrical input. You will probably see 1000-1100W with pumps included…

If we assumed 20,000 kWh of gas.

If we use the divide by 2900 rule of thumb that suggests ~ 7kW

The dive by 2900 rule on my own heat pump would suggest a capacity requirement of ~4kW (I have a 5kW heatpump) and max heat requirement closer to 3.5 kW.

What does your heat loss come out with from heatpunk with an air change rate of 0.5 for the rooms?
One way to clarify the air change rate question is to get an blower door test done, but that’s obviously another cost and delay.

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4.2 - 14kW seems a pretty good output range that’ll encompass most seasons/circumstances. Presumably once my heating requirement falls below 4kW and if the pump starts cycling frequently I could elect to have some off periods rather than have it running on/off for 24hrs/day

Sorry - can’t see how to change this on Heatpunk. Is it related to the ‘build date’?

You may have to set it to a custom room e.g?
Ah when you select the room type you can change the air change rate in the custom room section: