Newbie question about rated heat pump output

This seems such a simple question that I can’t believe I’ve not been able to Google the answer. But I’ve just got myself more and more confused :laughing:

The nameplate of an ASHP model might say 8kW. Am I right in thinking that is the output at +7C?
What happens if the outside temperature is -5C. Will the ASHP have an output less than 8kW? Or will it still give out 8kW but with less efficiency?

Most modern heat pumps have an inverter compressor, which allows it to vary the heat output to match demand.

Best to look at the manufacturers’ published datasheets. For example, this 11.5 kW Mitsubishi can deliver 11.5 kW of heat at nominal power at almost all temperatures:

It can deliver more than its nameplate on max power. There is also a minimum output that it can deliver. Performance typically goes down as temperature drops, for a given flow temperature.


Thanks, Tim, for the great info. That graph is so useful. I’ve not found anything like that from other manufacturers. I had to look up what nominal meant in this context. That led me to the MCS Certified guide and that has just confused me more. It seems to be saying that the output often does fall substantially below the nameplate at colder temperatures. How can I square this particular circle? I know I am missing something obvious :slightly_smiling_face:

“For instance, the standard rating condition for ASHPs used to be air at 7C and water flow at 35C. However, in practice they may be required to produce water at approximately 55C with design ambient temperatures of circa -2C. At these latter conditions the output of many heat pumps could be as little as 60% of the output at ‘standard’ (nominal) rating. For example a 13kW ASHP may provide 13kW of heat at 7C and 35C flow temperature but it is unlikely to do this at an air temperature of -2C and water flow at 55C.
The actual output could be as little as 7-9kW and hence, if relied upon for the design of the system, it will be vastly under-sized incurring the cost of expensive supplementary heating and/or lead to issues of poor comfort.”

Unfortunately there’s no standard way of deciding what the nameplate is across manufacturers. They all choose their own conditions to suit their product / mask it’s weaknesses.

Unfortunately MCS obfuscate the source data for their certificates and estimated sCOPs so you can’t use those either.

There is an ERP (European energy efficient standard) label for most heat pumps. This will state the sCOP for various climates running radiators weather compensated at up to 55C, and underfloor at 35C. It will also state the rated output at -10C. That’s not as useful in the UK as say the rated output at -2C.

Curves should, if unrestricted by software, show increasing output as air temperature rises and reducing as it falls. Flattened curves are due to vendors software limiting max output at higher temperatures.

There should be a dip in the curve around 0C if defrosting is accounted for in the output.

Good distributors will advise what the rates output is at each temperature. They probably won’t advise consumers though.

Units without ratings are generally not fit for purpose.


Thanks, Marko. That is so interesting. It seems that buying a 7kW ASHP from three different manufacturers might produce three machines with vastly different capabilities. I had no idea that was the case. I’d assumed that ASHPs would all roughly be the same! It also makes me feel a little less stupid for not being able to find the simple answer I had been searching for.

I’d suggest that ASHPs are roughly the same, as they use the same refrigerants and are governed by the same laws of physics. I don’t think you’d see vastly different heat outputs.

What really sets them apart is the quality of the controls, and how well designed the overall heating system is. What really matters is finding a great installer who really understands low-temperature heating, and let them worry about which heat pump to fit.

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Is the info from MCS wrong then, or at least out of date? Unless I have misunderstood, the MCS guide seems to be saying that many heat pumps will only give 60% of their rated output at the not particularly low temperature of -2C. Whereas other heat pumps, as you demonstrated, will give 100% of their rated output right down to below -10C. That seems like a pretty major difference to me.

MCS seems to be somewhat out of date, both with that 60% figure and expecting to need 55°C flow temperatures when it’s below freezing. Heatpump technology and system design has evolved considerably; MCS has not.

You may find this topic interesting, which discusses how much to oversize a heatpump to ensure it can provide enough heat to compensate for regular defrosts. It includes data from several manufacturers, as well as real-world measurements from real systems:

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Thanks, that’s really interesting. I think what the OP is hinting at is that the 11kW and 16kW models are mechanically identical, but the 11kW has been hobbled with software. So, when you look at the -7/8C output for the 16kW, you see it is only 66% of the rated output. That is probably closer to the heat pump’s inherent mechanical performance and more in line with what MCS says. Whereas for the 11kW model, the equivalent figure is 82% because mechanically it is a bigger heat pump than 11kW. That’s just my own interpretation and may, of course, be complete nonsense :laughing:

The 60% figure is for an old school (state of the ark) fixed speed unit; or an inverter drive unit that runs without any software restrictions

The units that give “100% of nameplate down to -10C” are throttled at higher ambient temperatures. It is a rational thing to do. (throttling output at higher temperatures)

Ignore the badge. Looks at the rating at design condition AND the minimum output at say +7C or +12C. Better yet don’t buy a unit that you don’t see is capable of working well on the OEM dashbaord.


Thanks, that’s really clear. But how do you actually get that information? In the thread linked by @Timbones, the OP said the Daikin rep showed him a page in a book but wouldn’t give him a copy. I thought that was very strange.

The datasheets I have read don’t make a lot of sense. They talk about Tj values and pdh values at partial load but without any explanation. I think most people would just like a simple graph or table to show output against temperature, like the one above for the Ecodan. I am surprised that manufacturers don’t provide this information as a matter of course in the brochure or on the website. Surely, it’s the most basic information that a consumer needs i.e. will this heat pump actually provide enough power when it’s cold outside? It seems to be treated like a state secret!

All manufacturer’s do indeed provide the data you are looking for.
Granted, it can often be difficult to find, sometimes you have to go as far as contacting their technical departments and request it.
More often than not, it is within the installation manuals.




Some manufacturers have it in the installation manual, but even then it is often presented in a very obscure way and is extremely difficult to understand for a layperson.

But why should consumers, who are often paying upwards of £15K, have to hunt for this most basic information? Until I started looking into heat pumps properly, the main thing I had heard was that they didn’t work as well when the outside temperature was low. So, the first bit of information I wanted to know was how well will my heat pump work when it is cold outside. I am sure many others are in the same position, and would appreciate this basic information being presented in an easy to understand manner in the consumer materials.

It is all very well being told we need to just trust the installer. But bearing in mind most heating engineers still struggle to specify a gas boiler of appropriate size, I think it is more than reasonable for consumers making a large investment to have easy access to this information. Not just to be given a glimpse of a table, as was the experience with the Daikin rep as recounted above.

Consumers are not expected to understand. Nor are the installers in the UK. That’s why all the marketing and the installation instructions in the UK are dumbed down.

On the continent the information you’re asking for is on the first page after the contents…

“Plan Your Work! - here are all the grapshs howing the maximum outputs for the 3.5 / 5 / 7 / 10 / 12 kW units at various combinations of outdoor temperature/flow temperature”

kurzanleitung-arotherm-plus-2105-2146405.pdf (5.4 MB)

Followed by size, noise, and restricttions on locations due to propane.

Then how much hot water you get for each tank size and what the available pressure is for puping the water around the heating circuit.

Then what you need to know about soakaways for the defrost water.

Fairly simple; but considered WAY beyond your average thicko in a colourful van working for a major utility company. And WAY beyond most of your average sales reps in the UK.

Then there’s also a full spec sheet:

kl-06-e2-verze-01-01062021-2108745.pdf (6.0 MB)

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They shouldn’t have to.

The first step any installer will do is a heat loss survey and calculate how much heat is required at “design temperature” (typically -3°C in UK). (You can do this yourself using HeatPunk, if you’re interested.) This will determine the flow temperature, radiator sizes for each room, and which heat pump will be best suited. This is required by MCS.

The point is, the heat pump will work well when it’s cold. The most common issue with installs is the heat pump being oversized, resulting in less good performance when weather is mild.

If you want a high quality, super efficient system, Find A Heat Geek.

Check out HeatGeek website or YouTube channel for consumer advice on heat pumps.

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Samsung offer peak output tables (enough for “Planen SieIhre Arbeit” purposes) to all distributors as part of the “EHS Technical Data Book”

Their authors are not as installer minded as Vaillant though (there isn’t a quick summary of just what you’re affter presented in a nice easy way in the booklet with the appliance; there’s just a huffing great PDF with the info buried halfway through)

Fun stuff you can see in this?

All of them run the compressor incrementally harder at lower outdoor temperatures in order to maintain capacity at the expense of COP (there are some internal re-injection ports and control valves that make this possible without burning up the compressor that are not fitted to other models)

8 kW @ -25C with a leaving water temperature of 55C sounds impressive / is genuinely impressive and the COPs when it’s genuinely cold outside are impressive. Irrelevant in a UK context - but useful to you if you are trying to make hot water in -25C with a COP of over 1.75, and heat your house via underlfoor heating in -25C with a COP of over 2.0.

The highlighted numbers at +7C (where all their efficiency labels are tested) are also impressive. It’s not quite as blow your socks off impressive as you might first think though. The Samsung rep will compare against others using max ouptut. COP 5 at 35Cthey will say. Vaillant is only 3.9; 3.9 at 45C but Vaillant is only 3.0; COP 3.2 at 55C but Vaillant is only 3.8 etc.

That’s missing the point though. Houses don’t need 8 kW at 35, 45, and 55C. They need less heat the warmer it gets. Compare the outputs at either minimum or the best efficiency point at each outdoor temperature and it’s much closer (5.0/4.9, 3.9/3.8, 3.2/3.0)

Compare.xlsx (11.4 KB)

The Vaillant can turn down usefully lower than the Samsung. The latter unit gets those high “max output” efficiencies by actually being a MUCH bigger unit that’s then software limited ot only run at a “max” output" that is pretty much the best efficiency point.

You need to know this when designing.

The Vaillant “7 kW” means about 7 kW (net of defrost) at about -7C; and it is designed to run at less than this by being conencted to rads/underfloor and allowed to “tickle along” at a relatively low output of it’s own managing.

The Samsung “8 kW” means about 8 kW (net of defrost) down to -25C; and it appears* pretty much designed to run with “on / off” controls with their software artifically imiting the output of the unit to just 8 kW at +7C. (they don’t bother limiting it some much above this because…drumroll…that doesn’t affect the score they get on the efficiecy label)

I guess what I’m saying here is…this is not simple. Any data that you do will be misrepresented against you by marketing asshats from the other brands…so it’s easier not to play the game and to rely on the installers to specify something appropriate / rely on independently verified results for how the units perform in the field with various designs.

*Though it’s far from obvious because nothing useful is published in terms of part load capability; and the internal controls appear to be as dumb as a rock with respect to managing output. (run as hard as you can and keep running until setpoint is breached and you can’t turn down any more?)


I did ask Samsung UK Helpline for Gen 7 part load data, and was told bluntly “not available”. Pity, because my ASHP spends most of its time at half speed. I can only estimate part load compressor characteristics by assuming that both energy developed and power consumed vary linearly with speed (roughly true for scroll compressors in general, but who knows whether the Gen 7 is typical?)

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One of the Dutch techies.confirmed that the highlighted figures were as good as the best efficiency point on the performance map.

I don’t think you can use pure compressor maps with this unit; given all the bypasses and cycle economising etc that are going on in the background. (not a simple compressor with variable orifice type setup)

I’m sure you’re right, Marko, I was just reflecting the published data from a couple of Daikin scroll compressors:

If you plot Qo (energy generated in BTU/h) or Pe (input power in kW) against the three given speeds (at any given ambient temp To and LWT Tc) you’ll see that they are pretty linear.

Thanks, Tim. It makes sense to me now.

Also, I’ve realised now that heat pumps are much more complicated than I had imagined. I knew they were complicated, just not how complicated. I can see why it is not possible to present the information I wanted simply in a meaningful way.