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We are looking at air to air heat pumps (aka reverse cycle AC units) to replace our existing electrical heaters.

Air to air heat pumps claim a COP of 3 or 4 to 1 in the manufacturing literature. However when I was looking at the graph presented to me by the salesman it only showed about 1.25 to 1 at 11 degrees C and less for lower temperature.

Since people do not generally heat their homes until the temperature gets low, I'm wondering what the true efficiency of there units is in the range of say 10 degrees C to -5 degrees C (the typical UK winter).

I would like to know if there is a standard benchmark by which COP is measured and also if there is proper data of how COP varies with temperature for different heat pumps.

Many thanks in advance.

  • 1
    Fantastic question Russell! You can't pump what's not there, so typical R/C ACs become increasingly inefficient as it gets colder outside (when you want/need heating the most). Heat pump hot water systems (that work on the same principles as heat pump ACs) break-even in Mild Temperate climates in Australia (winter overnight lows of around 5°C / daily highs of around 13°C) and are uneconomic and discouraged much below that. Marketing and advertising, however, is powerful, and convenience is valued by the masses. I wish I could offer more than anecdotes. – Tim Jun 22 '18 at 9:06
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    This will vary from unit to unit. Apart from the usual efficiency variation between brands and models, there are heat pumps optimized for different temperature ranges (by using a different refrigerant, operating at different pressures, etc). Given this, I'm not sure what a general answer to this question actually looks like. For example, it sounds like the unit in my home is very different from the one you've looked at: the literature suggests 2 to 1 at -20C. – Jean-Paul Calderone Jun 22 '18 at 12:52
  • Thanks for the comments Jean-Paul. The sort of answer I was looking for would be data showing how COP varies according to temperature for various manufacturers. I'll amend the question to make it clearer. – Russell Jun 22 '18 at 15:25
  • There's a bit of data here academia.edu/1073992/… that you could selectively analyse to get a few numbers, perhaps? In the summary: "Linear fits to each data set suggest that COP falls by 0.67-1.07 for every 10°C temperature rise." – Tim Jun 22 '18 at 17:01
  • Tim, this sounds like an answer to me. Would you consider pasting it into the answers section if you are happy with it? – Russell Jun 22 '18 at 17:23
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There's a bit of data in a paper titled A Review of Domestic Heat Pump Coefficient of Performance (by Dr Iain Staffell, Imperial College London, Centre for Environmental Policy, 2009-04) that you could selectively analyse to get a few numbers.

His approach was to collate heat pumps with reported COPs and see how many °C they would boost the inlet air temperature by. Although this information is not immediately useful, if you know you have to raise the air temperature by a certain number of °C to make the internal temperature comfortable, then you can read the charts backward to work out what the COP would be of heat pumps capable of doing it.

In the summary he says: "Linear fits to each data set suggest that COP [of Air Source Heat Pumps] falls by 0.67-1.07 for every 10°C temperature rise."

If you want an easy-to-remember rule-of-thumb, then you could probably just use ΔT/10. Applying that to an example: An ASHP/airconditioner required to pump heat from -5°C (outside) to 20°C (inside) would thus see its COP drop by (20 - (-5))/10 = 25/10 = 2.5.

An interesting consequence of COP reductions of this magnitude is that if the ASHP/airconditioner in the example only had a heating COP of 3.5 to start with, then its effective COP would be 3.5-2.5= 1.0 which is no better than a (direct) electric heater (e.g. baseboard, radiant, wall, bar) anyway. When it's really, really cold outside it might make more sense to turn on an electric radiator, or a bunch of incandescent light bulbs, than run the airconditioner.

Whilst rules-of-thumb are great for estimating things in 'normal' ranges, they aren't so good when you approach operational limits. If an AC's operating range goes down to 5°C then don't expect linear degradation of performance past or even near that limit. Our AC, for instance, actually ices up and stalls at about 3°C — so COP has rapidly reduced to zero (or perhaps even a negative number) at that point. As always, results and observations in the real world trump formulae presented on Internet forums. ;)

  • This seems to imply that the rated COP for a unit is only valid when the temperature rise is 0° (this is the only ΔT where your equation gives a COP drop of 0). I don't have evidence to contradict this but it seems odd to me that units would have a COP rating under conditions where you would never even use them. I suspect that the real COP drop equation should be something more like max(ΔT - N, 0) / 10 where N is some per-unit value that gives an efficient operating range within which you get the rated COP and not some de-rated value. – Jean-Paul Calderone Jun 23 '18 at 13:49
  • The linear fit (and hence COP drop rate) was determined from data between different units, not the same unit at different temperatures — thus would only be of use provided it didn't stray too far from a unit's declared COP. Different refrigerants in different units would produce different, non-linear curves within their operational ranges — something with far too many variables to reduce to a simple equation with any meaningful level of accuracy. I don't think any rules-of-thumb should be trusted when the outdoor temp nears the limits of the AC — it'll probably just ice up and stall. – Tim Jun 23 '18 at 14:38
  • I think my point was that it doesn't seem that the referenced paper supports the conclusion that you should expect a COP drop of 2.5 when there is a 25°C temperature difference between input and output. Instead, you should expect a COP drop of 2.5 when you increase the temperature difference by 25°C. Thus "increase it from what?" becomes a rather important question. The referenced paper presents data which shows no unit with a COP below 1.67 even for a ΔT of 60°C. Therefore, I find the last paragraph of your answer significantly misleading. – Jean-Paul Calderone Jun 23 '18 at 15:05
  • I've had another look at the paper and it still makes sense to me. I cannot understand your confusion. Lines of best fit are easily extrapolated. Since COP 0–1 is likely to be very close to the operational limits of a given AC I added a paragraph to the answer to warn folks about using rules-of-thumb near operational limits (where the performance curve is likely to be very non-linear). – Tim Jun 23 '18 at 16:32
  • What is the "initial" COP based on? Is there some standard operating condition that mfgr's use to calculate that? Also, I'd recommend including the paper title, and perhaps a snapshot of the most relevant graph, in case that link breaks, or if others don't have login access to adacemia.edu. – LShaver Jun 24 '18 at 16:34

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