I was a doing a placement at a company, and they were designing a new heat pump. I was told the more compact you can make a heat-pump, the better it is for efficiency, and they mentioned something about this was because the power density was increased.

I get there are benefits from using less material and making something lighter which would improve efficiency in that regard, but I don't get the benefit of a higher power density in a heat pump. I understand that in a heat engine like a combustion engine you want a high compression ratio so you'd want to make the thing as compact as possible, but why is a better power density also good for a heat-pump? Is it this just the thing with all types of machinery which have some sort of volume which is compressed? If so Why?

Edit: I didn't really want to say COP but rather efficiency to keep the question more general. These guys seemed pretty certain that having a small high rpm heat pump gave improved performance, whilst I assume they meant COP could be something else they are looking at. Maybe a better way to ask this is: Are there any theoretical benefits to the COP of a heat pump by making it smaller?

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    $\begingroup$ The term "efficiency" is used inconsistently a lot, are you sure they meant it in terms of, say, coefficient of performance? $\endgroup$
    – Carlton
    Commented Nov 18, 2015 at 14:20
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    $\begingroup$ To add on to what @Carlton said, "efficiency" is terribly generic. All that really means is what you get out for what you put in. But those measures of input and output could be anything - cost, power, size, etc. And all too often, "more efficient" means "better" but that depends on what your definition of good is. You can only optimize for one variable at a time. This is getting ranty, but the tl;dr is that we need to know how you're measuring efficiency to give a good answer. $\endgroup$ Commented Nov 18, 2015 at 17:51
  • $\begingroup$ Theoretically the pumping losses at least increase for smaller units, due to the square-cube rule: with a linear increase in size the volume of e.g. a tube increases by the cube, while the surface area (which causes the friction losses) increases by the square. And assuming a fixed power output, larger pipes mean lower flow speeds, decreasing the friction losses in a larger unit even more. Though usually a larger unit will also have more power output. $\endgroup$
    – JanKanis
    Commented Apr 6, 2018 at 19:16

1 Answer 1


As far as I know the COP of a heat pump doesnt vary greatly with the size of the unit only with the technology used, quality of components, and the correspondingly price. As a general engineering rule, its easier to make something cheaper/more effective when you have less constraints. So having no footprint constraint would generally make the unit cheaper/higher COP.

I do not work in HVAC, but generally a large building will invest in two large heat pump air handlers integrated into one system. They have two for redundancy purposes and no more than two because maintenance increases with increased number of units.

If the reliability of small units were very high, they could potentially be a better solution because of the elimination of the large ducting required. With current technology however, small units increase facility maintenance. Another aspect is control. One or two large units is easy to control. Many small units, all with different set points and pressure differentials can cause a lot of maintenance headaches.

  • $\begingroup$ Thank you for the maintenance perspective I wouldn't have thought about it from that angle, but I'm looking for something coming from theoretical knowledge. $\endgroup$
    – dezza3192
    Commented Nov 21, 2015 at 13:56
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    $\begingroup$ At the theoretical thermodynamic pump level, for the same technology used, smaller units will be no more efficient than large units. Usually the opposite is true, because a single large unit employs higher quality components. Also larger units can instead employ more efficient technology that may not have been economical at a small scale; such as a screw type compressor. trane.com/commercial/north-america/us/en/products-systems/… $\endgroup$
    – ericnutsch
    Commented Nov 22, 2015 at 23:14
  • $\begingroup$ Thank you what you are saying is true, just like how it is more efficient to have one large air-conditioner for a building instead of heaps of small ones. But I believe there are some theoretical thermodynamic reasons why one would want to make the part of the heat pump with the compressor smaller. Anyway I'll accept your answer, if someone does come up with any reasons why smaller is better, I may change it though. $\endgroup$
    – dezza3192
    Commented Nov 23, 2015 at 3:45

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