Rated and useful capacity: how much a heat pump heats when it is cold
On a heat pump data sheet the first figure is the rated capacity. But that figure is measured when it is 7 degrees outside, and the day the machine is really needed is the cold one. Here is how much it changes, with the figures from our data sheets.
01
Rated capacity: a laboratory figure
It is measured under fixed conditions, the same for all manufacturers: outdoor air at +7 °C and, for air-to-water units, water at 35 °C; for air-to-air units such as the Kita Air, indoor air at 20 °C. It is used to compare different machines, not to choose the right one for a house or an industrial building.
02
Why capacity drops when it is cold
With colder air the machine has less heat to take. The refrigerant evaporates at a lower pressure, the compressor draws in less dense gas and moves less heat with every turn. On top of that come the defrosts: while it defrosts, the machine does not heat.
03
How much our machines lose
These are the figures from our certified data sheets. Our propane air-to-water units of the SP, MP and LP ranges deliver on average 76% of their +7 °C capacity at -7 °C, and 61% at -15 °C. An SP 10, for example, delivers 10 kW at +7 °C, 7.4 kW at -7 °C and 5.9 kW at -15 °C. The Kita Air Premium delivers 42.7 kW at +7 °C, 30.8 kW at -7 °C (72%), 27.5 kW at -15 °C (64%) and 24.3 kW at -20 °C (57%); the basic Kita Air keeps 82% at -7 °C.
04
Useful capacity: the one the machine is chosen on
It is the capacity the machine delivers at the outdoor design temperature of the area and with the water at the temperature the system needs. With radiators the water is hotter and capacity drops further: at +2 °C an SP 10 delivers 8.8 kW with water at 35 °C and 8.1 kW with water at 55 °C. The machine is chosen so that it runs at full power at that temperature and at half capacity for the rest of the winter, as explained in the tip on sizing.
05
What to check before buying
- ask for the capacity at the design temperature of your area, not only at +7 °C;
- for air-to-water units, ask for it with the water temperature of your system;
- be wary of data sheets that give a single figure: those of our air-to-water units give the capacity from +12 °C to -20 °C.
The data sheets of our propane air-to-water units are on the air-to-water heat pumps page; the Kita Air one on the Kita Air page.
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Frequently asked questions
What is the difference between rated and useful capacity?
Rated capacity is measured at +7 °C outside and, for air-to-water units, water at 35 °C: it is used to compare machines. Useful capacity is the output at the design temperature of the area and with the water of your own system: that is the one to choose the machine on.
How much capacity does a heat pump lose at -7 °C?
On our propane air-to-water units, on average 76% of the +7 °C output remains, and 61% at -15 °C. An SP 10 gives 10 kW at +7 °C, 7.4 kW at -7 °C and 5.9 kW at -15 °C.
Why does output drop in cold weather?
The refrigerant evaporates at lower pressure, the compressor draws in less dense gas and moves less heat per turn. On top of that, while it defrosts the machine does not heat.
Does capacity change with radiators?
Yes, it drops further because the water has to be hotter: at +2 °C outside an SP 10 gives 8.8 kW with water at 35 °C and 8.1 kW with water at 55 °C.
See also: How much electricity does a heat pump use: per day, per year and by size, with the data sheet figures · Heat pumps and radiators: does it work? Yes, and better with propane · Heat pump COP: what it is, how it is measured, and why it is not the same in the fog · Air to air heat pump: how it works, pros and cons, and when it makes sense for heating · Best air source heat pump: the 10 things to check before you buy · Heat pump in cold weather: down to what temperature does it work? Below freezing, at -20 and -25 °C
All the technical tips are on the series page. The next one in two weeks.
