Heat pumps
The nameplate is a July number
A pool heat pump moves heat out of the outdoor air and into your water. That is why it is cheap to run and why its performance collapses exactly when the season gets interesting. Every capacity and every coefficient of performance you will see advertised is measured at one warm condition, and the gap between that condition and an April morning is the whole story.
The rating point
AHRI Standard 1160, Performance Rating of Heat Pump Pool Heaters — rating air condition 80.6 F dry bulb, 71.6 F wet bulb, 80.0 F entering water.
Read that again: the air is 80.6 °F. The rating condition is a warm afternoon in which almost nobody needs pool heat. A unit advertised at 125,000 BTU per hour and a coefficient of performance of 5.5 is telling you the truth about that afternoon and nothing at all about a 58 °F morning in April.
The rating point is a warm summer afternoon. Every published pool heat-pump COP you will see quoted is measured there, which is why quoted COPs describe July and not April.
What happens as the air cools
Two things degrade together, at different rates. Capacity falls fastest: there is less heat available in colder air, and the refrigerant cycle has to work across a wider lift. Coefficient of performance falls more gently, because as the machine moves less heat it also draws less power. Below a cutoff temperature the unit either shuts down on its own control or ices its evaporator and stops being useful.
| Air temperature | Delivered capacity | Coefficient of performance | Cost per million Btu |
|---|---|---|---|
| 80.6 °F | 125,000 Btu/h | 5.50 | $8.12 |
| 75.0 °F | 112,750 Btu/h | 5.10 | $8.75 |
| 70.0 °F | 101,813 Btu/h | 4.74 | $9.41 |
| 65.0 °F | 90,875 Btu/h | 4.38 | $10.18 |
| 60.0 °F | 79,938 Btu/h | 4.03 | $11.08 |
| 55.0 °F | 69,000 Btu/h | 3.67 | $12.16 |
| 50.0 °F | 58,063 Btu/h | 3.31 | $13.48 |
| 45.0 °F | off | — | — |
Between the rating point and 50 °F air, delivered capacity falls to 46% of nameplate. If your sizing calculation said you needed 110,594 BTU per hour in the coldest month of your season, and that month runs near 50 °F air, then you need a nameplate rating of roughly 238,093 BTU per hour to actually deliver it.
Derate your own unit
If your datasheet publishes capacity at two air temperatures, work out the real slope between them and type it in — that replaces the default assumption outright.
Delivered capacity at that air temperature 84,313 Btu/h67% of nameplate
Coefficient of performance there 4.17delivered heat divided by electricity in
Cost of a million delivered Btu $10.70at your electricity price
Nameplate you would have to buy 163,965 Btu/hto cover that load at that air temperature
Your season, month by month
| Month | Mean air | Coefficient of performance | Delivered capacity | Electricity used |
|---|---|---|---|---|
| April | 69.1 °F | 4.68 | 99,839 Btu/h | 3,102 kWh |
| May | 78.7 °F | 5.36 | 120,748 Btu/h | 2,029 kWh |
| June | 89.5 °F | 6.14 | 143,750 Btu/h | 1,089 kWh |
| July | 93.0 °F | 6.32 | 143,750 Btu/h | 305 kWh |
| August | 91.3 °F | 6.27 | 143,750 Btu/h | 320 kWh |
| September | 85.9 °F | 5.88 | 136,694 Btu/h | 1,179 kWh |
| October | 74.2 °F | 5.04 | 110,904 Btu/h | 2,888 kWh |
Season-average coefficient of performance across the whole run: 5.32, against a rated 5.5. That difference is not a defect and it is not a manufacturer exaggerating. It is what happens when you take a machine rated on a warm afternoon and run it through a real spring.
What this means when you shop
- Size on your coldest season month, not on the rating point. A unit that exactly meets your load at 80.6 °F air will be roughly half a heater in April.
- A titanium heat exchanger is about water chemistry, not efficiency. It does not change any number on this page; it changes how many seasons the unit survives salt and unbalanced water.
- The dedicated circuit matters. Pool heat pumps draw a large, continuous load for many hours. That is an electrician's line item and it belongs in the install cost you take to the payback page.
- Slow is the design, not a fault. A heat pump sized to the maintain load will hold your pool all season and will take a day and a half to bring it up from cold. If that is unacceptable, the honest comparison is against gas, not against a bigger heat pump.