Pool & spa
Pool Heat Pump Size Calculator
Sizes a heat pump, which is efficient and slow and depends on air temperature.
Result
A heat pump's COP is quoted at a warm air temperature and falls as the air cools, so the same unit delivers less heat and costs more per degree in spring and autumn. Below roughly 50 degrees F most pool heat pumps become ineffective.
Get a Free Business AuditHow the calculation works
The same surface area and temperature rise drive the requirement, but a heat pump is specified differently from a gas heater because it moves heat rather than making it. That makes it far more efficient per unit of energy and considerably slower to raise temperature, and its output falls as the air gets colder because it is extracting heat from the air. Sizing therefore has to consider the coldest air temperature at which you expect it to work, not just the heat needed.
What moves the number
- Surface area and temperature rise, as with any heater.
- Minimum ambient air temperature during the swimming season.
- Whether a cover is used, which matters more with a slow heater.
- Electrical supply, since a heat pump needs a substantial dedicated circuit.
Where this one goes wrong
A heat pump cannot heat a pool quickly, and that changes how it is used: it suits maintaining a temperature over a long season rather than warming a cold pool for the weekend. An owner expecting gas-heater responsiveness from a heat pump will be disappointed regardless of sizing. Output also drops as air temperature falls, so a unit sized at a mild ambient will underperform at the shoulders of the season, which is where the rating conditions matter. A cover matters more with a heat pump than with gas, because a slow heater cannot outrun large evaporative losses.
Before you quote
- Size against the coldest ambient you expect it to work at.
- Set expectations that it maintains rather than rapidly heats.
- Confirm the electrical supply can carry a dedicated circuit for it.
- Pair it with a cover, which a slow heater needs more than a fast one.
Where the rules come from
Questions
About this calculator
Is a heat pump better than a gas pool heater?
Far more efficient per unit of energy and much slower to raise temperature, and its output falls as the air cools. It suits maintaining a temperature across a long season. For warming a cold pool quickly or extending into cold weather, gas does that and a heat pump does not.
How long does a heat pump take to heat a pool?
Considerably longer than a gas heater, often days rather than hours for a meaningful temperature rise. That is inherent to how it works rather than a sizing problem. It is why heat pumps are run continuously to hold temperature rather than switched on for a weekend.
Does cold weather affect a heat pump?
Yes, substantially, because it extracts heat from the air and there is less available as the air cools. Output falls at the ends of the season, which is exactly when more heat is wanted. Sizing against the coldest ambient you expect, rather than a mild rating condition, avoids disappointment.
The formula
BTU/h = surface area x temperature rise x 12; kW = BTU/h / 3412 / COP
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How to use this calculator
- Pool length (ft) (ft). Enter your own figure; the page starts at 32 ft.
- Pool width (ft) (ft). Enter your own figure; the page starts at 16 ft.
- Temperature rise wanted (F) (F). Enter your own figure; the page starts at 10 F.
- Coefficient of performance. Falls as the air gets colder.
- Electricity per kWh ($). Enter your own figure; the page starts at $0.16.
- Hours run per day. Enter your own figure; the page starts at 10.
The result updates as you type, so you can try a figure, see what it does to heat pump size, and change it back. Nothing you enter is sent anywhere or saved.
A worked example
These are the figures the calculator opens with. Every number in the results column below is produced by the same calculation the page runs, so you can follow it through before putting your own figures in.
What goes in
| Input | Value |
|---|---|
| Pool length (ft) | 32 ft |
| Pool width (ft) | 16 ft |
| Temperature rise wanted (F) | 10 F |
| Coefficient of performance | 5.5 |
| Electricity per kWh | $0.16 |
| Hours run per day | 10 |
What comes out
| Result | Value |
|---|---|
| Heat pump size main result | 61,440 |
| Electrical draw | 3.3 |
| Running cost per day | $5.24 |
| Per month | $157.15 |
About 61,440 BTU/h, drawing 3.3 kW at a COP of 5.5. That is roughly $157.15 a month to run.
How sensitive the answer is
Holding everything else steady and changing only electricity per kwh, this is what happens to heat pump size. It is worth knowing which of your inputs the answer actually hinges on, and which barely move it.
| Electricity per kWh ($) | Heat pump size |
|---|---|
| $0.08 | 61,440 |
| $0.12 | 61,440 |
| $0.16 example above | 61,440 |
| $0.24 | 61,440 |
| $0.32 | 61,440 |
Every other input is held at the example value while this one changes, which is not how a real job behaves. Treat it as a guide to which number matters most, not as five separate quotes.
What each figure means
- Pool length (ft) (ft)
- Measured in ft, starting at 32 ft.
- Pool width (ft) (ft)
- Measured in ft, starting at 16 ft.
- Temperature rise wanted (F) (F)
- Measured in F, between 0 and 40, starting at 10 F.
- Coefficient of performance
- Falls as the air gets colder.
- Electricity per kWh ($)
- A money amount, starting at $0.16.
- Hours run per day
- A number, between 0 and 24, starting at 10.
What the results tell you
| Result | What it is |
|---|---|
| Heat pump size | The headline result. A number. |
| Electrical draw | A decimal figure. |
| Running cost per day | A money figure. |
| Per month | A money figure. |
Keep going
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