A Heat Pump Isn't Built
to Chase Large ΔT.

A heat pump doesn't generate heat the way a resistance or gas heater does — it extracts heat that already exists in the surrounding air and concentrates it into the pool water. That's what makes it efficient to run, but it also means its output is entirely dependent on how much heat the air actually has to give up. As ambient air gets colder, there is simply less heat available to extract, so both capacity and efficiency fall together. This is a fundamental characteristic of the technology, not a product defect — every air-source heat pump behaves this way to some degree.

Because of this, heat pumps are fundamentally maintenance devices, not rapid heat-up devices. Their natural role is holding a pool at a set temperature against ongoing losses — evaporation, wind, radiation — not driving a large, fast temperature rise from a cold start. This is exactly why, on projects that genuinely need to carry a large ΔT — bringing a cold pool up to temperature quickly, or holding target temperature through the coldest weeks of winter — a heater is paired alongside the heat pump. A resistance or gas heater's output does not fall as ambient temperature drops; its efficiency stays essentially flat regardless of outside conditions, so it becomes the workhorse for the hardest days while the heat pump carries the easier load the rest of the season.

In practice, ΔT and derating move in opposite directions across a season, rather than both being at their worst simultaneously. At the start of winter, ambient temperatures are still relatively mild — derating is modest — but the pool may need a larger ΔT (commonly 10–15°C) to bring it up from an unheated, cold-start condition. By peak winter, ambient temperatures are at their lowest and derating is at its worst — this calculator assumes up to 50% capacity loss — but by that point the pool is typically already close to its target temperature from ongoing heat pump operation, so the ΔT actually being carried on any given day narrows to something much smaller, commonly 4–6°C. Because no single day sees both the season's worst derating and its largest ΔT at once, this calculator couples the two directly rather than applying one flat derating figure across every ΔT — see Section 4.

Worth knowing directly: most manufacturers state that heat pump output becomes unreliable, or the unit shuts off automatically, below roughly 5°C ambient air temperature — for outdoor pools, and in some cases for indoor pools too, where the heat pump unit itself sits outside. Below this point, many standard units simply stop contributing. This is why the equipment derating used in Section 4 is calibrated to a deliberately demanding 3°C design point, and explicitly assumes a cold-climate-rated unit — a standard heat pump may not function at all at that condition.

This calculator sizes a heat pump's rated (nameplate) capacity to raise an outdoor swimming pool's water temperature by a given ΔT within a chosen time window. It is an initial heat-up sizing tool, not a continuous running-cost or steady-state maintenance-load calculation — once at target temperature, day-to-day heat loss (evaporation, wind, night radiation) is a separate, ongoing load that a correctly sized heat pump also has to carry, but that isn't what this specific curve solves for.

The derating assumption below is calibrated for outdoor pools exposed to ambient air, wind, and night sky radiation. Indoor or enclosed pools lose heat differently (primarily evaporation into conditioned air, minimal wind/radiation loss) and would need a different derating basis entirely.

Separately from the heat pump itself, the pool loses energy continuously to its surroundings through four mechanisms while it's being heated:

• Evaporation — the dominant loss mechanism for any outdoor pool, commonly cited as roughly 70% of total heat loss. Every litre of water that evaporates carries away its latent heat, and evaporation accelerates with wind, low humidity, and larger surface area. A pool cover sits directly on or near the water surface and blocks most of this loss while in place.
• Convection (wind) — moving air strips the thin warm boundary layer off the water surface. A cover largely blocks this too, since it removes direct air contact with the water.
• Radiation — the pool surface radiates heat to the night sky as long-wave radiation, particularly on clear, cloudless nights. Also substantially reduced by a cover sitting over the surface.
• Conduction — heat lost through the pool shell into surrounding soil or structure. Unaffected by a surface cover, and generally the smallest of the four factors for a well-insulated shell.

This calculator assumes a pool cover is used, but the pool itself remains outdoor and exposed (not enclosed or indoor). Since a cover blocks most evaporative, convective, and radiative loss — together the large majority of total heat loss — a modest +10% buffer is applied on top of the heat-up load, rather than the much larger buffer an uncovered outdoor pool would need. Industry sources commonly cite covers cutting overall heating energy demand by up to roughly half relative to an uncovered pool; +10% reflects a pool that spends most of its heat-up period covered, with some allowance for periods when the cover is off.

If the pool in question will typically run uncovered, this buffer should be increased substantially — this figure is not conservative for an uncovered exposed pool.

Separately from pool heat loss, a heat pump extracts heat from ambient air, so its actual output falls below its rated capacity as ambient air gets colder — the same reason an air conditioner works harder on a hot day. Because Section 1 argues that a large ΔT belongs with a milder, early-winter ambient and a small ΔT belongs with the coldest, peak-winter ambient, this calculator applies that same logic directly to the derating figure, rather than pairing every ΔT with one flat number:

• ΔT ≥ 15°C (cold-start, early winter, milder ambient) — 30% capacity loss (70% retained).
• ΔT 10–12°C (mid-season) — 35–39% loss (61–65% retained).
• ΔT 6–8°C (late-season top-up) — 43–46% loss (54–57% retained).
• ΔT ≤ 4°C (peak winter maintenance, coldest design ambient of 3°C) — 50% capacity loss (50% retained), the most conservative point on the curve.

3°C is a genuinely demanding design point at the coldest end of this range. Several manufacturers cite roughly 5°C as the point below which standard air-source pool heat pumps stop operating reliably altogether — only cold-climate-rated (EVI/inverter) units are typically specified to maintain meaningful output at or below that threshold. This entire table therefore implicitly assumes a cold-climate-rated unit is being specified, not a standard model, and should be read as a conservative planning framework rather than a verified manufacturer curve — no single authoritative source publishes exact percentages across this range.

• Assumes a pool cover is used for most of the heat-up period — if the pool will typically run uncovered, the +10% heat-loss buffer is not conservative enough and should be increased substantially.
• Does not itemize wind, humidity, or cloud cover for a specific site — both the heat-loss buffer and the equipment derating are blanket assumptions, not site-specific calculations.
• Does not size for ongoing/steady-state heat-loss compensation once at target temperature — only the initial heat-up load.
• Not applicable to indoor or enclosed pools, which lose heat through a different balance of mechanisms.
• The 30–50% coupled derating range assumes a cold-climate-rated (EVI/inverter) heat pump — a standard unit may not operate reliably at all at the coldest end of this range.
• Should not replace the manufacturer's actual performance curve for the specific unit being specified before final equipment selection.

This methodology is built for the harder case — an outdoor pool exposed to open air. An indoor, air-conditioned pool starts from a warmer, steadier baseline and loses no heat to wind or night sky, so it performs comfortably within this outdoor-calibrated sizing.