Pool Heat Pump Capacity vs Air, Water, and Humidity

A rating is stated at one air temperature, one humidity and one water temperature -- commonly a summer combination -- and the shoulder season an owner bought the unit for is a different set of numbers entirely. THE THREE FACTORS MULTIPLY AND THEY DO NOT MOVE TOGETHER. Cooler air cuts capacity because the evaporator has less heat available to collect. Warmer water also cuts it, because the condensing temperature rises with the water temperature -- so pushing the setpoint up reduces the very output that has to reach it. And higher humidity RAISES capacity, because latent heat is available at the evaporator, which is why pool heat pumps perform well in humid climates and why a dry spring evening is worse than a damp one at the same air temperature. That third term is the one nobody expects. THE FACTORS ARE ENTERED FROM THE MANUFACTURER'S OWN CAPACITY TABLE and are not derived here. Published tables give capacity across a grid of air, humidity and water temperatures, and the shape of that surface differs between units and refrigerants enough that a generic correlation would be wrong for most of them. Reading the three factors off the table and multiplying is the honest operation; inventing a curve is not. THE COP FALLS ON THE SAME CONDITIONS, so the running cost per BTU rises at the same time the output drops. A unit advertised near 5 or 6 can be near 3 in the shoulder season, which roughly doubles the cost of every BTU exactly when the pool needs the most of them. AND A COVER DOES NOT ADD CAPACITY -- it removes the loss the heater is fighting. On a slow heat-up that distinction is the whole result: an uncovered pool in cool weather can run a heat pump continuously and gain almost nothing, because the surface sheds heat as fast as the unit adds it. The heat-up figure here is the pool's own thermal mass and takes no account of concurrent loss, so it is a floor rather than an estimate -- and it uses the DERATED capacity rather than the nameplate, which is the whole difference between this and a heat-up computed from a rating. The same pool and the same unit give quite different hours depending on which capacity goes in. This applies entered factors to an entered rating. It does not supply capacity or COP factors for any unit, model the refrigerant cycle or predict the frost-limited cutoff (commonly around 50 degF, and a manufacturer figure), compute the concurrent surface loss during heat-up, size a heat pump against a heat loss calculation, evaluate electrical service or defrost behaviour, or address the flow rate the unit requires. The manufacturer's capacity tables and the pool professional govern.

delivered capacity = rated capacity x the air, humidity and water derate factors multiplied together, all three read from the manufacturer's own capacity table; heat required = gallons x 8.34 x the temperature rise, and the heat-up time divides it by the DERATED capacity.

Pool heat pump performance from the manufacturer's published capacity tables. The factors are ENTERED because the shape of the capacity surface differs between units and refrigerants enough that a generic correlation would be wrong for most of them.

Three entered factors and one thermal mass relation.

Estimate. AHJ and licensed professional govern.

Field names used by the API: rated_capacity_btuh, air_derate_factor, humidity_derate_factor, water_derate_factor, rated_cop, cop_derate_factor, pool_gallons, temperature_rise_f, cover_loss_reduction_pct, combined_factor, derated_capacity_btuh, capacity_pct_of_rating, derated_cop, heat_required_btu, heat_up_hours, heat_up_hours_with_cover

• Factors are entered from the manufacturer's table the capacity surface differs by unit and refrigerantthe manufacturer's capacity tables
• Heat-up uses the DERATED capacity not the nameplate, which is a summer figurethis tile versus a rating-based heat-up
• Frost-limited cutoff not modelled commonly around 50 degF, and a manufacturer figurethe manufacturer's data