Free Swimming Pool Heat Pump Calculator
Size swimming pool heat pumps: calculate surface evaporation heat loss, required BTU/hr output, daily electrical running costs, and initial heat-up hours.
🏊 Pool Dimensions & Climate
📊 Heating Performance & Cost
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Tested hardware and components for high reliability
How Swimming Pool Heat Pumps Work & ASHRAE Sizing
Unlike gas pool heaters that burn fossil fuels to generate raw flame heat, a pool heat pump uses a vapor-compression refrigeration cycle to extract free ambient heat from outdoor air and transfer it directly into circulating pool water.
1. Understanding the Coefficient of Performance (COP)
Heat pump efficiency is rated by its Coefficient of Performance (COP). A COP of 5.5 means that for every 1.0 kW of electricity consumed by the compressor and fan, the unit transfers 5.5 kW (or ~18,700 BTU) of thermal energy into the water. In comparison, electric resistance heaters have a COP of 1.0, and gas heaters operate at ~0.82 COP.
2. The Physics of Pool Water BTU Demand
Water has one of the highest specific heat capacities in nature ($1.0 ext{BTU / lb / }^circ ext{F}$). One gallon of water weighs approximately 8.34 lbs:
For a 18,000-gallon pool, raising the water temperature by just 10°F requires:
18,000 × 8.34 × 10 = 1,501,200 BTU!
3. Evaporative Heat Loss (The Dominant Heat Sink)
Every pound of water that evaporates takes 1,050 BTUs of latent heat with it. On breezy dry nights with a 20°F temperature difference between warm water and cold air, an uncovered 500 sq ft pool will easily evaporate 50 gallons of water overnight, losing over 400,000 BTUs into the atmosphere.
Frequently Asked Questions
Do pool heat pumps work when outside temperatures drop below 50F?
Standard pool heat pumps lose efficiency rapidly below 50°F (10°C) because there is less ambient heat in the air and evaporator coils begin frosting up. Cold-climate inverter heat pumps with electronic expansion valves and hot gas defrost can operate down to 35-40°F, but output drops significantly.
How long does it take for a heat pump to warm up a cold pool in spring?
Unlike gas heaters that heat a pool in 8 to 12 hours, heat pumps warm pools gradually at a rate of 1.0°F to 1.5°F per hour of continuous operation. A spring startup from 60°F to 84°F typically requires 24 to 36 hours of continuous running.
Is it cheaper to leave the heat pump running 24/7 or turn it on only when needed?
Because water has massive thermal mass, allowing pool water to cool down 15°F requires running the heat pump flat-out for 20+ hours to recover. It is much more energy-efficient to maintain your target temperature continuously (with a solar cover on whenever not swimming).
What plumbing flow rate does a pool heat pump require?
Most 110,000 to 140,000 BTU pool heat pumps require a minimum water flow rate of 30 to 45 GPM (gallons per minute) to prevent the internal high-pressure refrigerant switch from tripping. Variable-speed pool pumps should be programmed to run at least 2,000 to 2,400 RPM while heating.