Heaters & Covers

Heating is the largest line in a pool's running cost, and the only one where the equipment choice changes the bill by a factor of four rather than a few percent.

What it costs to raise the water

Water takes 8.34 BTU per gallon per °F. A 20,000 gallon pool raised 10 °F needs 1,668,000 BTU delivered into the water, before any of it escapes.

At 80% efficiency, a gas heater burns about 2.09 therms per 100,000 BTU delivered. That 10 °F rise is roughly 20.9 therms — call it $25 at $1.20/therm.

Where the heat goes

Almost all of it leaves through the surface, and most of that is evaporation. An uncovered pool loses on the order of 4–8 BTU per square foot per hour per °F of difference between water and air, and evaporation accounts for around 70% of it.

Which is why the cheapest piece of pool heating equipment is a cover. A solar or bubble cover cuts evaporative loss by 50–70%, costs $80–200, and typically pays for itself inside one season on a heated pool. Nothing else on the equipment pad has that payback.

Heater types

Gas — 200,000 to 400,000 BTU/hr, heats fast, indifferent to air temperature, expensive to run. The right answer for a pool used on demand and for spas.

Heat pump — moves heat rather than making it, with a coefficient of performance of 5 or 6. Roughly a quarter of the running cost of gas, but output collapses below about 50 °F air temperature and it raises water slowly — a few degrees a day rather than an hour.

Solar — panels on the roof, near-zero running cost, extends the shoulder season rather than heating on demand. Wants roughly half the pool's surface area in collector.

Sizing

Rule of thumb for a gas heater: surface area × temperature rise × 12 gives BTU/hr for a roughly 24-hour heat-up. A 16 × 32 pool wanting a 20 °F rise: 512 × 20 × 12 = 123,000 BTU/hr, so a 150,000 BTU unit with the cover on.

Undersizing a heat pump is the more common error, because the same rule of thumb assumes gas-like output the unit cannot produce on a cold morning.

The other pad decision

A variable-speed pump. Pump power scales with roughly the cube of speed, so running at half speed for twice as long uses about a quarter of the energy for the same turnover. On a pool running 8 hours a day, that is usually the second largest saving after the cover.