Pool Heater Cost: Gas vs Heat Pump, Worked Out on a Real Pool

One degree has a price tag

Water is stubborn about temperature. That's what makes it a nice pool and an expensive one to warm. A gallon weighs 8.34 pounds, and it takes one BTU to raise one pound by one degree Fahrenheit, so an ordinary 15,000-gallon pool (a 16 × 32 at an average depth of about four feet) needs roughly 125,000 BTU for every single degree. Warm it from 70°F to 82°F and you're buying 1.5 million BTU before any of it leaks back out.

Now price those BTU at the meter. Natural gas is sold by the therm, which is 100,000 BTU, and a new gas heater turns about 84% of it into warm water. At $1.60 a therm, that 12-degree lift burns about 18 therms. Call it $29. Propane carries roughly 91,500 BTU a gallon, so at $2.80 a gallon the same job takes about 20 gallons and $55.

A heat pump plays a different game. It doesn't make heat, it moves heat out of the air and into the water, and in 80°F air a decent unit delivers five or six units of heat for every unit of electricity it draws. Use a more honest 4.5 for a spring afternoon and the same 1.5 million BTU costs about 98 kWh. At 17 cents, that's $17. The catch is the air. Around 50°F the efficiency sags toward 3, and plenty of units simply stop working somewhere between 45 and 50°F.

The warm-up is cheap. Holding it is the bill.

Nobody heats a pool once. The water sheds heat every hour it sits uncovered, and the biggest leak by far is evaporation, not the walls or the ground. That's why an open pool in a dry, breezy climate can lose several degrees overnight even in May.

Say our 15,000-gallon pool drops 4°F on a typical cool spring night and you want it back at 82 by the afternoon. That's 500,000 BTU a day, every day. Over a 30-day month, natural gas comes to about $285, propane about $550, and the heat pump roughly $165, since it does most of its work in the milder daytime air. Your rates and your weather will move those numbers. The ratio between them holds up surprisingly well.

Put a solar cover on at night and the whole table shrinks. The Department of Energy puts the heating savings from a cover at 50 to 70%, because it stops the evaporation that's doing most of the damage. A decent cover for a 16 × 32 costs $100 to $200 and lasts three or four seasons.

Buy the cover before the heater. A heater running against an uncovered pool on a cold night is mostly warming the sky.

Getting one on the equipment pad

A 400,000 BTU gas heater is $2,500 to $3,500 for the box, and most installs land between $4,000 and $6,000. The wild card is the gas line. If the meter sits on the far side of the house, or the existing line is too small to feed 400,000 BTU, the plumbing alone can add $1,000 to $2,000. Propane stacks a tank purchase or lease on top of that. A heat pump in the 110,000 to 140,000 BTU range runs $3,500 to $5,500 for the unit and usually $5,000 to $8,000 installed, with most of the difference going to a dedicated 240-volt, 50 or 60 amp circuit. Look at your electrical panel before you shop. A panel upgrade can double the install bill.

Speed matters more than people expect. A 400k gas heater pushes roughly 336,000 BTU an hour into the water, which is about 2.7°F an hour on our 15,000 gallons. A 125k heat pump manages closer to 1°F an hour in good weather. That's a 12-degree warm-up in four or five hours against a day and a half. If the plan is "heat it Saturday morning for the kids," that difference decides it for you.

Expect 7 to 10 years from a gas heater and 10 to 15 from a heat pump, less for both if the water is abusive. The part that fails is the heat exchanger, and chemistry is usually what kills it. Low pH eats copper exchangers, and the first clue is often blue-green staining on the plaster before the heater itself complains (our stains guide covers telling copper from iron). The other classic is a trichlor puck in the skimmer, which sends concentrated acidic chlorine straight through the heater every time the pump starts. Pucks belong in a floater, or in an inline chlorinator plumbed after the heater. Never before it.

Which one I'd put in

Heat pump, if you swim most days and your spring and fall afternoons stay above about 55°F. That covers most of Florida, Texas, Arizona, Southern California and the Gulf Coast. You leave it running, it holds temperature cheaply, and the extra $1,500 to $2,000 up front comes back in about three seasons against natural gas and in roughly one against propane.

Gas, if you heat on demand. Weekend swimmers, the occasional party, a spa you want at 102°F in half an hour, or a northern pool you want open in early May while nights sit in the 40s. You pay more per degree, but only on the days you actually swim, and it doesn't care how cold the air is.

Propane-only house? Get the heat pump. It isn't close. And if you only swim mid-June through August anyway, skip both for a season. A solar cover on every night often buys several degrees on its own, for the price of one tank fill.