Help Me Choose a Pool Heater

Nothing else is close. A gas heater burns fuel to make heat on demand, so a heater rated at 400,000 BTU/hr of gas input, at the 82% to 84% thermal efficiency typical of the category, still delivers more than twice the hourly heat of a large heat pump, and it does that whatever the air outside is doing. A spa holds a small volume of water, so that output turns straight into temperature — usually well under an hour from cold, against many hours for a heat pump.

The tradeoff is what it costs to run, and for a spa that tradeoff is the right way round. You run a gas heater hard, briefly, and then it sits on the pad costing nothing until the next time.

A heat pump does not make heat, it moves it. It pulls warmth out of warm, humid air and puts it into the pool, which is why it returns several units of heat for every unit of electricity it draws — the COP figure on the spec sheet. On the Gulf Coast, where the air stays warm and wet most of the year, that is as close to cheap powered heating as this equipment gets.

What you give up is speed and cold-weather output. Heat pumps warm a swimming pool gradually over many hours. Actual heating speed depends on pool volume, heater output, starting water temperature, outdoor air temperature, humidity, wind conditions and whether the pool is covered. Proper heater sizing and the use of a pool cover can significantly affect recovery time. It is equipment for holding a temperature through a season, not for surprising the family on Saturday morning.

By July a Florida pool in full sun can sit in the low nineties, and a dark interior finish makes it worse. A reversible heat pump runs its refrigeration cycle backwards and takes heat out of the water instead of putting it in. It is the only practical way to get a pool back down to something that feels like swimming rather than bathing.

Most manufacturers sell the heat-only and the heat/cool version of the same machine under nearly identical model numbers. The difference is a reversing valve inside the cabinet, not a badge on the outside, so check the exact SKU on the quote before you assume a unit cools.

Salt chlorine generators and coastal air are hard on metal. On a heat pump the exchanger is normally titanium, which is effectively indifferent to pool water chemistry. On a gas heater the choice is real: plain copper is the cheapest exchanger and the first thing to go on an aggressive pool, while cupro-nickel and nickel-alloy exchangers hold up far better for a modest difference in price.

Material is not a substitute for chemistry. A corrosion-resistant exchanger buys margin for the days your water drifts out of range; it does not buy permission to stop testing it. Manufacturer warranties generally have something to say about out-of-range water, and that paragraph is worth reading before you buy rather than after a failure.

If you heat for a long weekend three or four times a winter, how fast the water comes up matters more than what each degree costs. A gas heater is idle at no cost and then delivers heat on demand. A heat pump warms a pool gradually over many hours, which makes it the wrong tool for a decision made on a Thursday night.

The arithmetic flips the moment heating stops being occasional. If it turns out to mean most weekends from November through March, price both: over a season the running-cost difference is large enough to pay for a fair amount of equipment.

A headline warranty number is usually the longest single term a manufacturer publishes, and that term is usually attached to one component. A ten-year heat exchanger is not a ten-year warranty: if labor runs out at year two, then in year three you are paying a technician to fit a free part.

So read the coverage as four numbers, which is how every model page here lists it. Labor is the one that decides what a repair actually costs you, and it is almost always the shortest. Then ask who honors it — coverage you can only claim through an authorized service center is worth exactly what the nearest authorized service center is worth.