Gas heaters
Buying speed, and what speed costs
A gas pool heater burns fuel and puts a fixed percentage of it into your water. A heat pump often costs less to run because it moves heat instead of making it, but the result depends on your gas price, electricity price and seasonal coefficient of performance. What gas sells is raw output — enough BTUs per hour to take a cold pool to swimming temperature between Friday night and Saturday morning.
Efficiency is a floor you can look up
You do not have to guess a gas heater's efficiency, because federal law sets a minimum, in one sentence:
Gas-fired pool heaters manufactured on and after April 16, 2013 and before May 30, 2028, shall have a thermal efficiency not less than 82%.
What it settles here This is where the 82% default on this page comes from, and it is the reason gas is the one heater efficiency on this site that is a published figure rather than a judgement — but read it as a floor and not a rating, because a real unit meets or beats it and a condensing model reaches the high eighties, so if you have a model number use its own certified efficiency instead.
The clause has an end date in it, and the rule that sets the end date says when the replacement bites:
The effective date of this rule is July 31, 2023. Compliance with the new and amended standards established for consumer pool heaters in this final rule is required on and after May 30, 2028.
What it settles here The standard was settled in 2023 and does not apply to anything on a shelf until manufacture on or after 30 May 2028, so a heater bought today is an 82 percent floor unit and the practical consequence for a buyer is timing: model lines will turn over around that date, and clearance stock is only a bargain if the units replacing it are not meaningfully better.
What replaces the 82 percent floor is a different measure entirely: 10 CFR 430.32(k)(2) — gas-fired and electric pool heaters manufactured on and after May 30, 2028 must meet an integrated thermal efficiency, computed from certified input capacity for a gas-fired model and from certified active electrical power for an electric one. That change of measure matters more than it sounds. Thermal efficiency describes the burner while it is firing; the integrated figure that replaces it is built on top of that and also counts what the heater consumes in standby and off mode, which is where a standing pilot and an always-live control board show up. A unit that looked good on the old measure can look ordinary on the new one without its burner changing at all.
Input rating against output rating
Gas heaters are advertised by input. A 400,000 BTU per hour heater consumes that much fuel energy and delivers 328,000 BTU per hour into the water at 82%. Every sizing number on the sizing page is an output requirement, so divide by the efficiency before you shop.
It also sets the gas supply you need. At 1,036 BTU per cubic foot, a 400,000 BTU per hour heater wants 386 cubic feet an hour at the appliance. That is a real constraint: an existing meter or pipe run may not deliver it, and a larger meter, a new run or a utility service upgrade can add work that never appears in the heater's shelf price.
Cost per hour, per degree, and the gas line it needs
The two outputs people actually want from a gas heater are how much an hour of running costs and how long a degree takes. Both fall straight out of the input rating.
Delivered output 328,000 Btu/h82% of the input rating
Cost of an hour of running $7.17at $1.79 per therm
Cost of one degree on this pool $3.50159,815 Btu of water, before losses
Time for one degree 29 minutesat full output, ignoring loss
Gas supply required 386 cu ft/hcheck this against your meter before you buy
The case gas actually wins
Run the default pool for a whole season at the published prices used here and gas costs $4,291 against $1,662 on a heat pump — 2.6 times as much. That is this pool, climate and price pair, not a universal ordering; the energy-price worksheet lets either input change and shows the resulting cost per delivered million BTU.
Gas can answer a different requirement: rapid intermittent heat. A pool used two weekends a month, brought up from ambient each time and allowed to drift back down, is a heat-up problem rather than a maintain problem. A gas heater rated at 400,000 BTU per hour costs $7.17 an hour to run in this example and supplies a twelve-degree lift, before losses, in 5.8 hours.
An attached spa presents the same trade: a small volume and a large, fast temperature rise can make output and response time more important than the seasonal efficiency comparison. The right result still depends on the actual heater ratings and energy prices.
What gas costs that the sticker does not say
- Gas supply. 386 cubic feet an hour is often beyond an existing residential run. Price the pipe and the meter before you price the heater.
- Venting and clearances. An outdoor unit still has combustion clearances, and a heater tucked against a wall or under an overhang may be non-compliant where it sits.
- Water chemistry. Poorly balanced water can shorten a gas heat exchanger's life. Follow the manufacturer's permitted chemistry range rather than assigning this page's operating-cost result an assumed equipment lifetime.
- Standing pilot models. If the unit has one, it burns fuel all season whether or not you swim. Electronic ignition is worth insisting on.
If you burn propane instead
Propane carries 91,452 BTU per gallon, so the same season load costs $6,995 at $2.674 per gallon — the national residential figure, and one you should replace with your own delivered price, because propane pricing varies more between two suppliers in one county than the national average moves in a year.