250K vs 400K BTU Gas Pool Heater: Which Size Does Your Pool Need?

The 250 or 400 in a gas heater's name is BTU input: the gas it burns at full fire, not the heat that reaches your pool. Pentair's MasterTemp manual says so in plain terms, defining the model size as BTU input in thousands of BTU per hour. The heat delivered to the water is the input multiplied by the thermal efficiency, and the rest goes up the vent.

Because the efficiencies match, a 400 doesn't cost more than a 250 for each BTU it puts in the pool. It simply burns gas faster: at full fire a 400,000 BTU heater uses about 386 cubic feet of natural gas an hour and a 250,000 BTU heater about 241, using EIA's 1,036 BTU per cubic foot. What you pay depends on how many hours it runs, and the bigger heater reaches setpoint in fewer of them. The one efficiency exception worth knowing is Pentair's cupro-nickel MasterTemp HD, rated at 82% instead of 84%.

Here is what that looks like across the eight heaters in our heater comparison in these two sizes. The output column is our arithmetic from each manufacturer's published input and efficiency; the minimum flow is what each one needs through it before it will fire.

The Department of Energy sizes a gas heater the same way it sizes a heat pump: on the pool's surface area and the difference between the pool temperature you want and the average air temperature in the coldest month you swim. Its formula is surface area times temperature rise times 12, which gives the output in BTU per hour for a rise of 1 to 1-1/4°F an hour in a 3-1/2 mph wind. For 1-1/2°F an hour multiply by 1.5, and for 2°F an hour by 2. Our heat pump sizing guide walks through the formula step by step.

Two quick examples show where the line falls. A 15 by 30 foot pool (450 square feet) asking for a 20°F rise needs 450 x 20 x 12 = 108,000 BTU/hr, or 216,000 BTU/hr to come up at 2°F an hour, which is right at the output of a 250K-class heater, roughly 210,000 to 222,000 BTU/hr. A 20 by 40 foot pool (800 square feet) with the same 20°F rise needs 192,000 BTU/hr at the base rate and 288,000 BTU/hr at 1-1/2°F an hour, which is 400K territory.

Hayward's own chart recommends its H250 for up to about 750 square feet of pool surface and its H400 for about 1,200 square feet, based on a 30°F rise, a 3-1/2 mph average wind and elevation up to 2,000 feet. That is more generous than the DOE formula, which for 750 square feet and a 30°F rise would call for 270,000 BTU/hr of output (750 x 30 x 12); Hayward doesn't state the heat-up rate behind its chart. Neither is wrong; they answer different questions, which is why we ask how you want to use the pool before we pick a size.

On a pool with an attached spa, the spa usually makes the decision. Hayward publishes how long each size takes to raise an insulated, covered spa by 30°F, and Pentair's estimate for the MasterTemp 400, 27 minutes for a 600-gallon spa, lines up with Hayward's 400.

Start with the idealized math, which assumes no heat is lost while the pool warms. Raypak's AVIA manual gives the formula: heater output in BTU per hour divided by the pool's gallons times 8.33 gives the approximate rise in °F per hour, and it notes that this does not account for heat lost to the weather. That follows from EIA's definition of a BTU as the heat that raises one pound of water by 1°F.

A real pool gives heat back while you heat it. The Department of Energy calls evaporation the largest source of heat loss from a pool, and wind, cool nights and an uncovered surface all speed it up. A heater also only delivers its rating while the pump is running, the flow is inside its window and the gas supply holds pressure at full fire. So on a breezy January evening an uncovered pool will come up noticeably slower than the zero-loss figures, and with a cover on it gets much closer to them. For a weekend, start the day before rather than the hour before.

Pentair's lab estimate for the MasterTemp 400, 4 hours 58 minutes to lift a 20,000-gallon pool by 10°F, is almost exactly the zero-loss figure below, and Hayward's spa times match the same arithmetic, so treat all of them as best cases. Here is the zero-loss math, using the outputs from the table above:

There's no single pipe size, because the answer depends on four things: the heater's BTU input, the length of pipe from the meter or tank regulator to the heater, the fuel, and the supply pressure. Fittings count as length; Pentair and Jandy both add 3 feet of straight pipe for every elbow. The manufacturer tables rate natural gas at 1,000 BTU per cubic foot and propane at 2,500, and the Jandy and Raypak tables are built on a 0.5-inch water column pressure drop. The 3/4-inch connection on the heater is not the pipe size either: Pentair notes the line from the meter is usually larger and should be reduced as close to the heater as possible.

The table below shows rows we read in the installation manuals for natural gas, as maximum equivalent length for each pipe size; Jandy's natural gas figures for the JXi match Raypak's for the AVIA. Read it this way: a 400K heater on natural gas gets 1-inch pipe only if it sits within about 20 equivalent feet of the meter, and needs 1-1/4-inch out to roughly 95 to 100 feet. A 250K-class heater can run 15 to 20 feet on 3/4-inch and 50 to 70 feet on 1-inch. That gap is why swapping a 250 for a 400 on an existing line can turn a heater replacement into a gas-line job as well.

Pipe size is only half of it. Each heater needs a minimum inlet pressure while it is firing: Pentair calls for at least 4 inches of water column with no more than a 2-inch drop from static to running, Hayward 4.5 to 10.5 inches on natural gas, Raypak a 3.5-inch dynamic minimum, Jandy 4.0 inches. Jandy says to take the reading with the heater, and if possible every other gas appliance on the same supply, running, and if it is low, to look for an undersized pipe, a restrictive fitting or an undersized meter.

This is not a homeowner calculation. Pentair's manual requires the gas supply to be installed to the National Fuel Gas Code, ANSI Z223.1, and local codes, by a qualified installer, service agency or the gas supplier, and Raypak notes that installers may be required by some states to be licensed. A licensed professional measures the actual run, counts the fittings, confirms the meter or regulator capacity with the utility or propane supplier, then tests pressure with a manometer at full fire. The table shows why the answer changes; it is not a substitute for that measurement.

Propane carries more energy per cubic foot, so the same heater can run on smaller or longer pipe. In Pentair's table a MasterTemp 400 on propane gets 60 feet of 1-inch pipe against 20 feet on natural gas, and Raypak gives the AVIA 404A 55 feet of 1-inch on propane against 20. Propane also runs at a higher inlet pressure, 10.0 to 13.0 inches of water column on a Hayward and an 8.0-inch dynamic minimum on a Raypak, so the regulator setup is different. Heaters are built for one fuel or the other, which is why Hayward sells the H400FDN for natural gas and the H400FDP for propane.

The tank is where size shows up on propane. EIA puts a gallon of propane at 91,452 BTU, so at full fire a 399,000 BTU heater burns about 4.4 gallons an hour and a 250,000 BTU heater about 2.7; Hayward's own product Q&A puts its 400 at about 4.5 gallons an hour. That means 100 gallons in the tank is roughly 23 hours of full fire on a 400K-class heater and about 37 hours on a 250K. Pool heating is lumpy, nothing for weeks and then a lot during a cold front, so talk to your propane supplier about tank size, whether the tank and regulator can deliver the heater's full input, and getting a delivery scheduled before the cold weather rather than during it.

Placement matters more on propane. Pentair warns of an explosion risk if a propane heater is installed in a pit or other low spot, because propane is heavier than air, and requires installations to meet NFPA 58 and local codes on the relationship between the heater, the tank and filling equipment. Our natural gas versus propane guide covers the fuel choice itself.

On most brands the cabinet doesn't change. The MasterTemp 250 and 400 share the same 23-inch-wide cabinet, the Raypak AVIA 264A and 404A have identical dimensions, and every JXi size uses the same cabinet. Hayward is the exception: the H400 is 34 inches wide against 25 inches for the H250, which can decide whether a swap drops straight in. Flow can change too, from 25 GPM minimum on a MasterTemp 250 to 40 on the 400, which matters on a variable-speed pump.

The real cost difference is often in the gas supply: a longer or larger line, a meter upgrade, or a bigger propane tank. Removal of the old heater, the pad, access to the equipment and anything unusual about the site all move the installed price as well. Our gallery publishes what two specific gas jobs cost: a Hayward gas heater set on its own pad plus a Jandy TruClear salt system, finished in May 2026, at $4,500 to $5,500, and a Naples job that added a gas heater and redid the pool equipment plumbing at $5,500 to $6,500. Those are those jobs, not a quote for yours.

Going bigger than the pool needs doesn't buy efficiency either. As our sizing guide notes, an oversized gas heater brings a larger gas line, larger venting, a bigger meter or tank and more short cycling, with nothing gained per BTU.