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Gas Pool Heaters

How a gas pool heater works

Pool water leaves the filter and enters the heater, where it flows through a heat exchanger made of tubes. Below or around those tubes, burners fire when the heater senses flow and a call for heat. Hot combustion gases pass over the exchanger, transfer their energy to the water and leave through a vent. Safety controls check water pressure, flame presence, high temperature and, on many units, venting. When the water reaches the set point, the burner shuts off while the pump keeps circulating.

Ratings explained

Gas heaters are sold by input BTU per hour, which describes fuel burned, not heat delivered. Output is input times thermal efficiency. A 250,000 BTU/h input heater at 84 percent efficiency delivers about 210,000 BTU/h to the water. Some regions publish minimum efficiency rules for pool heaters, so the efficiency figure on new units is generally higher than on older ones. Higher efficiency models often use different exchanger designs and venting.

Sizing

Size from your surface area and the temperature gap you want to hold, plus how fast you want the first heat-up. Owners who heat only for weekends often lean larger so a cold pool warms in a day. Run the pool heater size calculator for a starting range. Your gas supply then sets a practical cap: the meter, pipe size and length from the meter must deliver the heater's full input at the right pressure. Only a licensed gas professional can check that.

Natural gas or propane

Most heaters are made in natural gas and propane versions, or can be converted with a kit installed by a qualified technician. Natural gas comes through a utility line; propane comes from an on-site tank that you refill. Per BTU, propane often costs more. Compare with the operating cost calculator and our natural gas vs propane guide.

Installation requirements

• Gas line: sized for the heater's full input over the distance from the meter, with a shut-off valve.
• Venting and clearances: outdoor units need clearances from windows, doors, overhangs and property lines set by the manufacturer and code. Indoor units need proper venting and combustion air.
• Pad: a level, non-combustible base.
• Plumbing: usually after the filter and before any chlorinator, with a bypass and check valves as the manufacturer specifies, so concentrated chemicals do not flow back into the exchanger.
• Electrical: control power and bonding as required.
• Permit and inspection: many jurisdictions require them for gas appliances.

All of this is licensed work. There is no safe DIY gas installation.

Running and maintenance

Gas heaters run hard for short periods. Keep leaves, nests and debris away from the vent and burner area. Keep water balanced: low pH and high sanitizer levels corrode exchangers, while high calcium hardness leaves scale that insulates tubes and makes the heater overheat. Book an annual service. If the heater short-cycles, will not light or smells of gas, stop using it and call a pro; if you smell gas, leave the area and call your gas utility. See pool heater not heating.

Who gas heaters suit

Gas suits owners who swim irregularly and want quick heat, those in cool climates who stretch the season, spa owners who want hot water within an hour or so, and pools in shade where solar is weak. It is less attractive where gas is expensive or unavailable and the pool is used daily through a warm summer; a heat pump may be cheaper to run there. Read heat pump vs gas for the full comparison.

Worked example

Figures below are illustrative examples (2026-10-29), not quotes or guarantees.

Assumptions: 20,000 gallons, warm from 70°F to 82°F before a weekend party, 250,000 BTU/h input heater at 84% efficiency, 25% surface loss allowance. Energy = 20,000 × 8.34 × 12 ≈ 2.0 million BTU. Net heat ≈ 250,000 × 0.84 × 0.75 ≈ 157,500 BTU/h. Time ≈ 12.7 hours. Fuel ≈ 2.0 million ÷ 0.84 ÷ 0.75 ÷ 100,000 ≈ 32 therms.

Signs a gas heater needs attention

Soot around the vent, yellow lazy flames, repeated lockouts, rumbling or banging, water leaks under the cabinet and corrosion on the header all justify a service call. Do not remove panels or adjust gas valves yourself.

Ignition types and controls

Older gas pool heaters sometimes used a standing pilot that burned all season. Most current models use electronic ignition, which lights the burner only when needed and saves fuel. Digital controls display water temperature, set point and fault codes and can connect to pool automation, letting owners schedule heating around swim times. Some models offer separate pool and spa set points. When comparing models, check whether the control works with your existing automation system and what happens after a power outage, such as whether settings are kept.

Low-NOx and efficiency features

Some regions limit nitrogen oxide emissions from gas appliances, and many pool heaters are certified as low-NOx to meet those rules. Ask your installer which certifications your area requires. Higher-efficiency units may use different combustion designs and may need specific venting materials. Your installer must follow the manufacturer's venting instructions and local code exactly; venting mistakes can create carbon monoxide hazards, which is why this is licensed work. Keep the manual for reference.

FAQ

Can a gas pool heater be installed indoors?

Some models are approved for indoor use with proper venting and combustion air; the installer follows the manual and code.

Do gas heaters need electricity?

Most use electronic ignition and controls, so they need a power supply and bonding.

How often should it be serviced?

Most makers suggest an annual inspection by a qualified technician.

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