What is a pool heater?
Updated August 2026
A pool heater is the appliance plumbed in after the filter that raises pool water temperature, either by burning natural gas or propane, by pulling heat out of the outside air with a heat pump, or by circulating water through solar panels. Which type you own decides nearly everything about how the pool gets used: how fast it warms, what a heated weekend costs, and how far into fall the water stays swimmable.
Where it is
The heater is the last piece of equipment before water returns to the pool, sitting after the filter and any chlorinator. A gas unit is a boxy cabinet with a flue on top, a gas line entering one side, and a small digital control panel. A heat pump looks like an air conditioner condenser: a large fan facing up and a coil wrapped around all four sides. Solar systems put the panels on the roof and leave only a three-way bypass valve and a temperature sensor down at the pad.
What it does
The performance gap between the three is the whole story. A gas heater raises an average residential pool 1 to 2 degrees per hour and does it at 40 degrees outside or 80, which suits a spa spillover or a pool heated on demand. A heat pump manages about 0.5 degrees per hour and loses efficiency once ambient air drops below roughly 50 degrees, so it fits daily-use pools in mild regions. Solar costs the least to run and offers the least control, typically adding 5 to 10 degrees during sunny stretches. Failures cluster around chemistry and flow: low pH eats the copper heat exchanger from inside, hard water scales it over, and a dirty filter starves flow until the pressure switch shuts the burner down.
Maintenance
Two habits protect a heater more than any service call. Hold pH between 7.4 and 7.6 with total alkalinity around 80 to 120 ppm, and keep the filter clean so flow stays strong. Beyond that, book an annual burner and heat-exchanger inspection at $150 to $300, leave 3 feet of clearance around a heat pump so it is not swallowing its own cold exhaust, and drain the exchanger before the first freeze. The pool heater maintenance checklist covers open and close, and service life runs 5 to 10 years for gas, 10 to 15 for a heat pump.
What it costs to replace
A replacement pool heater runs $2,000–4,500 installed for a gas heater, or $3,500–6,000 for a heat pump.
OnOtto's photo scan identifies parts like this in your home and builds the maintenance schedule for you. See how the scan works.
Frequently asked questions
Gas heater or heat pump for a pool?
Match the equipment to how you swim. If you heat the pool for occasional weekends and want it warm by tomorrow afternoon, buy gas: it works in any weather and recovers quickly. If you swim most days from spring through fall in a mild climate, buy a heat pump and hold a steady temperature, since operating cost is a fraction of gas. Either way, add a solar blanket at $100 to $300, which is the cheapest heating upgrade available.
How much does it cost to heat a pool?
For an uncovered 20,000-gallon pool, plan on roughly $200 to $600 a month of gas during active heating, or $60 to $200 a month for a heat pump doing similar work in mild weather. Propane costs more than natural gas. The factor most owners overlook is evaporation: an uncovered pool sheds most of its heat off the surface overnight, so a cover cuts heating cost substantially without changing anything else about the system.
Why does my pool heater turn on and shut off right away?
Short cycling on a pool heater traces to flow or exhaust far more often than to the burner. Clean the filter and open every valve first, then check the pressure switch, which cuts the burner when it senses weak flow. On a gas unit, also look for a blocked flue or a scaled heat exchanger, both of which trip the high limit inside a minute or two. If the display shows an ignition lockout code instead, the fault is gas supply or the igniter.
Should I repair or replace an aging pool heater?
Let the age of the unit break the tie. A gas heater past 8 years with a corroded heat exchanger does not justify a $900 to $1,600 exchanger, since that is a third of a new heater that arrives with a warranty. Under 5 years, repair almost anything: control boards, igniters, and pressure switches run $150 to $500 and the cabinet has plenty of life left. Work through repair or replace a pool heater for the full call.