Quick answer
A little water near your pool heater usually isn’t a leak. Heat pump pool heaters normally produce condensate — pool-industry sources put it around 6-10 gallons per hour on hot, humid days, the same way a window AC unit drips — and it should drain harmlessly through a hose or fitting at the base of the cabinet. Gas heaters condense too, but the mechanism is different and more serious: water vapor from combustion can condense inside the heat exchanger if return water runs colder than the heater’s minimum rated temperature, and that condensate is mildly acidic enough to corrode metal over time. Two quick tests — check for chlorine/salt in the water, and see if it stops when the heater shuts off — tell you which situation you’re actually looking at.
Heat pumps: condensation is normal, and it can be a lot of water
A heat pump pool heater works like an air conditioner running backward — it extracts heat from the surrounding air to warm your pool water, and in the process, moisture in that air condenses on the cold evaporator coil, the same way beads of water form on a cold glass. Pool-industry sources including AquaCal and Madimack put normal output around 6-10 gallons per hour on hot, humid afternoons, tapering off close to nothing on cool, dry days. That’s genuinely a lot of water on a muggy day — enough that some owners assume something’s wrong when it’s actually the heat pump doing exactly what it’s designed to do.
Most units have a drain hose or fitting built into the base of the cabinet to route that water away, and it should be piped or hosed to a drain, gravel bed, or at minimum away from foot traffic and the electrical disconnect. AquaCal even sells a retrofit condensate-drain kit for older units that shipped without one — a sign this is a known, expected byproduct of how the equipment works, not a defect.
| Heat pump condensate | Gas heater condensation | |
|---|---|---|
| Where it forms | Outside, on the evaporator coil | Inside the heat exchanger |
| Cause | Extracting heat from humid air (like an AC) | Cold return water below the heater's minimum rated temp |
| Is it normal? | Yes — expect 6-10 gal/hr on humid days | Brief startup condensation is normal; sustained condensation is not |
| Risk if ignored | Low — pooling water near the pad, wiring, or foundation | High — acidic condensate corrodes the heat exchanger from inside |
WiFi Water Leak Detector
- Placing one of these under or near your heat pump's drain path flags a real problem early — a sudden alert rather than discovering a puddle days later.
- Useful as a general equipment-pad safeguard too, since it catches pump-seal and plumbing leaks, not just heater condensate.
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Gas heaters: condensation is an internal corrosion problem, not just water on the ground
Gas combustion produces water vapor as a byproduct, and if the metal inside the heat exchanger — or the return water flowing through it — is too cold, that vapor condenses on contact, the same way warm breath fogs a cold window. The difference from a heat pump is that this condensate forms inside the heater, and it’s mildly acidic (trade sources cite roughly pH 3.5-5.5), which means it slowly corrodes the heat exchanger and surrounding components from the inside rather than just pooling harmlessly outside the cabinet.
Manufacturers publish specific minimums for exactly this reason. Pentair’s MasterTemp installation manual states the heater’s minimum operating water temperature is 68°F, and running it continuously below that “will cause harmful condensation and will damage the heater and void the warranty.” Raypak addresses the same issue with its Unitherm Governor, a thermostatic mixing valve that holds internal heater temperature in a safe 105-115°F band, because Raypak’s own guidance flags condensation damage risk if the heater runs below roughly 100°F for any length of time — see our bypass valve guide for how that mixing hardware works. Brief condensation on a cold start is considered normal across gas heaters generally; the real risk is sustained operation on cold return water, which shows up most often during spring pool startup or short spa-only heating cycles where a small volume of water heats up fast and the bulk of the system stays cold.
If you notice rust or corrosion around the burner tray, gas valve, or heat exchanger area specifically — not just light surface rust on the cabinet’s exterior — that’s worth a professional look rather than a DIY sand-and-paint job, since it can point to chronic condensation that’s already eating into the exchanger. Our pool heater not heating guide covers heat-exchanger failure as one of the costlier causes of a heater that’s stopped working, and corrosion from chronic condensation is one of the ways it gets there.
Condensation or a real leak? Two quick tests
Pool pros use two simple checks before assuming the worst:
- The chemical test. Dip a pool test strip in the water pooling near the unit. Condensate is essentially distilled water — no chlorine, no salt. If the strip picks up chlorine or salt, you’re looking at pool water, which points to a plumbing or pump-seal leak rather than condensation.
- The shutdown test. Turn the heater off but leave the filter pump running, then watch for 10 minutes to a couple of hours. Condensate drainage tapers off and stops once the unit isn’t actively running; a real leak keeps dripping at the same rate regardless of whether the heater is on.
Continuous dripping that doesn’t respond to either test, or water that shows up even when the heater has been off for a while, is a sign to call a pool tech rather than keep monitoring it — especially on a heat pump, where it could also point to a refrigerant-side issue rather than the condensate drain itself.
The bottom line
Don’t panic at water near your heat pump on a humid day — 6-10 gallons per hour of condensate is normal, and the fix if it’s pooling somewhere inconvenient is usually just redirecting the drain hose, not a repair. Gas heater condensation deserves more attention, since it happens inside the heat exchanger and can corrode it if return water regularly runs colder than the manufacturer’s minimum — keeping your bypass valve properly set is the usual fix. When in doubt, the chemical test and shutdown test take a few minutes and tell you definitively whether you’re dealing with normal operation or something that needs a repair call.
Sources: Pentair — MasterTemp 125 Installation and Users Guide, Raypak — The Long Lost Art of Bypass Valves, AquaCal — Why Is My Swimming Pool Heater Leaking?, TroubleFreePool — Heater Condensation, Madimack — Leakage vs. Condensation