Water heater sweating: is it condensation or a leak?

Why water heaters sweat

Bradford White compares it to a cold drink sweating on a summer day. Condensation forms when moist air or gas meets a surface below its dew point, the temperature at which water vapor turns to liquid. A water heater has three ways to get there:

• Inside the flue (gas heaters). A. O. Smith's Bulletin 15 says low incoming water temperatures cool the piping and heat-transfer surfaces, and when the burner comes on, the hot flue gases condense on them. That water drips down onto the burner, which is why you may hear sizzling.
• On the outside of the tank. During heavy hot water use, cold water rushing in cools the tank's outer surface, and A. O. Smith says it can condense moisture from the air. High humidity makes this more likely.
• On the cold pipe. Bradford White notes that cold incoming water in winter can make copper pipes sweat in a warmer basement.

When it's most common

• Winter. Bradford White says cold incoming water in winter reacts with the warmer tank and makes it condense.
• A new or refilled heater. A. O. Smith says condensation is common when a heater is first filled or refilled, because the tank is full of cold water. Its gas manual adds that you may hear drips or sizzling and see a small puddle after first lighting, and that it goes away once the tank reaches operating temperature.
• After heavy use. Back-to-back showers or laundry fill the tank with cold water; A. O. Smith says this condensation is usually temporary.
• Newer, efficient gas heaters. A. O. Smith says newer heaters condense more than older ones, because they pull more heat out of the flame, which lowers flue gas temperatures closer to the dew point. Bradford White says the same about energy-saver models.

The 8-hour test: condensation or leak?

A. O. Smith's Bulletin 15 gives this test for gas heaters:

1. Wipe up any water under the heater.
2. Turn the thermostat knob to the PILOT position so the burner stays off.
3. Wait 8 hours and check for water under the heater.

If the floor stays dry, the heater was condensing. If water collects anyway, A. O. Smith says to look for a loose fitting, and if every fitting is tight and the tank itself is leaking, the heater needs to be replaced; a leaking tank can't be repaired. Our guide to a water heater leaking from the bottom walks through the other places water comes from, including the drain valve and the relief valve.

Signs it's probably not condensation

• Water keeps appearing with the burner off, as in the test above.
• Drips trace back to a pipe fitting, the relief valve discharge pipe or the drain valve.
• The heater is electric. Electric heaters have no burner, so there is no flue gas condensation; water under one is more likely a fitting, valve or tank leak, though a cold tank can still sweat in a humid room.

How to stop or manage sweating

• Give it time. A. O. Smith says condensation should stop once the entire tank is above about 115°F.
• Lower the humidity. For condensation on the outside of the tank, A. O. Smith suggests a dehumidifier or better ventilation.
• Insulate. For older tanks in cool or unheated spaces, A. O. Smith says an insulation blanket can reduce condensation by keeping the outer surface warmer. Follow the heater's label and the blanket's instructions.
• Use a drain pan. Bradford White suggests setting the heater in a drain pan to collect condensation until it evaporates, and notes that some gas utilities and local codes don't allow a plastic pan under a gas heater, so use a metal one there. The 2018 International Residential Code already requires a pan where a leak would cause damage, at least 1-1/2 inches deep with a drain of at least 3/4 inch (see water heater drain pan).
• Keep the venting in good shape. A. O. Smith's advice is to make sure vents and flues on gas models are working properly (see venting and backdrafting).

When to call

Call a technician if water keeps collecting after the 8-hour test, or if you can't find where it's coming from. A tank that's leaking needs replacing, and the sooner it's caught the less damage it does (see signs your water heater is going bad).