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What causes water dripping down the flue (condensation) at a gas water heater?
Thursday, June 21, 2018
Although water dripping down the flue can be caused by a missing or damaged rain cap at the roof, it is more likely that the wetness is caused by condensation of combustion gases on the interior surface of the flue. You may see it puddled on top of the water heater or under it. Corrosion of the water heater jacket just below the draft hood (flared bottom of flue above it), like in the photo above, is another sign of condensation.
Water vapor is one of the main byproducts of the combustion of natural gas. A water heater produces about a half gallon of water per hour of continuous operation. Normally, the hot combustion gases heat the flue enough so that the water remains in vapor form as it rises through it and out into the atmosphere. But any of the following problems can cause the vapor to condensate on the inside of the flue and drip back down:
- The air supply for combustion is colder, usually during a winter day.
- The water in the tank is not hot, due to the cold start-up of a new unit, large flow of water during a brief period, especially if the incoming water is very cold, or the thermostat is set below 110º F, any of which can cause condensate on the internal flue.
- Flue too large and does not evacuate air before it starts to cool.
- Too long a flue run and/or too many elbows.
Since the condensate is acidic, it accelerates corrosion of metal wherever it puddles. The drip can also extinguish the pilot light and corrode the burner. You might see black or red/orange granules around the burner or under the draft hood, which is residue from evaporated condensate.
Fixing any of the listed causes will stop the problem but, if you suspect it might be tank leakage instead of condensation puddling, here’s the way a technical bulletin from Rheem Manufacturing Company recommends to determine which it is:
“While attempting to verify a suspected leaking water heater, first take a good look at all the fittings and attachments to the tank. Check for the tell tale signs of evaporated water around the fittings and outer jacket of the heater. Also, check the pattern of the water that has pooled into the drain pan. Normal condensation will accumulate and then evaporate. A leaking tank will always leak leaving larger accumulations of water with little time to evaporate.”
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