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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Here’s links to a collection of more blog posts about WATER HEATERS:

Are water heaters required to be raised off the floor? 

Can a Temperature and Pressure Relief (TPR or T&P) valve be mounted to discharge horizontally?

Where are gas water heaters not allowed to be installed?  

Is the minimum size water heater inlet pipe 1/2" or 3/4" according to the building code? 

Can you use a light switch for a water heater disconnect? 

How can I tell if a water heater is HUD-approved for mobile/manufactured homes?

Can you wire a 240-volt water heater with 120 volts? 

Is it alright to have a shut-off valve on both the hot and cold water pipes at a water heater?  

What is the minimum clearance to doors and windows for an outdoor tankless gas water heater? 

What is required clearance for access and working space in front of an electric water heater? 

Why is the water heater older than the house? 

Does a water heater need a shut-off valve? 

Why should a tankless water heater have an isolator/service valve kit installed?

When was a gas water heater first required to be elevated 18 inches above a garage floor?  

Can the temperature and pressure relief (TPR) valve discharge pipe for a mobile/manufactured home water heater terminate under the home? 

What is the purpose of a thermostatic mixing valve above a water heater? 

Does a tankless gas water heater still work with no electricity during a power outage?  

•  Why do water heaters have a sacrificial anode? 

What is the difference between a manufactured/mobile home water heater and a regular water heater? 

Does a tankless water heater require a pressure relief valve?

When is a water heater drain pan required?

Why is there water in my water heater drain pan?

What does it mean when a water heater Temperature/Pressure Relief (TPR) discharge pipe is "trapped"? 

Can I leave a gas water heater in place when remodeling a garage into a family room or bedroom? 

Where do I find the water heater in a mobile home?

Does a tankless water heater in an attic require a drain pan? 

Does an electric water heater require a disconnect? 

Is a catch pan and drain piping required for a replacement water heater?

What is the difference between a single element and dual element electric water heater? 

What is an FVIR water heater?

What is a heat pump water heater?

What is a dielectric union?

What's that powdery crust on the pipe connections at the water heater? 

What are the most common installation mistakes with water heater replacement?

Why is my water heater making strange (rumbling, gurgling, knocking or banging) noises? 

What can I do to make my water heater last longer?

How can I determine the age of a water heater if the serial number is missing or decoding it is impossible? 

How does a hydronic heating system work?

What is the difference between a regular water heater and a direct vent water heater? 

What is the difference between a regular water heater and a power vent water heater?

What is backdrafting at a gas water heater? 

How do I determine if a water heater is gas or electric?

What does it mean when a gas appliance (water heater, furnace, or range) has been "red tagged"?  

• What's the valve with the flip-up handle on the water heater for?

• Why is an older water heater an insurance problem?

   Visit our WATER HEATERS page for other related blog posts on this subject, or go to the INDEX for a complete listing of all our articles. 

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