How To Look At A House

McGarry and Madsen's home inspection blog for buyers of  

site-built, mobile/manufactured and modular homes

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.”

• • • • • • • • • • • • • • • • • • • • • • • • • • •

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? 

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

•  Why do water heaters have a sacrificial anode? 

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?

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.

Water Heaters

Water Heater Age

"What Are The

Signs Of..."

Septic Tank Systems

Structure and Rooms

Plumbing Pipes

Termites, Wood Rot

& Pests



When It First

Became Code

"Should I Buy A..."

Park Model Homes


Shingle Roofs




Wind Mitigation

Roof and Attic

"Does A Home


Pool and Spa

"What Is The Difference Between..."




Concrete and

Concrete Block

Metal Roofs


Modular Homes

Rain Gutters

Mold, Lead & Other Contaminants


Older and

Historic Houses

Crawl Spaces

Mobile-Manufactured Homes

Building Permits

Life Expectancy

Clay Soil





Exterior Walls

& Structures


Common Problems

HUD-Code for

Mobile Homes

Garages and Carports

Flat (Low Slope) Roofs

Electrical Panels

Sprinkler Systems

Electrical Receptacle Outlets

4-Point Inspections

Hurricane Resistance

Frequently Asked Questions (FAQ)

Home Inspection

Heating and Air Conditioning

Building Codes

Fireplaces and Chimneys

Inspector Licensing

& Standards

Energy Efficiency

Washers and Dryers



Doors and Windows



Electrical Wiring

Click Below  

for Links

to Collections

of Blog Posts

by Subject

Plumbing Drains

and Traps


Smoke & CO Alarms

Aging in Place

Top 5 results given instantly.

Click on magnifying glass

for all search results.






Air Conditioner & Furnace Age/Size


Electrical Switches





Water Intrusion

Electrical - Old

and Obsolete


Foundation Certifications

Tiny Houses

About McGarry and Madsen



Buying a home in North/Central Florida? Check our price for a  team inspection by two FL-licensed contractors and inspectors. Over 8,500 inspections completed in 20+ years. In a hurry? We will get it done for you.

Moisture Problems

Crawl Spaces