Why is there a stain in the ceiling around the exhaust pipe (flue) above the gas furnace?

Monday, May 27, 2019

A circular stain around the flue above a furnace where it penetrates the ceiling can be caused by a roof leak or condensation of the hot combustion gases inside the flue. Roof leakage around the flashing is a fairly common problem. Examining the underside of the roof sheathing in the attic around the flue penetration will tell you if a roof leak is the culprit. There will be dark staining of the sheathing around the flue and/or drip stains running down the sides.

    Condensation of the combustion gases in the flue can be caused by several different problems or a combination of them. Water vapor is one of the gases produced by burning natural gas and, ideally, the gases rise up quickly through the flue and, assisted by a blower in the furnace, exit into the outdoor air before cooling enough to cause the water vapor to condense. Here’s some issues that can cause the water vapor to condense too early and drip down the inside of the flue:

  • A flue diameter that is too large for the manufacturer’s rating of the furnace. An oversize flue reduces the velocity of the air flow, and can allow cold air to come down from the top and trap it in the flue. This may happen when an older gas furnace with a lower energy efficiency rating is replaced with a new, higher-efficiency furnace without verifying that the existing flue matches the manufacturer’s specs. Older furnaces send 55% to 72% of the heat produced into the home, and the other 28% to 45% goes up the flue as very hot combustion gases. Newer furnaces keep 80% to 83% of the heat in the home, so the combustion gases going up the flue are cooler and not rising as vigorously.
  • The flue cap is missing. Flue caps don’t just keep our rain and small animals. They also help prevent downdraft gusts in the flue and generate a venturi effect to give the gases an extra push upward.
  • Too many elbows or inadequate rise. Too much horizontal run and too many elbows builds up back pressure on the gas flow, slowing it down. Minimum backwards slope toward the furnace is 1/4” per foot. See our blog post What is the minimum slope of a flue connector for a gas furnace or water heater? for more on this. A horizontal run that slopes downward towards the flue termination is the worst: condensate forms and then puddles in it.
  • Single-wall flue used in unconditioned space. A double-wall flue is necessary in any unheated space to reduce heat loss through the flue wall and resulting condensation.
  • Short-cycling of the furnace. When the furnace only runs briefly each cycle, the flue does not get hot enough to avoid condensation.
  • No connector fitting between flue connector running from draft hood to B-vent flue above and/or no pipe collar. Allows air leakage around openings and condensation. Shown below is missing fittings and a correct installation.


  • Vent connector turns too soon above the furnace. Although there is a blower in modern furnaces to speed up the air flow, they are essentially "natural draft” appliances that operate on the principle of hot air rising, and need at least 18” of vertical flue connector run above the furnace before any turns are made, to allow the gases to build up some speed before meeting any resistance further up the flue. Shown below is a flue connector that just barely meets the 18” standard, with minor staining at the ceiling around the flue collar.

    Long-term condensation drip can corrode the flue or flue connector, like the one shown below. Evenutally, rust holes will open and allow the carbon monoxide in the combustion gases to escape into the house or attic. There is also sometimes a residual powder below it in the furnace compartment. For the statistics on carbon monoxide poisoning, go to our blog post Why are carbon monoxide (CO) alarms required by law for homes in Florida? 


    What to do? If you are able to safely climb into the attic to check for a leak at the flue penetration of the roof, that’s a good start. And, if it’s not a roof leak, call an HVAC professional to evaluate and fix the condensate drip problem.

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  To learn more about heating and air conditioning systems, see these other blog posts:

How can I find out the SEER of my air conditioner? 

My air conditioner won't turn on. What's wrong? 

How can I find out the size of my air conditioner? 

How can I find out the age of my air conditioner or furnace?

How can I tell whether the condenser (outdoor unit) is an air conditioner or heat pump? 

Where is the air filter for my central air conditioner and furnace? I can’t find it? 

Does an old air conditioner use more electricity as it ages? 

How did homes stay cool in Florida before air conditioning?

What is wrong with an air conditioner when the air flow out of the vents is low?

Why has the thermostat screen gone blank? 

Why does it take so long to cool a house when an air conditioner has been off for a while? 

Why is my air conditioner not cooling enough? 

What are the most common problems with wall/window air conditioners?  

Will closing doors reduce my heating and cooling costs? 

    Visit our HEATING AND AIR CONDITIONING 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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