Heating system return duct filling up with water

World Plumber

Another contractor just called me with a dilemma. Meeting room for ministry building only heated when needed. The return duct 1" duct-board and flex-duct are filling up with water. The return duct is in the unheated attic and the supply ducts are buried under the floor.

     Any suggestions would be appreciated. I'm at a loss as to what to tell him.

 1)     Would more insulation be enough to remedy this? They got 5 gallon of water out of one flex duct today.

  2)   Could warm air be migrating into the duct from the sun through the windows and the part of the building that is heated. Would a damper that closes the return off at the furnace when the system shuts-down  be of any benefit? 

3)  Do they make a humidity sensor that could be used to turn the fan on. Would that help?

Wayne_16

Sounds like a leak from the condensate trap, hoses, heat exchanger etc.

I am going to assume this is a high efficiency gas furnace.  Check for condensate leaks.  Start at the condensate trap and connecting hoses.  The trap plugs up on the outlet and overflows some place else.  Don't forget the pipe/hose from the trap to the building drain.  Many times a piece of garden hose is used and the hose plugs up.  Code states the smallest diameter to use for condensate drain is 3/4 inch. 

If all this checks out then start to look at a failure of the secondary heat exchanger.  A gasket may have let go, the header body cracked or an adaptor to the inducer is cracked, leaking water into the bottom of the furnace/return duct.  A 5 gallon pail is not out of the question for how much condensate a furnace can produce in a 24 hour period.

Take the covers off and start looking.



Regards Minnesota Wayne

Wayne_16

sorry double post

World Plumber

Ceiling

  I tend to think if it was anything to do with condensate it would get in the supply ducts especially when the return is above the ceiling with the fan pulling the other way.

   I've seen the condensate in supply ducts and the bottom of furnaces buy never climb 14 feet against the air flow.

TonyS

My central air system

use to be in the attic. One winter I forgot to close the registers, the warm moist air from the house went right up the open vents to the attic and condensed, when water started to run out the vents I went to the attic to investigate and couldn't believe how many gallons of water were in my flex duct. I just closed my dampers and that finished it. Since your return is part of your heating system it wont be that easy. I would try running the fan continuously to keep the duct heated, that may solve the problem.

World Plumber

Foam

I talked to him late last night. He is going to spray the entire duct system with foam. Hopefully that will keep it warm enough to prevent the condensation.  He says to me why don't I have that problem. He as a building the same size with similar duct set up but no problem. Go figure, there is always that one that has issues.

Wayne_16

Sorry guys

I missed the sentence stating the return duct was in the attic.



Minnesota Wayne