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Why does water vapor drip back down a vent pipe.

29K views 22 replies 13 participants last post by  vernonbrown 
#1 ·
What causes the water to drip down a vent pipe on a forced air furnace? We installed many furnaces and water dripped back down the pipes and literally destroyed several furnaces. I am curious about the physical or chemical reaction that causes the problem.
 
#3 ·
leaky bent pipe



When we replace an existing FAU, and connect with single wall pipe, and connect to a transite pipe, water will pour backwards into the furnace and flood the controls. Often, there is an oily substance that comes down the pipe. At one time I thought the customer stored a can of motor oil on the furnace and the can leaked. It looked exactly like 3-in-1 oil. Most people think their roof jack is leaking, but these leaks occur when we haven't had rain in over a leak.


The furnace instructions require us to install double wall metal pipe from the outlet of the furnace through the roof.
 
#6 ·
I'd check with the mfg about what the flue temp should be. 400 degrees seems off to me for some reason. It's possible that the fan speed is too high;that would allow more heat in the house then goes up the chimney.

Are you using chimney liner kits? The HO keeping the the temp low in the home would also cause stack to fall below dew point of burnt fuel.

Another thing is over sized duct work will contribute to this problem
 
#23 ·
There's a wide range of issues that affect the health of your chumney including the age of your home, what type of chimney you have, and the quality of the weather. The presence of small animal or bird nests during the Summer or dry leaves in the Fall seriously dangerously impact chumney health. If you notice your chimney is not ventilating fireplace smoke so well you should get it cleaned as soon as possible.
 
#7 ·
Oily substance? Are you venting into an unlined masonry chimney that was previously used for oil. In my area the venting of the furnace needs to match the chimney liner and the liner has to be accurately sized for the firing rate of the furnace and water heater. We use B venting or a smaller single liner inside a existing larger one sometimes.
 
#9 ·
pcplumber
can you give the exact set up of these furnaces.
it sounds like there has been more then one, but are they in different houses or all in the same house.
What furnace came out, what went in.
Chimney configuration.
Use brand names and models #'s whenever you can.
 
#10 · (Edited)
The flue tmep is defiantly the issue here. If it's an 80% furnace with out a metal chimney liner you will get condensation in that oily form you describe.

I had it happen to me as a rookie. I called the local utility and was informed that is was some type of additive put into the gas along the transmission rout .
To burn it off the flue temp must be at optimum temp.


I think you are due for a combustion analysis. If you don't have the equipment then hire an hvac friend to do it for you. You will get a wealth of knowledge from it.
 
#11 ·
Get a copy of the GAMA vent tables. Plus a copy of the IGFC.
It will have vent size tables.

As others have said. It sounds like your flue gas is cooling too much as it goes up the flue.

Oversize flues/chimneys cause much of this.

While not as often. If you install a properly sized furnace in a house that had amplely sized duct work. Its also posible to have too much air moving across the heat exchanger. And also cool the flue gasses too much.

When ever possible. If you can find an extra install manual for a furnace, weather the brand you install or not. Grab it, and keep it.

The install instructions for one, are not always the same as the next brand.
 
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