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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.
 

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Flue gasses condensing in the flue. for non condensing appliances the flue gas must be above 400 degrees. Long flue runs will also cool the gasses and cause the problem.
 

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Discussion Starter · #3 ·
leaky bent pipe

Flue gasses condensing in the flue. for non condensing appliances the flue gas must be above 400 degrees. Long flue runs will also cool the gasses and cause the problem.
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.
 

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Size your venting properly. Too large and you won't get a proper "stack effect", the gasses will cool off and condensate inside the vent. End result? New Furnace.
 

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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
 

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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.
 

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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.
 

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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.
 

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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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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.


This is the reason we aren't allowed to vent back into transite....Some manufactures also advise venting into transite.. When I run into transite . I line it with DW vent.. If it's oval transite.. I install a 90+ furnace
 

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This is the reason we aren't allowed to vent back into transite....Some manufactures also advise venting into transite.. When I run into transite . I line it with DW vent.. If it's oval transite.. I install a 90+ furnace
I probably know it by another name, but what is transite?
 

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Size your venting properly. Too large and you won't get a proper "stack effect", the gasses will cool off and condensate inside the vent. End result? New Furnace.

Out here in De. we are allowed to use Pvc for the High 90% over above. I am gathering that is the type of heater in use. If that is the case, then, there sounds like you have a pitch pblm.
 

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Out here in De. we are allowed to use Pvc for the High 90% over above. I am gathering that is the type of heater in use. If that is the case, then, there sounds like you have a pitch pblm.
No its not a 90% furnace as any body in the HVAC industry can plainly tell from the posts.

Are you employed in the hvac business or just a civilian dabbler?Mo:confused1:re
 

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Sounds like he's just getting started so we will see. Apprentices are welcome on the site as with anyone who works full time in the industry. I'll post more about who is who is not allowed soon.

Thanks.
 
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