I’m doing some work on a row of townhouses, built around 1997. One of the homeowners asked me about the function of a pipe that exits the rear of each unit. It’s a piece of 2″ sch40 PVC, insulated with foam, terminating in a downward-pointing ell. The units all have natural gas furnaces with A/C, and the “mystery pipe” terminates at the furnace.
Okay, obviously the pipe carries condensate from the A/C coil, as evdenced by water dripping from the outside end. But one customer tells me the pipe occasionally emits a puff of steam when his furnace is running during the winter. (Again, this is a gas furnace, not a heat pump.) So, what gives? My curiosity has gotten the better of me here. Does the pipe possibly supply make-up air for the furnace? Where does the steam come from in the winter?
Any enlightenment here will be much appreciated!
-Jon
Replies
It would be interesting to hear from some HVAC types here, but my guess is that it's probably the vent for a high efficiency condensing furnace. I would expect to see more than an occasional puff of steam in the winter though.
Are they 90+ condensing furnaces? They would have PVC flues, not metal, and a series of drainage tubes connected to the flue, usually behind the casing door, and that 2" pipe you see is most likely the flue, not the A/C condensate line.
If it is a puff of steam, I'd be really concerned. However, I'd guess it's just condensing water vapor.
90+ high efficiency furnaces use a secondary heat exchanger which actually recovers heat from the process of some of the heated water vapor in the flue gases condensing, and the flue gases are fairly cool as they hit the colder air outside and more condensation occurs.
And you will see condensate water dripping from them in heating mode, but shouldn't see anything in A/C mode. In colder climates it's even common to see icicles hanging from the flue or a mound of ice building up below the flue.
They typically use the same condensate lines as the A/C unit to drain that condensed water vapor away, but that is usually 3/4" PVC
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Yeah, that's the flue for a high-efficiency furnace. Be sure nothing blocks it.
Only 2"? They are all 4" here.
Mine is 4" also, but the furnace stubs out with a 2", which is immediately increased to the larger size. Maybe the newer ones have stronger fans and don't need that extra size.
I think the 4" exit is to prevent frost build-up from covering the opening. Winter can get pretty cold here.
We have a new house built across the street from us. They have one of these PVC vents right next to their front door. They spent tons of money putting rock siding on their house, not to mentiop all the other architectural details, and then stuck this ugly PVC pipe out the side. It really sticks out like a sore thumb, especially in the winter when a cloud is coming out of it.
I wonder if there is a more attractive alternative? A bronze ell, or something like that. It's right at eye level.
I wonder if there is a more attractive alternative? A bronze ell, or something like that. It's right at eye level
Sure ... scuff with sand paper and spray paint.
For low ... low slope roofs ... where the roof can be seen ... I spray paint the vent stack pipes to "match" the shingles.
Even had customer say once ... while standing out in the back yard ...
Hey, great! ... You got rid of that ugly pipe ...
He thought I cut it off.
JeffBuck Construction, llc Pittsburgh,PA
Artistry in Carpentry
>>Only 2"? They are all 4" here.
Depends on make, model, run and # of angles.
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The condensate from a condensing furnace or water heater is acidic.
Be careful what it drips on. Don't run it to a metal gutter.
A chimney flue full of limestone set into the ground?
Ask the HO if he has the manual for the furnace, and if not, download it and read it. It will tell you the function of the pipe and whether or not it should puff. I've seen some Munchkin condensing boiler and similar in use, and the puffs of steam are at the roof, not out the condensate drain. If it were my house I'd be having the HVAC guy stop by at beer:30 to give me a quick look.
But once again, this probably isn't a condensate drain, but rather the flue for a HE condensing furnace.
The pipe can be "obscured" with shrubbery or some sort of "lawn ornament", but any such stuff must be kept some distance (one or two feet, I suspect) from the pipe.
OK.... now I'm remembering a job where the furnace had a 4" white PVC outlet that ran horizontally (downhill, actually) and was for combustion air, flue, and condensate drain. It terminated outside into a white plastic cone-shaped fitting, about 6" in diameter where the 4" pipe entered, and about 3" at the narrow end... it exhausted both water and CO thru the narrow end, and sucked in combustion air around the edge. Steam did come out of that thing. The first time the installer ran it he did not know that it needed drainage slope, so it filled partly with water and made a gurgling sound when the furnace was running. He came back and re-ran it, and then it dripped water whenever the furnace was on.
Yep, when they installed the HE furnace in my MIL's house many years back (one of the first HEs available) the installer let the flue pipe sag, and water collected there, causing the unit to sense back pressure and shut down. They blamed it on frost on the outside and tried all sorts of things on the outlet to prevent it. I came along and tied up the pipe with a piece of wire and it started working fine. The wire is still there.