Furnace Vent Pipe Advice

[wddossett]

Hi, we had our furnace replaced with a condensing one (I think thats what they are called, I lived in the UK for a long time and thats what they called them there...) anyway it vents water and CO2 out a pvc pipe. The pipe leaks at a join. we have finished our basement since/during this so saw a wet patch in the ceiling and cut it out and exposed it all, cut some bits out put couplers in, but it was very tight space, multiple couplers there and it started leaking again after a year. So, its a straight shot outside, going to cut out the bit with couplers and replace it all the way to the vent outdoors so there will be only one coupler and I can get a clean start...

While I am doing this, I am curious. It is 3" PVC pipe right up to nearly the outside vent and then its reduced to 2" and connects to the outside vent assembly... on the charlotte pipe website it says "DWV application diameter cannot be reduced downstream " this vents water out, so downstream would naturally be towards outside of house in my small mind, and it is being reduced from 3" to 2" on its way outside is this legal/correct or am I thinking backwards... the vent pipe rises on the way out, so the furnace is lower that the vent, so maybe that is downsteam. While I am ripping this out, if something wasn't done right, seems like the time to fix it... but the vent on the outside of the house is obviously made for 2". Thanks for any advice!

 

[FishScreener]

What do the furnace manufacturers installation instructions say. They dominate.

 

[wddossett]

exactly what I was reading.... I have to track down the installer manual as I only have operator and see what it says. Thanks!

 

[JG plumbing]

Yes follow manufacturer instructions. Normally they don't reduce in size as they exit the building, and they slope back toward the furnace. The furnace drains the condensate seperately normally.

 

[wddossett]

ok, found the manual and yes if you are using 3"a pipe, then it needs to be reduces to 2" 18 inches before it leaves the building which it does... and it slopes up the correct amount, I know when he was installing it he explained it all to me. Not sure why it goes from 2" to 3" and back to 2" but thats what the picture in the manual shows... Not sure what you mean by the furnace drains the condensate separately normally, is that inside the furnace? there is a drain on the combustion air intake which goes to a pump and there is also another pipe coming out that goes to the drainage pump (cou9dl be that one I guess)... plus also a humidifier that goes to the pump... which all works.. But as I said, I have a leak in the exhaust at one of the joins, I assume that some condensation goes out the exhaust, enough that it one of the joins that wasn't glued properly started to drip.

 

[JG plumbing]

The reason they change sizes is to decrease friction loss. You must be going quite a ways and have some bends.

 

[wddossett]

yes, its got a couple of right angle bends and a couple of 45s on the way up to the ceiling and about 50 - 60 ft I'd guess with the rise to ceiling. So, seems like its all done correctly, I can saw off the two couplers, and replace with one and do a proper job of gluing it as I can push from outside now to make sure it seats it it completely in the coupler with some help from a friend!

 

[Stout Mechanical]

Make sure you use sch 40 pipe, not foamcore

 

[wddossett]

yep... just wondering, I've had miserable luck with glue on this, mostly cos its hard to get at and get it spread all around evenly... this says its a PVC coupling what are your thoughts on using one of these? Or is it even legal or a good idea?

 

[Stout Mechanical]

Not legal where I am
It has to be primed and glued.

 

[wddossett]

ok, thanks... it shouldn't be an issue this time, I was trying to do a repair and sliding the coupler back and forth on pipe that was in place. Now that I am replacing it I will be able to set each piece properly.

 

[breplum]

You can definitely use a coupling, but use a "shielded" no-hub type of coupling. I've got many decades of experience and guarantee it is fine.
Even though 3" PVC is bigger than 3" no-hub pipe which it is designed for, the coupling will stretch and as long as it is plastic to plastic, the band will hold it all together.
If you could put good support where you want to use that non-shielded coupling then even that would be ok. But, they can sag with differential forces exerted without support. Hence, the shield.

 

[Twowaxhack]

I use these for cast iron to plastic or plastic to plastic most of the time.

Not on combustion gas vents though. I don’t like the idea of having the pipe being squeezed. However it may be fine, I’ve never been in a situation where it came up.

 

[JG plumbing]

There are furnaces where a no hub band type of connection is used at the furnace itself. Boilers too.

 

[FishScreener]

The reduction at the discharge is probably to increase the velocity/decrease the lag time of the moist air as it transitions the cold zone, where it can condense.

You should run the condensate through a whole house filter filled with marble chips to neutralize it. The pH can be quite acidic, and if your on a septic system, can get the pH too low for the bacteria in a septic tank to survive.