Thread: HVAC Venting

I was on a roof yesterday and this was the venting for the furnace. I cannot find the code call out for venting through the roof.

Ken

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Is that the black abs (pvc) or cast?
Assuming that is a 90+ install ...
My understanding, at least around here, is that the black abs isn't allowed only the actual white pvc. Don't know about CA
All the install guides I have seen rec/req either an elbow on a vertical termination or a drain tee at the bottom to allow the moisture that will get into the pipe to drain to somewhere besides into the furnace.
Jerry McC, your turf, CA

It is abs. The house was built 2001. It has the drain tee. I am just not sure about the abs.

ABS is not allowed for HVAC. Look at the manufacturer's installation manual. The install manual can be found on the net.

Thank you. That is what I thought.

Ken

Installation manual for the Lennox G26 furnace states that PVC, CPVC & ABS can be used for venting...

I have installed several 90+ gas furnaces with PVC pipe. Most everyone around here paints the PVC with flat black paint. You should make sure it's not painted before you call it out as ABS. Usually if the pipe does not have an elbow turned down there is a drain at the furnace, you will need to look inside the furnace cabinet where the PVC? pipe enters the unit.

Ken - that looks awfully small to vent a high efficiency gas furnace. To do that they would have to use a joint (or even two) that downsizes from the larger diameter at the unit to that one, kind of like a 4 to 2 to 1 exhaust pipe for a motorcycle (sorry, the analogy works for me!). That can cause pressure problems inside the unit and a possible break at the unit or in the plastic - CO can escape! Where was that downsizing done? And HOW? Where is the intake? And IF that vents a furnace, it should not point upward as rain water will get in. The installation guidelines recommend a horizontal exhaust with a slight downward pitch toward the outdoors to allow condensation to escape away from the unit.

Personally I have never seen ABS used to vent such a furnace, or water heater for that matter. Does it glue to PVC somewhere in the installation? Remember, the glue for PVC does not work on ABS and vice versa.

You should suggest that a qualified installer look at that - the whole thing looks potentially dangerous.

Trent - you can see writing on it so it is not painted pipe.

Also, Ken, no cricket on that chimney? I couldn't see much flashing...

Jay, the exhaust termination is typically 2", reduced at the furnace, using a reducer.
The horizontal exhaust should slope back toward the furnace, not toward the exterior.
The vertical termination should point straight up to allow rain to get in, and drain away with the condensate.

Some have suggested that the water running back to the system would be ok. I have to go along with the manufacture about not doing that. Sure there are lines to a condensation pump. But I am sure many of you have seen what I see, even on proper venting, that those lines leak and rust out the interior of the heating system cabinet. I would still advice the client to prevent that from happening by changing the vent pipe.

If a drain tee is installed properly any water/condensate will never make it back to the unit.
The pipe size is determined by the BTU of the unit. Typically, <90K gets 2", >100K gets 3". The install guide for that unit would specify.

Hey Neal! I have not noticed the reduction at the unit - always the same sized PVC exiting the unit and continuing to the outside. Around here anyway (Northern VA) they incline the lines toward the outside. Every HVAC guy I have asked, on the few occasions I have had opportunity, says the same thing. It is not uncommon to see icicles on the outside at the ends of the pipes! I have seen many T's as described by Markus, along with the clear tube to divert as much water as possible into the condensate pump, but I often also see rust inside the units. But, as we all know, I don't be no installer...!

Wow what a response. There is no size difference on the vent. There is a "tee" at the bottom of the installation only on one of the vents. I believe that was for water getting into the vent. I did not see a way for it to escape from there. There was a horizontal vent into the roof for escape of gases. I did suggest a HVAC professional not only for the venting but there were a few other issues with the system.

Thank you

Ken

Originally Posted by Jay Markanich

Around here anyway (Northern VA) they incline the lines toward the outside. Every HVAC guy I have asked, on the few occasions I have had opportunity, says the same thing.

Jay, instructions from an Amana Cat IV gas furnace installation manual.

"Horizontal runs of vent/flue piping must maintain a 1/4 inch per foot downward slope, back towards the furnace, to properly return condensate back to the furnace's drain system."

This one also says ABS is acceptable.

Hey Neal - That's good info! Do they say anything there about vertical discharges through the roof?

Jay, didn't read that part, but here's a source for all Goodman and Amana installation manuals.

Furnace Info

Thanks Neal - you da man! I will put that in my furnace info file. Gracias hombre!

Thinking further about it, I don't think I have ever seen an Amana or Goodman condensing furnace. I see a lot of Lennox's and Carriers though.

It doesn't really matter what the manufacturer's manual says about material, only the AHJ. If that house was built in 2001 then ABS would have been fine around here. Now ABS is not allowed.

From the photo, it looks fine. But this doesn't show the interior piping. One special note: most mfrs. state the venting must be air-tight. When consulted, the usual response is that means cemented. I find a lot of furnace venting where one or more joints will be friction tight, but not cemented. That can lead to problems if the joint comes apart due to vibration or something else.