PVC flue termination

**Mike Schulz** · 10-19-2008, 01:09 PM

Home had a 90 plus unit in the attic. The PVC flue was installed through the roof and finished off just like a plumbing vent stack. I wrote it up because the flue points straight up (pipe vertical). Reason being because foreign debris can enter the pipe. Is it required or just best practice to have installed a elbow on it to point it down toward the roof and have a grate installed in the opening? This was new construction and first time I have seen it vertical and open. I just sent the report off but figured I better arm myself for the call back about it.

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**Billy Stephens** · 10-19-2008, 01:49 PM

Originally Posted by Mike Schulz

Home had a 90 plus unit in the attic. The PVC flue was installed through the roof and finished off just like a plumbing vent stack. I wrote it up because the flue points straight up (pipe vertical). Reason being because foraging debris can enter the pipe. Is it required or just best practice to have installed a elbow on it to point it down toward the roof and have a grate installed in the opening? This was new construction and first time I have seen it vertical and open. I just sent the report off but figured I better arm myself for the call back about it.

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**Mike Schulz** · 10-19-2008, 02:41 PM

Well I'll be. You turn the supply air down and the flue vent can be straight up. It seems to me that the flue should also turn down for not the sake of water but leaves, pine needles etc. For example my oak trees have produce the most acrons this year then ever before. They have been dropping on my roof like hail for the last two weeks. If any got into the flue it seems it would cause problems. I don't have the flue but using it as a example.

**Bob Harper** · 10-20-2008, 06:48 AM

As with ALL appliances, refer to the listed instructions for that specific make and model appliance.

The general principle is that water will eventually get pumped out with the condensate. They apparently have not considered other obstructions as you mentioned. Once they get sued enough, they will make changes to their manuals. In the meantime, you can make practical recommendations such as cutting back overhanging trees.

If you look at the charts in the manul linked, you will see how a seemingly minor change adds considerable effect on the static pressure and therefore the overall vent layout. Just the using a hard 90* ell instead of a swept 90* subtracts 3 feet off the maximum vent length. Now, imagine how much reduction you'd get by putting mesh over the vent termination.

**DavidR** · 10-20-2008, 04:41 PM

Originally Posted by Bob Harper

As with ALL appliances, refer to the listed instructions for that specific make and model appliance.

The general principle is that water will eventually get pumped out with the condensate. They apparently have not considered other obstructions as you mentioned. Once they get sued enough, they will make changes to their manuals. In the meantime, you can make practical recommendations such as cutting back overhanging trees.

If you look at the charts in the manul linked, you will see how a seemingly minor change adds considerable effect on the static pressure and therefore the overall vent layout. Just the using a hard 90* ell instead of a swept 90* subtracts 3 feet off the maximum vent length. Now, imagine how much reduction you'd get by putting mesh over the vent termination.

There's only one problem with your post Bob, it makes too much sense.

You know we don't do things in the HVAC business to improve them.

If you don't go over the maximum length of piping you could put a different termination on it if acorns were a problem.

Haven't had that happen yet but I'm sure it's happened somewhere.

**Dan Hagman** · 10-22-2008, 09:24 AM

I have installed 100s of these and I never have gone through the roof. I always go out the gable end, side wall termination because who is going up on that roof in the winter when there is snow and ice on the roof when the vent is plugged over with ice,not me! First of all, if you add additional common sense features to the vent termination it can void the warantee on the furnace.
The direct vent system has both the vent and combustion air together and the in-direct vent termination has just the vent, so the combustion air is drawn from the basement. In order to have a in-direct vent termination you have to have a furnace that has a semi-sealed combustion heat exchanger like the Ruud or Rheem. The exhaust is always straight and the combustion air is next to it and in some cases turned down with an 90 elbow. On the roof you don't want to turn the exhaust vent down because if the chance of recirculating flue gas back into the combustion air intake. Bad idea ! See the manufacturers instruction. Will it work if you add your own common sense design, maybe, it can effect the pressure switch so I won't do it.
It dosen't matter that water runs down the exhaust because it will drain out with the condensate. Now you don't want water to go down the intake because that will just go into the furnace, unless a PVC "T" with a bottom drain has been installed close to the furnace to keep water from getting into the furnace. Not all manufactures require that.
This is not a code violation, the biggest problem around here in Iowa is not leaves or pine needles but birds and bees get into the vent in the summer and like to build nests. I always would leave the customer with a vent cap to put over the exhaust and intake for when the unit is not in operation and then they would just take it off when they were ready to use it in the fall. That would keep all the junk out and it will stay clean until they need it.
So.... if you want to recommend a solution, install a PVC 2" vent cap during the shut down period, thats why it is too bad it is on the roof. Mine were always down low from basement installations and the few we put in attic went out the gable end and not the roof. Just a choice I made. Never had any problems, ever!

Except when......... I installed a 90 % furnace and water started to gush and I mean gush into the furnace so I ran outside and a 5 year old boy had put there garden hose into the exhaust, filled the inducer fan and heat exchanger and inshot burners with water. His mom said she was sorry but so was I.

Dan
Rheem Dealer/Now Home Inspector

**Mike Schulz** · 10-22-2008, 11:56 AM

(On the roof you don't want to turn the exhaust vent down because if the chance of recirculating flue gas back into the combustion air intake. Bad idea ! )

The only thing terminated through the roof was the exhaust. The intake was just stubbed off the unit in the attic. The exhaust flue was 2" pipe with two elbows and about 6' in length.

**Dan Hagman** · 10-22-2008, 02:36 PM

Hey Mike, I must have missed that part and read through it to fast.

I would not have a problem suggesting a PVT "T" being installed on the exhaust pipe. A PVC 2" "T" would protect from other junk getting into the pipe and also would not be a concern of pressure drop that can effect the pressure switch like a 90 degree elbow or 2 elbows. On a in-direct vent application it is sugested in the manufacturers installation book to install a "T" at the termination of the vent 12" out from the house. I know some people use 2- 90 degree elbows and point it down. That could still work as long as the total length and allowed elbows were used.

I don't like the vents on the roof for all of these reasons.

Dan

**Markus Keller** · 10-23-2008, 06:51 AM

Some of the furnaces ship with perforated pvc termination screens that glue into a coupling at the end of the run.

**Dan Hagman** · 10-23-2008, 09:55 AM

Can't use a screen in cold climate as it will freeze over with ice because of the condensation in the flue gas. I have never seen a screen come with a termination kit, but that's with Rheem, that's not to say they don't. I have seen a baffle to be inserted into a 2" coupling at the beginning of the intake.

dcbritt · 11-01-2010, 08:38 PM

I put a furnace vent cap on my exposed PVC pipes and it is in my opinion the best solution. For under $10 you can get one from Welcome Save Pipey. It is very durable and only takes a second to install.

Fred Herndon · 11-04-2010, 08:14 PM

Originally Posted by dcbritt

I put a furnace vent cap on my exposed PVC pipes and it is in my opinion the best solution. For under $10 you can get one from Welcome Save Pipey. It is very durable and only takes a second to install.

Now that is slick. I found a plumbing vent clogged with acorns last year; when we emptied the whirlpool tub the toilet flushed. This is a perfect solution.

Back on topic, I completely agree with Mr. Hagman about not going through the roof. Around here they put a plastic plumbing vent boot on them, lasts about 6-8 years in this climate. Then water runs down the outside of the pipe directly into the furnace cabinet. The one in the picture was 9 years old. Luckily the HVAC contractor said damage was still superficial.

David Bell · 11-05-2010, 04:26 AM

I agree on sidewall terminations, they have less problems. I have done some thru the roof though, and have always installed a tee/wye on top. As for a pvc mesh, the only ones I have seen were meant to be installed on the furnace at intake opening.

**Ken Amelin** · 11-05-2010, 04:55 AM

CAUTION on ATTIC UNITS

Installing high efficiency condensing type furnaces in attics is not considered good practice is my part of the country. (freezing winter temps). The condensate trap on these units freeze up and break, creating leakage and interior water damage.

People put heat tape on the drain trap and piping, but this is just a bandage repair, as the heat tape doesn't last forever, it doesn't work in a power outage, or they can't get it wrapped around all exposures.