Thread: PVC flue reduction fitting

Hello all.
I'm Murph in the Charlotte area of North Carolina.

I saw a PVC flue yesterday from a high efficiency furnace in a crawl space that was reduced down from 2" to 1 1/2 ". A reduction fitting was used just inside the foundation wall as it exited the crawl space. So far I hadn't had a chance to look at code or the manufacturer's instructions. Does anyone know of a code compliance problem? It seems to me that the induced fan may have a problem and CO could build up. This is new construction and of course the builder quickly informed me "it passed the code inspection so there is no problem!"

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Reduction in size is usually not good and I doubt that the manufacturer would approve it, but with a HE system you never know for sure. They are all a little different depending on the manufacturer. You need to look up the install guidelines at the manufacturers website or call their technical desk.

Don,

I was at a South Face Seminar on High Performance Homes and HVAC Systems yesterday.In attendance was Building,Fire Code inspectors,Architects,Engineers,AIA
Members and RESNET certified raters. HI's and HVAC installers,Locale utility, TVA
officials and counterparts from another state.

I asked your question and was told not Allowed. No reduction is ever allowed.

Doesn't mean something New might come along and be allowed.

Styrofoam is not allowed as supports for furnace.

If anyone has different info please advise.

The high efficiency furnaces must activate several limit switches prior to allowing the furnace to operate (unless they have been bypassed). Too much back pressure and the furnace will not allow the gas valve to open as it assumes that the vent is plugged.

The manufacturer states in the installation manual the proper size of the vent pipe (verticle run, horizontal run, reductions for 90's, 45's, etc.)

Could be that the furnace could be installed with 1 1/2", but around here they are usually 2". I would at least refer the buyer to the installation guide and most likely a HVAC tech eval.

Thanks to all who responded.

I did some research as well. The Furnace is a Lennox G51MP-70 series. I pulled the installation manual and found that on this furnace it is supposed to be reduced from 2" to 1 1/2 " at the point of exit to the exterior. I also spoke with a HVAC tech who said that this is typical for this model of Lennox. Evidently the pressure is required for proper operation of the unit. Lots of techy talk involved here. Thanks again!

Don,

Thanks. Looks like as Scott was saying Manufacturing Install Instructions always not
blanket Statements.

Now if I could get this Styrofoam as furnace supports allowed or not clear in my wee brain.