Insulating PVC furnace vent pipe

Bob - MI · Oct 3, 2009

Had a new 96% furnace installed last year and the installers stopped the exhaust vent flush with the block wall on the back of the house. This is under the deck on the house and we get fumes rolling up through the deck and in the winter we get ice buildup on the bottom side due to condensation.

I want to extend the exhaust out past the deck (12 feet) and need to insulate the pipe. I am using 2" pvc which is what the installers have on it now.

Any good ideas? I had hoped to be able to find closed cell pre-formed wrap but so far am drawing a blank. I am guessing that wrapped fiberglass may get soaked through eventually.

Thanks in advance.

sheiserman · Oct 3, 2009

Would it be possible to route the exhaust/intake to another spot? There are limits to how far you can run the piping, so you"ll need to make sure you can add a 12" run. This will affect your warranty, and will be covered in the installation guide.

If you add the run, PVC needs to be supported every 4", and there"s no reason to insulate plastic pipe. If you decide you"re going to insulate it, exterior insulation is normally wrapped in an aluminum or stainless jacket.

My short answer would be to move the piping to a different spot. You may have to go to 2 1/2" or 3" PVC (exhaust)to to lengthen the run.

sheiserman · Oct 3, 2009

Need to add the exhaust should have adequate fall (drainage) back to the furnace so the ice build up isn't such a problem.

old · Oct 3, 2009

I know of 2 or 3 ways to do it. They make a foam pipe wrap that would work of you could take an put say a 4 inch pipe around that 2 inch and then fill it with that spray foam and that stuff will stop all the water problems plus do the rest of the stuff

Gun guru · Oct 3, 2009

When my 96% eff. furnace was installed 10 years ago the guy used 3" PVC and when it goes through the siding of the house it has a 90 degree elbow on it facing down, and some ice forms on it in winter but not enough to choke off the air exhaust flow.

showcrop · Oct 3, 2009

you should check to make sure that 2" will be big enough adding that much length, you probably should go larger.

Lamar Fudd · Oct 3, 2009

I hope he reads your post. You"re exactly right-no need to insulate- and for sure run the pipe so moisture runs back to the furnace. I have a "tee" fitting on the end of mine so a breeze won"t affect the draft. Maybe that is why he"s getting moisture buildup. Or his piping could be running downhill towards the outside, a definite no-no.

El Toro · Oct 3, 2009

I have a 90 plus furnace that has 3.0" pvc pipe installed on both the intake and the exhaust and its reduced to 2.0" at the furnace and where it
exit's & enter's the house. Its a Ruud and I had a problem with the furnace starting when the temp got to about -20F. That doesn't happen too often here in NE MD, but it did in 1994. Rudd told the installer to install a kit that would warm the intake air. Never had a problem after that was installed. I would need to read the owner's manual to get the kit number. You should keep a spare ignition ignitor on hand just in case too. They're not that hard to install if your furnace has one. They usually fail on the coldest night and on Sunday. Hal

guido · Oct 3, 2009

Hello Bob-MI,
Here is one setup on the pipe termination outside the house. About 12" from the house.
No help for your long run, but a good way to the ouside routing end.
Guido.

Lamar Fudd · Oct 3, 2009

Guido, that was what I was getting at in my reply to scheiserman. I took two half inch long pieces of the tube and split them for clearance so I could put a piece of screen wire in the top and bottom of the tee to keep out insects. Works real well.

don t-9n180179 · Oct 4, 2009

Is it possible to re-route the pipe inside to a better exit location? Granted, it may be more work, but might be a better overall option.
HTH..good luck...don t. ....

Bob - MI · Oct 4, 2009

Thanks for the tips guys.

I got the installation instructions out and although the 2" extension will be within the allowable overall length, I discovered a few installation mistakes that the HVAC guy made. I have them coming out for a checkup so I will see if they can't make the mod. while they are making the corrections.

Always great information available here.

Thanks again

Bob

bc · Oct 4, 2009

If your checkup is free and the mistakes are warranty work, then look for the price they charge for the mods to cover the entire trip to your house. But then if you do the mods yourself, they will say they aren't covered under the warranty. Catch 22.

T_Bone · Oct 4, 2009

Hi Bob,

Code for venting any type of residential combustion appliance is very simply stated:

"All vents shall be 2ft above and 10ft horizontal away of any structure opening."

This is too keep deadly CO from entering the residence.

Whether your furnace requires a powered vent kit to meet the above code, is determined by the unit mfg.

If you were not given a copy of the mfg. stated installation requirements, I'd call the mfg. and get a copy.

T_Bone

bc · Oct 4, 2009

Hey T-bone. Thanks. If I have a propane water heater vent coming out of the basement above the foundation on my MIL's house. Been that way for 50 years. I take it that it needs to be over 10' horizontally from a window since it can't be above the window without running a stack through the kitchen.

The vent is at the southeast corner of the house and close to the kitchen windows on the east side which are 3 feet from the corner. If I stick the vent out the south side at the southeast corner, it would be 10' (I think) from the south side windows. However moving the vent a couple feet around the corner would still leave it within 5 feet of the windows on the east side but at least they are around the corner. Is that kosher? I may just leave well enough alone to since the windows are rarely opened.

T_Bone · Oct 4, 2009

Hi bc,

If that window is a "fixed" window, meaning it doesn't open, as in a sheet of glass, then that window is not part of the consideration, and the vent would only have to be 2ft above the next 10ft horizontal obstruction. The side of the structure it's self then becomes the consideration.

If the window can be opened, then it's part of the consideration.

Example:

On a pitched roof, why does a vent have too terminate 2ft above any obstruction 10ft horizontal away?

If the vent is lower, then a negative pressure air flow can enter the vent stack, thus reversing the vent flow back into the structure, and that is caused from the air currents created by the slope of the roof.

I can tell you, that venting is the number cause of code failure upon inspection.

T_Bone

Guido. · Oct 4, 2009

Hello Lamar,
It works as you stated. Sometimes a picture
Helps.
Have good day............Guido.

bc · Oct 5, 2009

Thanks T-Bone. The windows will open and are on the east side of the house 3 feet from the corner. So does the 10' horizontal include going around the corner of the house or can I go around the corner of the house to the south side as long as I'm still 10' from any opening on the new/south side? The south side has a window about 12' from the corner.

This is an old farm house built around 1911 and has been vented this way since water heaters were invented. Probably been out of code since the water heater was installed but so far everyone has survived. The windows are rarely opened since central air and heat was installed. The present vent is 3" pipe with an elbow pointed down. At least the mud daubers and bugs avoid it. I'll look for a place or a wall to run something up through the ceiling depending upon how long of interior horizontal vent runs I can make. But this will have to continue to work for a while. We are installing new vinyl siding so if I can move it and be in code then I will do so.

old1850d · Oct 5, 2009

Bob if the length of the 2" exhaust is within manufacture specs go to a wholesaler and get the black foam rubber insulation and slide it over the top of the pipe, they come in 6' lengths and has a brand name of Armaflex. I use this all the time when I vent a 90 plus furnace thru a attic. If the furnace is a two pipe you need to keep the lenths fairly close and its more important to insulate the intake tha the exhuast

janjenny83 · Sep 16, 2010

Yes it is really important to insulate the intake and the exhaust pipe. The vent should be enough and the pipe diameter should be bigger.

Insulating PVC furnace vent pipe

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