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Can high efficiency pvc furnace exhaust and intake be insulated?

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Can high efficiency pvc furnace exhaust and intake be insulated?  I would think the R-value of pvc is next to zero and that there would be a considerable amount of heat loss transfer into the house from these pipes.  Offten times these pipes run fifty feet or more through a house.  So I was wondering if it is possible to wrap the pipes with insulation or if there is an insulation designed to fit over the pvc as it is being istalled?
asked in Heating and AC by ryan (120 points)
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1 Answer

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Hi Ryan,

Yes, the high efficiency furnace PVC vent / exhaust pipes can be insulated. There are certain installations and location conditions (climate related) in which they have to be insulated (always check with the appliance manufacturer for details. Insulating the PVC air intake pipe is not required under any conditions because its only purpose is to provide combustion air for the appliance’s burning process.

You can find the following paragraph in the PVC vent pipe installation manual of various high efficiency / condensing type furnace manufacturers:

When the vent pipe is exposed to temperatures below freezing, i.e., when it passes through unheated spaces, etc., the pipe must be insulated with 1/2 inch (22.7 mm) thick Armaflex-type insulation or equal. If the space is heated sufficiently to prevent freezing, then the insulation would not be required.

Some other examples of areas where the PVC vent pipe insulation might be required (cold and extreme cold climates of course) are unconditioned attics / garages / crawlspaces, extended runs on the outside walls of the house, or sections above the roof.

The reason behind the insulation requirement is to prevent freezing of the condensate flowing inside the PVC vent pipe towards the furnace and its drainage system. Therefore, insulating vent pipes installed / running inside the conditioned (heated) area does not increase the efficiency of your appliance. It is actually better to have the vent pipe exposed (e.g. in your basement) so it radiates some heat into the surrounding space.

Armaflex R value (1/2” thick foam) is slightly above 2 and on a longer runs it gets quite expensive to insulate a large diameter vent pipe, therefore, you may want to consider using some fiberglass insulation instead.

Let me know if you have more questions, just click "comment" below my answer.

answered by darekrudy (21,730 points)