Insulation on Pressure flue in attic?

[Rob Colecchi]

Have not seen this one before, but does anyone know why a pressure flue in the attic would be insualted?

[Scott Patterson]

Have not seen this one before, but does anyone know why a pressure flue in the attic would be insualted?

I don't know what a "pressure flue" is or I have never heard it called that, are you referring about a flue pipe from a CAT IV furnace?

[Rob Colecchi]

Scott,

Yes a CAT IV flue pipe.

[Scott Patterson]

Scott,

Yes a CAT IV flue pipe.

My best SWAG is that it was insulated in hopes to reduce the amount of condensation running back down the pipe. I have no clue other than that as to why.

[Rob Colecchi]

Scott,

Thanks for your input.

[David Bell]

Not to reduce condensation but, to reduce the risk of it freezing and creating a blockage. Many manufacturers require insulating horizontal runs in attics.

[Michael Thomas]

Both freezing and condensation are concerns which may dictate insulation, usually the manufacturer's wording is something like:

"Failure to follow this caution may result in water damage from condensate dripping. When combustion-air pipe is installed above a suspended ceiling, pipe must be insulated with 3/8-in. thick Armaflex-type insulation.Combustion-air pipe should also be insulated when it passes through a warm, humid space."

And

"When vent pipe is exposed to temperatures below freezing, such as when it passes through an unheated space or when a chimney is used as a raceway, pipe must be insulated as shown in Table "X" with Armaflex-type insulation."

where table "X" lists the allowable vent lengths with and without insulation.

Usually, this requirement is not restricted to horizontal pipe.

[Kary Krismer]

When it comes to freezing, I don't see the point of insulating an intake pipe. How often is the intake air warmer than the air in the attic? What are you insulating from what?

I could see it might make a difference in condensation.

[Michael Thomas]

The concern is that frozen condensation will eventually block the vent. in which case the furnace will shut itself down.

[Kary Krismer]

The concern is that frozen condensation will eventually block the vent. in which case the furnace will shut itself down.

But how is insulating it going to help prevent the freezing? Again the air in the attic is probably warmer than the air being sucked through the pipe, so insulating it will increase the potential for freezing.

Now if you had the intake and exhaust wrapped together and insulated, that could help.

[David Bell]

Insulating the intake prevents it from condensing on the outside. A cold outdoor temp and a warmer attic space can cause the intake to sweat just like a cold water pipe in a basement.

[Kary Krismer]

Insulating the intake prevents it from condensing on the outside. A cold outdoor temp and a warmer attic space can cause the intake to sweat just like a cold water pipe in a basement.

Right, I said I thought it would help with the condensation. I guess it helps prevent the freezing by preventing the condensation. Without water there's nothing to freeze?

[Rich Goeken]

I had a system like this. It was exiting from the basement and needed to go up a few feet with 90's to provide the proper clearence from the ground to prevent snow blockage due to drifting. Had to insulate the pipes outside the home due to ice buildup and shutdown the first year.