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Corey Degetaire Posted: Dec 09, 2009 09:51 PM	Sealing an attic with a gas furnace If an attic is retrofit with foam, sealing off the roof, gable, and soffit vents, creating an unvented attic, and the hvac unit uses gas for a source of heating. Does the hvac unit need to have a fresh air intake to make up for the air used when the unit heats? Or will there be enough air exchange from daily in and out of the house?
mason Posted: Dec 10, 2009 08:04 AM	You cannot use foam plastic in an attic that uses the air in the space (known as a plenum) in HVAC equipment unless it has a flame spread of 25 or less and smoke developed rating of 50 or less. I am not aware of any sprayfoam that matches these requirements. So in a attic space containing HVAC equipment you would need to specifically pipe in fresh air from outside to the HVAC units when using sprayfoam.
Corey Degetaire Posted: Dec 10, 2009 12:31 PM	thanks for confirming that for me
John Shockney Posted: Dec 28, 2009 01:12 PM	Any gas fired heating equpment needs/is required to have a combustion air supply. As a HVAC contractor I would requmend the use of a +90% gas furnice due to the sealed combustion chamber and the provision for an intake air supply. If you are not using a furnice with a sealed combustion chamber you will have to enclose the furnice into a sealed drywalled room with an outside combustion air supply sized to the furnice to meet NAFP code. cheek with a good local HVAC contractor hope I was helpfull
Robert Jordan Posted: May 16, 2010 08:15 PM	I am confused by your terminology. The plenum, as far as I know, is the ductwork immediately attached to the furnace. The concern with a furnace in an unvented attic is to be sure there is enough combustion air so the furnace has adequate air for combustion and drafts properly. In cases where there has been a furnace in the attic, I have put a grill in the pull down stairs to insure there is enough air getting into the house. The house was still leaky enough that there would be sufficient infiltration. If I understand your post correctly, this is not acceptable because of foam safety concerns.
mason Posted: May 17, 2010 09:58 AM	A plenum is the source of combustion air. If the air is not piped in directly to the furnace, the attic space would be considered the plenum. If you pipe the air in directly to the furnace and it does not use attic space air, then you can use sprayfoam.
John Shockney Posted: May 17, 2010 10:47 AM	I think that the confusion is how the word “plenum” is used, a plenum is any air distribution “box” (for lack of a better word) you can have a return air plenum, a supply air plenum, or combustion air plenum. The problem that you run into today is that code requires that combustion air be supplied from an outside source that is not connected to the living space. This is due to the possibility of back drafting and CO into the living space. If you follow code fully and install any heating equipment that doesn’t have a sealed combustion chamber it is required to be in an isolated room separated from the living space by a 30min fire wall and exterior grade door. And I know that the old way was to install the furnace in the hall closet and cut a vent in the door for combustion air and I’ve seen a lot of them done that way (replaced a lot with 90% furnaces too) but this does not meet code due to the CO danger and the fire danger. Hope this helps Airpro heating and air conditioning Airpro Spray Foam
Robert Jordan Posted: May 19, 2010 02:40 PM	There are many natural draft furnaces installed in attics and basements that are not connected to the outside for an air source. I suppose a furnace in an attic is connected to the outside in typical construction because of soffit vents. But Mason is also saying that there is a problem with foam and natural draft furnaces. So basically, we should not be sealing attics that have gas furnaces? Is there a code reference to show builders if this arises? Thanks
John Shockney Posted: May 19, 2010 04:42 PM	If the furnace is installed in a vented attic then it will get the needed combustion air from outside this would be ok, as long as combustible materials are kept required distances from the furnace. Seeing as most basements are sealed from outside air they do require combustion air vents and cannot pull combustion air from the living space. I have seen new houses where this was not provided and caused a lot of problems like: back drafting, CO poisoning, soot plugging the heat exchanger, burnt up heat exchangers, and burnt up burner assemblies. The National Fire Prevention Code under gas appliances has this requirement spelled out in detail including how many square inches of free air that is required per BTU, please note this also applies to all gas appliances like furnaces, water heaters, and gas fire places.

Author

Comments

Corey Degetaire

Posted: Dec 09, 2009 09:51 PM

Sealing an attic with a gas furnace

If an attic is retrofit with foam, sealing off the roof, gable, and soffit vents, creating an unvented attic, and the hvac unit uses gas for a source of heating. Does the hvac unit need to have a fresh air intake to make up for the air used when the unit heats? Or will there be enough air exchange from daily in and out of the house?

mason

Posted: Dec 10, 2009 08:04 AM

You cannot use foam plastic in an attic that uses the air in the space (known as a plenum) in HVAC equipment unless it has a flame spread of 25 or less and smoke developed rating of 50 or less. I am not aware of any sprayfoam that matches these requirements. So in a attic space containing HVAC equipment you would need to specifically pipe in fresh air from outside to the HVAC units when using sprayfoam.

Corey Degetaire

Posted: Dec 10, 2009 12:31 PM

thanks for confirming that for me

John Shockney

Posted: Dec 28, 2009 01:12 PM

Any gas fired heating equpment needs/is required to have a combustion air supply. As a HVAC contractor I would requmend the use of a +90% gas furnice due to the sealed combustion chamber and the provision for an intake air supply.

If you are not using a furnice with a sealed combustion chamber you will have to enclose the furnice into a sealed drywalled room with an outside combustion air supply sized to the furnice to meet NAFP code.

cheek with a good local HVAC contractor

hope I was helpfull

Robert Jordan

Posted: May 16, 2010 08:15 PM

I am confused by your terminology. The plenum, as far as I know, is the ductwork immediately attached to the furnace. The concern with a furnace in an unvented attic is to be sure there is enough combustion air so the furnace has adequate air for combustion and drafts properly. In cases where there has been a furnace in the attic, I have put a grill in the pull down stairs to insure there is enough air getting into the house. The house was still leaky enough that there would be sufficient infiltration.

If I understand your post correctly, this is not acceptable because of foam safety concerns.

mason

Posted: May 17, 2010 09:58 AM

A plenum is the source of combustion air. If the air is not piped in directly to the furnace, the attic space would be considered the plenum. If you pipe the air in directly to the furnace and it does not use attic space air, then you can use sprayfoam.

John Shockney

Posted: May 17, 2010 10:47 AM

I think that the confusion is how the word “plenum” is used, a plenum is any air distribution “box” (for lack of a better word) you can have a return air plenum, a supply air plenum, or combustion air plenum.

The problem that you run into today is that code requires that combustion air be supplied from an outside source that is not connected to the living space. This is due to the possibility of back drafting and CO into the living space. If you follow code fully and install any heating equipment that doesn’t have a sealed combustion chamber it is required to be in an isolated room separated from the living space by a 30min fire wall and exterior grade door.

And I know that the old way was to install the furnace in the hall closet and cut a vent in the door for combustion air and I’ve seen a lot of them done that way (replaced a lot with 90% furnaces too) but this does not meet code due to the CO danger and the fire danger.

Hope this helps
Airpro heating and air conditioning
Airpro Spray Foam

Robert Jordan

Posted: May 19, 2010 02:40 PM

There are many natural draft furnaces installed in attics and basements that are not connected to the outside for an air source. I suppose a furnace in an attic is connected to the outside in typical construction because of soffit vents. But Mason is also saying that there is a problem with foam and natural draft furnaces. So basically, we should not be sealing attics that have gas furnaces? Is there a code reference to show builders if this arises?
Thanks

John Shockney

Posted: May 19, 2010 04:42 PM

If the furnace is installed in a vented attic then it will get the needed combustion air from outside this would be ok, as long as combustible materials are kept required distances from the furnace.

Seeing as most basements are sealed from outside air they do require combustion air vents and cannot pull combustion air from the living space. I have seen new houses where this was not provided and caused a lot of problems like: back drafting, CO poisoning, soot plugging the heat exchanger, burnt up heat exchangers, and burnt up burner assemblies.

The National Fire Prevention Code under gas appliances has this requirement spelled out in detail including how many square inches of free air that is required per BTU, please note this also applies to all gas appliances like furnaces, water heaters, and gas fire places.

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