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Corey Degetaire Posted: Oct 09, 2008 03:00 PM	Gas furnace in unvented attic? When spraying under a roofdeck w/ cc spf is it ok to create a nonvented attic in an existing home if the heating unit is supplied by natural gas and located in the attic? How much ventalation is required if this is possible? My concern is the pilot light going out and the return duct pulling the gas back into the house, or having a leaky co2 vent and having that pulled into the house as well.
Eric Smith Posted: Oct 09, 2008 08:11 PM	Several things to consider. You will will need an approved ignition barrier over that foam if the area you are spraying is accessible for HVAC systems. You better have very well sealed ducts to prevent scavenge from the HVAC system. Most HVAC systems have sensors that shut off the gas if the pilot goes out. I would be more concerned with CO issues than natural gas. If you closed up that space I would do a worst case furnace evaluation for spillage and draft in the exhaust vent. That will tell you what you need to add as far as venting. If the attic space is going to be considered conditioned, you likely have enough make up air infiltration to run the unit even through a sealed roof.
mason Posted: Oct 09, 2008 09:14 PM	Ditto, good advice Blackhatch.
Corey Degetaire Posted: Oct 09, 2008 09:26 PM	What application of foam is generally recommended for these senario's with the gas furnace located in the attic? Is it better just to spray above the ceiling and encapsulate the duck work, around ceiling vents and the air handler? Rather than creating a nonvented attic. Thanks for the previous advice.
mason Posted: Oct 10, 2008 02:47 PM	You can do either, just make sure the furnace has its own make up air. Do not use the attic space as a plenum.

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Corey Degetaire

Posted: Oct 09, 2008 03:00 PM

Gas furnace in unvented attic?

When spraying under a roofdeck w/ cc spf is it ok to create a nonvented attic in an existing home if the heating unit is supplied by natural gas and located in the attic? How much ventalation is required if this is possible? My concern is the pilot light going out and the return duct pulling the gas back into the house, or having a leaky co2 vent and having that pulled into the house as well.

Eric Smith

Posted: Oct 09, 2008 08:11 PM

Several things to consider.

You will will need an approved ignition barrier over that foam if the area you are spraying is accessible for HVAC systems.

You better have very well sealed ducts to prevent scavenge from the HVAC system.

Most HVAC systems have sensors that shut off the gas if the pilot goes out. I would be more concerned with CO issues than natural gas.

If you closed up that space I would do a worst case furnace evaluation for spillage and draft in the exhaust vent. That will tell you what you need to add as far as venting. If the attic space is going to be considered conditioned, you likely have enough make up air infiltration to run the unit even through a sealed roof.

mason

Posted: Oct 09, 2008 09:14 PM

Ditto, good advice Blackhatch.

Corey Degetaire

Posted: Oct 09, 2008 09:26 PM

What application of foam is generally recommended for these senario's with the gas furnace located in the attic? Is it better just to spray above the ceiling and encapsulate the duck work, around ceiling vents and the air handler? Rather than creating a nonvented attic.

Thanks for the previous advice.

mason

Posted: Oct 10, 2008 02:47 PM

You can do either, just make sure the furnace has its own make up air. Do not use the attic space as a plenum.

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