Do I seal my attic with a furnace in it?

[gillstone]

We have a 1950's tri-level with a quirk of having the furnace in the attic space above the kitchen/dining room.  I want to seal the attic space and am working on some drawings of a hatch to pull down when I need to seal the space.  What I'm concerned about is that with the furnace in the attic, will it have enough airflow to continue workign properly if I seal the space.  Right now it has two air intake vents in the house and an intake going through the roof and it has air coming from the opening to the attic in the utility closet (vent on door).  If I sealed the attic, it would remove the airflow that comes from the vent on the utility closet door.  Its a Bryant 92% efficient forced air natural gas furnace. 

On a related note, if I can't rig something that lets me seal the space, should I even bother going around my attic and sealing all the lightfixtures etc...

[Greg]

The furnace must have a source of combustion air, period.

Even if you don't seal off the attic, air sealing prevents warm air escaping into the attic and cold drafts coming in the other way.

[Exflyboy]

92% AFHU efficiency I "believe" is a condensing furnace.. (so should have a small drain tube going outside. If so it will almost certainly have a schedule 40 PVC intake tube coming in from the roof for the combustion air and the same for the exhaust.. Mine are 2".

If this is the case then by all means you can seal up the attic.

If it does not have piped in combustion air then you need a seperate source and your local building code will tell you how many sq inches of opening you need per say 100k BTUs.. This will need to come from the outside usually so if you have a dedicated opening to the attic for the purpose then there is no problem with sealing the attic as long as you don't block the dedicated hole.

Frank

[gillstone]

Half the time I type these I end up realizing a solution either just before or just after I post.  It is a condensing furnace and it does have intake through the roof and a drain tube.  What struck me once I thought of the hassle of sealing it but having adequate access (I have to climb a ladder while squeezing between my water heater and its exhaust pipe without stepping on the heater or its water or gas supply lines) was that I could seal around the furnace so the rest of the attic is sealed off but the furnace and the small space above is still open and no potential combustion air sources are compromised. 

The catch is that there aren't studs around that area and its a tight fit. Could I nail up some strips of OSB to attach panels to and then seal the seams or do I need to install studs and a panel (OSB) to put insulation against.

Its pain in the ass to get up there, let alone get materials and tools up there) so I want to do this once and not have to worry about it again.

[Exflyboy]

If it has a 2" schedule 40 PVC intake air that IS the combustion air source so yes you could box the thing in.

Remember though that the heater will be rated as to how close it can be installed next to combustibles.. wood IS a combustible and I think most inspectors consider sheetrock to be a combustible if it is over wood studs, even though in theory it isn't.

You will need to find details on the heater to understand how close it can be installed to combustibles... My furnace is rated as a zero clearance on the back and sides.. Some furnances are not rated and I believe (last time I looked at residential code was 10 years ago) that means the closest combustible clearance is 6 inches minimum.

If you take this into account in building your box you will have no issues.

Frank

[Greg]

Good that it's got intake air.  Be sure to leave enough room around it so that it can be serviced as needed.

[gillstone]

Thank you! I 'm getting a better idea of what I can put in and where.  I checked and I have an inch of clearance for combustables according to manual by setting air seal wall 9 inches from back of furnace and providing an access panel, I should be more than fine by 2009 IFGC. 

[Exflyboy]

Good there is a small (but pretty unlikely) chance the Fuel gas code has not been adopted by your AHJ (Authority having Juristiction).. Namely your building code dept.

But it is extremely unlikely and if the heater has been constructed to 2009 FGC standards then its almost certainly OK to go by that.

Frank