# Furnace in crawlspace, how do I insulate and supply combustion air?



## WillK (Aug 29, 2010)

Well, the subject probably says it all, but here's what I'm starting with in more detail:

I have an 80% efficient gas furnace in my crawlspace, and at present it has a heat register into the crawlspace on the plenum. My ducts are probably 50% flexible insulated duct and 50% uninsulated metal duct pipes.

The crawlspace is 100% uninsulated. It is enclosed by wooden sheething most of which is rotted. This is covered by brick veneer.

The floor is dirt. I eventually plan to cover with plastic sheet, but that is not under consideration until I'm done digging and pouring footings.

I am considering putting up foam at the walls, but before I do that I need a strategy for supplying combustion air to the furnace. Can I box the furnace in and vent the inside of the box to the outside with PVC or something like that? Is there a better way?


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## AllanJ (Nov 24, 2007)

Yes you can box the furnace in and run a duct from one of the exterior walls. This will allow you to make most of the crawl space insulated space.

You will have to see the International Building Code to find out the needed clearance around the furnace for fire protection and maintenance, and the cross section area of the duct needed.


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## gregzoll (Dec 25, 2006)

I would not state that it is 100% uninsulated space. A 100% uninsulated space, would be a open pit fire trying to force the radiant heat into your home.

Now the big question is, what is keeping you from moving the furnace to either the attic or a closet, along with getting a new 94% unit, or even a all-in-one unit.


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## Marty S. (Oct 31, 2009)

Will that supply register in the crawl space serves 2 functions...1 it supplies the needed air for combustion...2 makes the area a conditioned space so the ducts do not have to be insulated. You can box in the furnace and run a duct from outside if you want but there's no real need.


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## WillK (Aug 29, 2010)

gregzoll said:


> I would not state that it is 100% uninsulated space. A 100% uninsulated space, would be a open pit fire trying to force the radiant heat into your home.


True enough that I might get an R3 from the crawlspace enclosure and it stays warm enough for working, I'm suspecting that at this point my greatest heat loss source is the crawlspace. I have at least 2 vents that don't close, and there are cracks here and there.



> Now the big question is, what is keeping you from moving the furnace to either the attic or a closet, along with getting a new 94% unit, or even a all-in-one unit.


It is definitely in the long-term plan, but definiteily not in the budget this year. The crawlspace should be insulated regardless, and even if I do this ahead of the foundation being poured, I'm assuming that the 2" foam I'd put in place can be reused at that point.


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## WillK (Aug 29, 2010)

Marty S. said:


> Will that supply register in the crawl space serves 2 functions...1 it supplies the needed air for combustion...2 makes the area a conditioned space so the ducts do not have to be insulated. You can box in the furnace and run a duct from outside if you want but there's no real need.


So it's an interesting point to consider, strictly speaking though I don't think that a supply register can act as combustion supply since ultimately doing so would mean pulling air from the house conditioned space, so either make-up air would have to draw through other leaks in the house or through a combustion vent, pulling CO2 into the house. Combustion supply air needs to come from outside, and currently it draws from vents in the crawlspace.

So what this does remind me is that I would need to ensure that when I box the furnace, the box would have to allow the plenum to pass through rather than enclosing the plenum inside the box. The crawlspace register would be outside the box, and if it was inside the box there would need to be another outside to condition the crawlspace to prevent water pipe freezing and heat loss from uninsulated ducts.


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