# Installing furnace in a closet



## djlazar (Jan 23, 2010)

I am going to install a furnace in a closet. The specs for the furnace call for at least 580 square inches of filter. The common way I've seen of putting a furnace in a closet is to build a boxed-in little stand for the furnace and put a vent at the bottom for air intake with the filter at the bottom of the furnace. The furnace is only 14X28 so the filter would be less than 400 square inches. In one .pdf file I've found, 
http://www.alpinehomeair.com/_viewresource.cfm/ID/150/p/Selecting and Designing Duct Systems
about halfway through the article it shows cutting in a filter into the side of the furnace and putting some vents on the closet door, using the furnace closet as a return air plenum. I've never seen this done but apparently it is or at one time was acceptable. Is there anything wrong with this setup? I'm looking for a common sense answer please, not a quote from the code book.


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## beenthere (Oct 11, 2008)

Common sense answer is: That is a code violation for the type of furnace they show.

Don't look for an answer that says. "Hey, as long as your only killing yourself and your own family go a head".

What type of furnace are you installing. 80% unsealed combustion. 80 sealed combustion. 90% unsealed combustion, or 90% sealed combustion.

Post brand and model number if you don't know what it is.


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## djlazar (Jan 23, 2010)

Goodman GMH80703AN


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## beenthere (Oct 11, 2008)

Thats an unsealed combustion unit.

Can't have return in closet with it.

Return MUST be ducted from the occupied/living space to the furnace.


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## Red Squirrel (Jun 29, 2009)

I would not do it, just because it would be a nightmare to try servicing that in the future if it breaks.


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## djlazar (Jan 23, 2010)

So what would be the effect of doing it like it shows in that link I posted? If the byproducts of the burning methane are piped up through the roof, isn't the only thing going on in the furnace closet air intake for the blower and the combustion?


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## beenthere (Oct 11, 2008)

The closet will be in a negative pressure. And pull the products of combustion back out of the furnace.
Many of us have seen it too often.

And that link you posted, is a bad example of the way to do many things.

I'm afraid in their zest to keep sells volume up. They are posting inaccurate, and some what dangerous info. that you and many other unsuspecting people would follow. And cause harm to themselves, and their families.

That furnace requires a negative draft system. With the blower drawing air from the closet, the flue pipe would not longer be in a negative draft.

It is a code violation. And is cause for great personal harm.

And I have removed the link because it does show unsafe methods.


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## djlazar (Jan 23, 2010)

So back to installing the filter at the bottom of the furnace. I'm pretty sure this is common practice, correct? The furnace in my house (the other is a rental property) has this setup and doesn't seem starved for air at all.


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## beenthere (Oct 11, 2008)

Yes. The filter at the bottom of the furnace is common.
Make sure it is installed either inside the furnace cabinet. Or, is installed with a tight filter door.

On that furnce, in heating mode, you can set the CFM between 850 and 950 CFM and still be well within safe temp rise. 

A 12X26 air filter at 950 CFM would give you a FPM of less then 450. Which is ok.
A 12X26 air filter at 850 CFM would give you a FPM of less then 310. Which also good.


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## Red Squirrel (Jun 29, 2009)

beenthere said:


> The closet will be in a negative pressure. And pull the products of combustion back out of the furnace.
> Many of us have seen it too often.
> 
> And that link you posted, is a bad example of the way to do many things.
> ...


I had not looked at it, but isn't there always two pipes, one for incoming air and one for the exhaust? Or do furnaces still need ambient air to combust properly? 

Come to think of it, how is it that HW heater flu's work if there is no air coming back to replace what is leaving? I'm just thinking of that since mine has no blower, so it's just using physics (hot air rises) to leave the house.


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## beenthere (Oct 11, 2008)

Building leakage. is what is relied on, for most nat draft water heaters. but, if no fresh air can enter the building. Then you get CO back into your house.

Next. A water heater generally doesn't have a blower drawing 800 CFM or more of air from it either.

80% nat draft water heaters don't have a fresh air intake of their own(except for some sealed combustion 89% units). So they need it to be provided into the room they are in. And return air can not be taken from that room. Nor can that room be used for a return air pathway.


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