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Furnace closet door vent....what size??

RichBeckman's picture

I have built a closet around a furnace that is in the corner of the dining room. All that is left is the door.

But the door needs to accomodate combustion air (the return air is through a duct in the side of the closet).

I don't like fully louvered doors because they are a pain to maintain (clean and paint). Also, the customer is anxious for any marginal gain in quitness that might be achieved (I guaranteed that they would still hear the furnace easily).

So I want to cut a hole in a flat slab and put a vent into it. But how big a vent does the furnace need?

I couldn't find a BTU rating on the furnace. I did find a label with several numbers on it including:

Input

100,000

Next to that is "Output" but without a number underneath.

The exhaust pipe is 4" diameter (but it gets bigger a few feet up).

Thanks.

Rich Beckman

Another day, another tool.

(post #112736, reply #1 of 8)

http://www.codecheck.com/pg21_22mechanical.html#combustionair

. William the Geezer, the sequel to Billy the Kid - Shoe

(post #112736, reply #2 of 8)

You will need two vents, on 12" from the floor and the other 12" from the ceiling.  "Each opening shall have a free area of not less than one square inch per 1,000 Btu per hour of the total input rating of all appliances in the enclosure, but not less than 100 square inches."  Wood louvres will have 20 to 25 percent free area and metal louvres and grills will have 60 to 75 percent free area. So you will need 2 grills that are at least 12" x12" if they are metal since you said the furnace was 100k btus, hope this will help, happy new year

(post #112736, reply #3 of 8)

Thanks for the english translation.

But I'm still unclear on one area.

What does 12" from the ceiling mean? 12" to the top of the vent? to the middle of the vent?

In this case, 12" from the top of the ceiling is the top of the door. I'd much prefer to put the higher vent in the wall above the door, but that would put the top of the vent within a few inches of the ceiling (assuming a vent wider than 12"...)

Thanks,

Rich Beckman

Another day, another tool.

(post #112736, reply #4 of 8)

>>"Each opening shall have a free area of not less than one square inch per 1,000 Btu ....

"I'm not sure where that "Each" came from, every rule of thumb I knw of calls for min 100 sq, and 1" per 1,000 btu, 1/2 high, 1/2 low.

Note, however, that simply meeting the rule doesn't guarantee adequate performance.

Jim Davis, the guru on this stuff, says that you never get sufficient combustion air though a louvered door in closet installations.

My experience is that sometimes you do, but it is not uncommon to have insufficient draft and insufficient combustion air in closet installations, -especially- where the return is right around the corner, which is what your installation sounds like.

The only way to know for sure is to test the draft with a draft gauge.

Adding a small supply register in the closet itself can cure the problem.

I wouldn't get too worked up about the 12" up and down, though, as long as the air supply's are near the top and bottom.

Can you get your combustion air from above and below? (Attic and crawl?)

Much better solution, and it isolates the draft from that nearby return.

======================================== "Man's capacity for justice makes democracy possible, but man's inclination to injustice makes democracy necessary." Reinhold Niebuhr: 'The Children of Light and the Children of Darkness' http://rjw-progressive.blogspot.com/ ========================================

(post #112736, reply #6 of 8)

" -especially- where the return is right around the corner, which is what your installation sounds like."

Yup.

"Can you get your combustion air from above and below? (Attic and crawl?)"

This would be possible, but I doubt there is sufficient open floor space in the closet to work a vent in. I'll look at that today.

Rich Beckman

Another day, another tool.

(post #112736, reply #7 of 8)

I checked and there is room for a vent in the closet floor, but after thinking about it, I decided that I'd rather go through the door.

I have no interest in taking whatever steps may be necessary to properly vent (or seal, or whatever might be required) the crawl and/or attic.

I'll put two vents in, one high, one low and I'll get a buddy to come by and check the draft for me after it is done.

Thanks for the info.

Rich Beckman

Another day, another tool.

(post #112736, reply #8 of 8)

>>I'll put two vents in, one high, one low and I'll get a buddy to come by and check the draft for me after it is done.

In testing, the house should be closed up; check the draft with the furnace running, especially when the blower comes on.

Then, fire up all exhaust fans and the dryer and the water heater. Keep checking the draft.

Also for good measure, open and close the garage door, and an outside door on another side of the house.

Note, you want the draft to be between 0.01 and 0.02 water column inches; less isn't good (especially in winter, when drafts tend to run higher: more, well, judgment call, but be sure the draft isn't so strong it is creating an "air curtain" across the discharge into the draft hood (more likely on the water heater, in my experience.)

======================================== "Man's capacity for justice makes democracy possible, but man's inclination to injustice makes democracy necessary." Reinhold Niebuhr: 'The Children of Light and the Children of Darkness' http://rjw-progressive.blogspot.com/ ========================================

(post #112736, reply #5 of 8)

I read this out of the AGA (american gas association) book. I will try and scan it for you. I don't have it right in front of me now but I do know that bringing make up air in  through the attic has its own set of rules.