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10 feet to return a straight line?


Richard Moore
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So...smallish, 1990 2 story on a slab. Natural draft furnace (47K BTU) and water heater are in a small utility room in the center of the house, along with the laundry facilities. The distance from the one return (the other return is at the same level at the other side of this room) to the combustion air vents is about 5 feet and then it would be about 6 feet back to the furnace. A definite problem with the door open but, if the door was weatherstripped and fitted with a closer (it had neither), would the distance be OK at 11 feet total...or do we have a trickier problem here? There really isn't anywhere else convenient to draw the air from or to move this return to.

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Can't say I'm happy about the dryer also being in the same small room. No appliances so I wasn't able to test it with a dryer running but the furnace and W/H seemed to be drafting just fine when run together. No indication that flue gases were getting sucked out.

Thoughts?

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I think it would be better off with the door open.

The proximity of the return air grille is insignificant compared to the depressurization of the room by the clothes dryer.

In order for the return air grille to have any effect on the pressure in that room it would first have to depressurize the entire space. With supply registers in the same space, that's unlikely.

On the other hand, with the door closed, those two little combustion air openings aren't going to prevent the dryer from depressurizing that space. That's the greater concern.

- Jim Katen, Oregon

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Just got back from the ball game. We lost.

In order for the return air grille to have any effect on the pressure in that room it would first have to depressurize the entire space. With supply registers in the same space, that's unlikely.

I've always been a bit "suspicious" about the 10' rule and how it truly affects drafting, that is unless the return was really right slap bang against the furnace intake. It's just too round a figure, with way too many variables involved. Jim...your point about the registers "resupplying" the pressure to the large area outside the room makes sense and the two units did seem to work fine, door open or closed. No mirror fogging, CO, etc.

Which leaves me with the dryer. Your observations about that also have me somewhat turned around. I'm actually now thinking that the room would be better off with a fully louvered door or, at least, much larger combustion air vents. It might not meet the 10' "code" that way, but it would seem to be the best all round solution. I'm guessing the door was often left open when the laundry was being used and probably at other times, nobody has died in the house since 1990 (to my knowledge), and I sure don't want to suggest something that would actually make matters worse.

However, I'm also going to be recommending a CO monitor (or two or three).

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I don't think the venting meets the minimum net free area prescribed in 1702.2 (100 square inches for each vent of net free area).

Frankly, it'd raise my eyebrows even if the dryer wasn't in the space.

As it is, with the dryer in the space I'd report the condition as dangerous. It doesn't take much of a stretch to imagine at some point the furnace will burn less than perfect changing the normally clean exhaust to a death inducing concoction exacerbated by starving it for combustion air.

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