I have a direct vent gas forced air furnace that currently has one of those PVC concentric pipe combination air supply and flue going horizontally thru my garage wall. I am interested in separating these two pipes and running them out thru two perpendicular walls. My local inspector says the two pipes need to be within a few feet of each other.
Does that sound right? If so, can anyone please explain what the reason is for that requirement?
Thanks a lot,
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Replies
They need to be within a few feet of each other (per the mfgr's specs) so that any wind pressure on one pipe is offset by the wind pressure on the other. I suspect mfgr's specs will keep the pipes a certain distance from corners as well, since air pressure won't be even near a corner. Definitely not on separate walls.
OK, I guess that makes sense. Thanks again Dan.
Interesting.....Please explain how systems that don't use exterior combustion air work?
Edited 1/24/2009 9:09 pm ET by sledgehammer
Edited 1/24/2009 9:10 pm ET by sledgehammer
Differently. And sometimes they backdraft.
The modern conservative is engaged in one of man's oldest exercises in moral philosophy; that is, the search for a superior moral justification for selfishness. -John Kenneth Galbraith
sometimes direct vent backdraft too. :)
Who knew induction fans backdraft.
You learn all kinds of things here.
Why do you feel compelled to separate them? Aren't they designed to be as they are (concentric pipes)? Part of the design is that combustion air is preheated by the flue gas ... I think.
It is kind of a long story, but all that heat blasting out of the side of the house is comes out about 18' under a roof overhang that this time of year normally has about 4' of snow on it. So the heat melts the snow on the edge of the roof at one particular spot and creates huge icicles. Also the dripping water splashes up on the siding and ices that over. So I thought if I put it thru the perpendicular (eave end) wall I would not have as much of a problem with icing. Maybe not. But I guess there is no reason I can't just move the concentric pipe arrangement to the other wall and see if that helps. This would mean the horizontal run of the intake & exhaust pipes would be about 6 to 8' longer. So I'd better check with the furnace manufacturer to see if this is a problem.
Can you run it up vertically through the roof? That would/should solve the problem. That's exactly what happens w/ ice damming ... but the culprit is often less obvious (often an unprotected recessed can light exposed in the attic).
There is no path to the roof (two stories up) and I prefer to stay away from any more penetrations in the roof anyway. All ceilings are cathedral. We have no can lights. Each spring , the snow starts sliding off slowly as the air warms up, so any pipes going thru the roof have to be very strongly braced or they just bend right over. It happened to a 1.5 inch PVC plumbing vent the first winter. So I will probably try going thru the eave end of the house wall.
I installed a concentric vent on mine a couple of years back, the center exhaust vent frosts up if the temp falls below mid twenties, I'm pretty sure that it wouldn't if it was surrounded by still air, instead of contiuously having cold air sucked in around it .. knowing what I know now, I would never combine the pipes again ...