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A quick furnace exhaust question


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I'm looking at two gas high efficiency furnaces with draft inducing fans in the attic. They are within 5 feet of each other and share a metal flue through the roof. There is no increase in diameter past the junction.

Normally I would call this out, but I'm wondering if this is something new with high efficiency units?

Anybody know for sure? I would think that being this close together and without the larger stack the fans could cause backdrafting?

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You say they are "high" efficiency. Does this mean they are condensing units or is that simply what's stated on the cover of the appliance.

According to my CodeCheck HVAC:

Multiple Appliances Venting in Common

IRC 2424.10.3.4; 2426.6.8.1X2/UMC 809.3 - Flue must equal largest connector plus 50% of others.

I'm lousy at math, but to me that means the bottom line is that the cross-sectional area of the common flue must be a minimum of 150% of one of them.

If these are condensing units that obtain their combustion air from outside, their proximity to one another won't make any difference.



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Yup, that's what I thought.

I just learned that one of the furnaces was added at the last minute.

That answers my question. They didn't change out the flue when they added the furnace. Tsk Tsk...

Thanks Mike. You were right on time. (I don't believe I have the most recent version of code-check. I just wondered if this was something new.)

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High efficiency furnaces--i.e. 90%--shouldn't be connected to metal flue pipes. They typically vent into PVC wrapped with foam insulation. The flue gases are damp and not terribly warm. Entering an extremely cold flue pipe that's uninsulated can cause a boatload of problems--so many problems that it used to be considered lunacy to even install high efficiency furnaces in attics.

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No doubt it did, Bain, but we all tend to be creatures of habit.

I suppose the inspector may have had it in his mind that he'd already inspected the flue and was simply there to approve the furnace. I'm willing to give the building inspector the benefit of the doubt, but the installer certainly knew better. I think he was just lazy and didn't give a rats a&&.

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Originally posted by mgbinspect

I think he was just lazy and didn't give a rats a&&.

Which pretty much sums up the average Chicago DOB inspector.

Anytime I get depressed about how little I know & how much harder I have to study, I just compare myself to the average muni-geek and I perk right up.

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And then Kurt, on top of it all we have sobering moments like I did a couple days ago and are reminded that our profession is at best an imperfect art. Sometimes we all screw up.

The day before yesterday I somehow overlooked a pretty rotten stair railing and the buyer had to mention it to me. I'd like to think that as I left I'd have seen it and went back in to document it. But, I guess I'll never know now.

Being human particularly in this profession sucks sometimes.

I have a bit more understanding for the building inspector than the installer. The unspoken responsibility that we all rely on lies with the installer, the expert in his field. When he is careless, everyone else is put at risk.

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