View Full Version : Condensation from the stack

12-11-2011, 06:21 PM
Hi guys, I've recently bought a small rental house with a hot air gas furnace in a first floor closet.

The exhaust stack comes out of the furnace at 3" and is adapted to an 8" pipe going through the roof.

The exhaust is condensing in the stack and running out of the pipe as well as dripping into the furnace.

What can be done to correct the condensation problem?

Thanks very much for your replies.

12-11-2011, 07:27 PM
Consult the manufacturers installation manual. That 8" flue pipe may not be approved. It is allowing the gas to cool off too much and the vapors condense.

12-12-2011, 05:21 PM
I agree with Jimbo because something similar happened to me. My flues were very old and had been installed initially with double walls which insulate the flues. Over time, the inner walls of the flues deteriorated an in cold weather, water came down the flues into the furnaces. Also, rusty flakes came down the flues.

It is possible that the inner walls of the flues have deteriorated and allow the condensation to form inside them if your house is older, or the 8" flues were not double wall type when installed. That would be my first suspicion.

12-12-2011, 06:46 PM
thanks for the replies.

The stack is probably original from the fifties and it is double wall insulated.

It probably has deteriorated but most of the neighboring houses have the same stack coming out of their roofs.

There is no manual in the house so maybe I can get one from Luxaire if I contact them.

12-13-2011, 10:32 AM
A 3" round vent has a cross sectional area of 7 square inches.

An 8" round stack has a cross sectional area of 50 square inches, 7x the size, so it has 1/7 the velocity.

There's no way you can have sufficient stack velocity to prevent condensation, even if the furnace is power-vented. A narrower liner or new B-vent stack of the same or slightly larger diameter would probably fix the issue. A 3" vent into a 5" stack will work, most of the time, but if it's the only appliance using the stack there's no need to go that big.

If it's a condensing furnace with a 3" stainless or plastic vent it can't be vented into a non-stainless (or non-plastic) stack.

Got a model name and/or input & output ratings on that Luxaire?

12-13-2011, 06:18 PM
It just reads Luxaire High Efficiency on the door. It is model # PCMD-LD08H057B
It reads 45,000 and 57,000 BTUs
I did find a plate inside the cabinet that lists part #s for 3" stack pipe

Do I need 3" stainless stack pipe for this?

Can I leave the existing stack there and run the new pipe inside?

How would I plug up the space between the two?

Thanks very much.

12-14-2011, 09:43 AM
45K/57K ==> ~79-80% efficiency ==> non-condensing, which means B-vent, not stainless, and definitely not plastic. Ideally you'd be using the same diameter as the stack coming from the unit. There SHOULD be a dilution air hood and flue damper built into it, which allows some air from the room to draft up the flue, which would also reduce condensation issues. (If not you may have to do your own.) Got pictures?

I'm not sure if it would meet code with a B-vent inside of single-wall stack. I'd rip it out and seal it all well.
[edited to add:]
Be sure to use the same stack vendor's cap and storm collar etc. where it goes through the roof to get a good fit.

If you have hot water heater or some other gas-burner using the same flue you'll have to up-sized the diameter to meet code-min, but I'm assuming that's not the case here.

Take a look at the table in figure 47 on p23 and Table 1 on page 27 of this primer (http://www.duravent.com/docs/product/L202_Sizing_Handbook.pdf). The BTU numbers you should be using is input BTUs (=57,000, in your case.) It looks like you may have to bump the B-vent diameter to 4" if it it's a an atmospheric-drafted variety (doesn't have fan assist on the vent), which I'm assuming it is at ~78-80% efficiency.

12-14-2011, 02:23 PM
Dana Thanks for the info. I'm starting to understand what is needed to fix the problem.

Yes it its fan assisted exhaust.

The current 8" stack is single metal with a 1/2" rigid insulation surrounding it.

So you think I should rip out all of the old stack?

Won't the new 3" stack have to be insulated since it will be going through an attic?

12-16-2011, 05:15 PM
Yes, get RID of that old stack, ALL of it. It's nothing but a big hole in your ceiling causing a condensation problem. It takes a HUGE furnace (may be 8-10x the size of yours) to ever need a stack that big, and is probably 20 minutes away from rusting through in spots from years of condensation, if it hasn't already. Use the narrowest B-vent that meets the spec in the tables, and it'll work better, last longer.

The new stack needs to be air-sealed where it penetrates the ceiling (preferably with a tight fitting collar) and IIRC code still requires a 3" clearance from combustibles. Given the huge gap where the 8" stack went through the ceiling, you may want to cut right-sized hole in a piece of fiber-cement board to air-seal the ceiling penetration. You can caulk/foam the fiber-cement board (Hardiboard, or similar) to the gypsum, and use a fire-resistance caulk where the vent meets the fiber-cement board. Where it goes through attic insulation you can use unfaced rockwool batts (Roxul) wrapped around it and you can then bring the other insulation snug up to the rock wool with no gaps.

B-vent is double-walled- be sure to seal it well enough at the bottom that it doesn't end up sucking out conditioned-space air 24/365 in the space between the inner & outer walls too. Automotive muffler sealer or similar can treat the seams, if you are air-leak obsessive.

12-22-2011, 06:36 PM

Thanks for all of your advice. It looks like I have some work to do.

12-23-2011, 10:06 AM
Yer welcome! (And yes, it's a real project...)