What's wrong with this flue system?

[midhvac]

What's wrong with this flue system and how would you remedy it?


11 yr old 2 story home zoned up and down. 2nd floor is 1/2 the size of first floor.
2 Carrier 80% 80,000 btu furnaces with 4" flues
2 40,000 btu water heaters with 3" flues
All flues single wall into single wall 8" which goes into 8" vertical double wall with 30' rise
Flue system all original except for 6 mo old 3" water heater flues which were replaced when orig. water heaters were replaced 6 mo ago.
(Single wall 8" continues about 3' to the right out of picture range before tieing into 8" double wall riser)
2 yrs ago large amounts of rust began falling out of furnace flues onto top of furnaces and you can hear dancing rust when inducer motors start.
Showing signs of acidic condensation & flue connections starting to rot.



[Edited by midhvac on 03-04-2004 at 02:47 PM]

[kmetal]

You can't vent all of that crap together!

[pmccune]

I think I would just start over completely that is dangerous I don't see how that could ever work.

[ls-1_eater]

Theres nothing wrong with 4 appliances hooked to a common vent. The first problem is the size: 30' of 8" double wall will handle around 470,000btu. The second problem is the way they are hooked together: The furnaces should be the last thing entering.

As for an actual rememdy, I couldnt give an actual answer without seeing the job firsthand. However if this would have been my job from the start, I would have done a better job of either placing the stack, or the appliances. Also I would have sized the stack properly instead of just guessing. From the info you have given I would have installed a 6". And last but not least, I would have used double wall connectors instead of single wall.

[midhvac]

Originally posted by ls-1_eater
Theres nothing wrong with 4 appliances hooked to a common vent. The first problem is the size: 30' of 8" double wall will handle around 470,000btu. The second problem is the way they are hooked together: The furnaces should be the last thing entering.

As for an actual rememdy, I couldnt give an actual answer without seeing the job firsthand. However if this would have been my job from the start, I would have done a better job of either placing the stack, or the appliances. Also I would have sized the stack properly instead of just guessing. From the info you have given I would have installed a 6". And last but not least, I would have used double wall connectors instead of single wall.

I've inherited the job of fixing it and am not relishing it. You're right, the placement is a major problem and all of the single wall has to go.

I don't think a good load calc was done as the 2nd floor is 1/2 the size of the first floor and both have 80,000 btu furnaces and 3 ton a/c units. First floor furnace never runs very long and 2nd floor furnace hardly ever runs. I also don't think they need 2 50 gal water heaters.

1st floor unit is on the left in the photo. When only it is running you can hold your hand on the 8" single wall flue just to the left of where the right furnace tees in and it's not even uncomfortable. A foot to the right of there just after the right furnace tees in is much cooler still, just barely warm at all. Every time I've been there I've never see more than one of the 4 appliances on at one time. I can't even imagine what happens when only 1 water heater is on and no furnaces.

[midhvac]

Originally posted by pmccune
I think I would just start over completely that is dangerous I don't see how that could ever work.

You should see the closeup photos. All 3 connections to the 8" single wall are home made. They just snipped tabs into the smaller ones and folded them over. One of the tabs was eaten by acidic condensation and fell off in my hand when I touched it.

It drafts real good though!

[ls-1_eater]

And here I thought I was the only one who ended up with jobs like this. Good luck. I'm sure you will come up with something better than what they have now.

[PaysonHVAC]

I see your point. With only one ot two appliances coming on at a time the gases will surely cool off too quickly and condense water in the flue. You almost need to run some seperate flues. At least the two WH together and the two furnaces together all in B-Vent.

I bet if you put a drain in the 8 inch just as it turned up for the 30' rise you would collect some water!

[midhvac]

Originally posted by PaysonHVAC
I see your point. With only one ot two appliances coming on at a time the gases will surely cool off too quickly and condense water in the flue. You almost need to run some seperate flues. At least the two WH together and the two furnaces together all in B-Vent.

I bet if you put a drain in the 8 inch just as it turned up for the 30' rise you would collect some water!

I know. This is one of those situations where something can be done according to venting tables and still produce tons of acidic condensation. I too thought about separating the wh and furnace flue systems and tieing them in closer to the 8" riser but there's still going to be lots of moisture.

Sometimes I think codes should require more than one flue out the roof for x number of appliances.

[haji7]

If you follw the 7 imes rule your waterheaters are a problem 8" is to big the waterheaters flue connections are only 3" flue connenctor correct they should be 4" I would sell them a bigger water heater.

[trgams]

Tell customer that they need to suffice with one water heater...or relocate one...that takes care of the sizing issue.

[midhvac]

Originally posted by trgams
Tell customer that they need to suffice with one water heater...or relocate one...that takes care of the sizing issue.

The 2 50 gal water heaters are 6 months old and were replacements for the 2 original 40 gal ones. HO wanted bigger ones. The new ones were put in 6 months ago by my plumber buddy who is the one who referred me to the ho.

There's no room anywhere nearer to the 8" run going up through the house for a water heater.

Everything drafts okay now. I know the single wall stuff has to go, but I'm just trying to figure out what configuration of 4 B vents would minimize the acidic condensation and cold flue problem and still allow everything to draft well. I'm supposed to give them a price on redoing it.

Any suggestions on the best way to hook these 4 things together with b vent?

[kb0vso]

Was this put in by a NATE installer ?