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Correct size for furnace flue pipe?

Posted by kellyeng (My Page) on
Sun, Nov 25, 07 at 10:26

I have a new construction house with a Bryant Evolution system. Friday morning we woke up to find the system was shut down and a message of, "SYSTEM MALFUNCTION CALL TECHNICIAN." The installer was out of town for the holiday so I called a local company that I have used before with 24/7 service. The technician comes over, runs a diagnostic, goes into the attic and then goes outside to look at the flue pipe on the roof.

He says that the flue pipe coming out of the roof is not venting correctly thus causing an air pressure build-up which caused the system to shut down. He also said we were probably getting a build up of carbon monoxide (runs on propane). He said that the flue pipe is correctly sized inside the attic (6") but on top of the roof it is connected to a 4" dryer vent pipe and we need a flue pipe that is 6" and made for venting the furnace. He also said this was a city code issue as well.

So we decided to change the flue pipe and are just waiting for Monday to call a roofer because the HVAC technician said they don't cut holes in roofs due to liability issues.

In the meantime, the original installer calls this morning. I tell him what happened and what the other company's technician said and he says that's bull. He says that the specs of the system calls for a 4" flue pipe and that the problem is probably a "low pressure switch" needs to be adjusted. The other HVAC company says that switching out the flue pipe not including cutting the hole and installing a roof jack will cost $300.

Any thoughts or advice?


Follow-Up Postings:

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RE: Correct size for furnace flue pipe?

Very rare for today's furnaces to need a 6" flue pipe. Should be all double wall from the furnace through the roof with that brand's flashing, storm collar & cap. What's with the dryer vent?

What BTU furnace? I'm guessing 4" is fine if done right.


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RE: Correct size for furnace flue pipe?

I would expect, the size that comes off the burner, should follow completely through to the top.

Obviously, one or the other of the techs, or both, are incorrect. I'd go for a third opinion.


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RE: Correct size for furnace flue pipe?

A Rheem furnace is a good example of why you have to know the btu ratings in order to properly size it. All come off the top of the furnace with a 3 inch opening and if you go to the 125,000 btu furnace it has to immediately transition to a 5 inch vent. I'm guessing that maybe this 6 inch was lined with a 4 inch (hopefully doublewall) to terminate. We use old 6 inch flashings with 4" all the time. A picture of the setup would be helpful to pass judgment.


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RE: Correct size for furnace flue pipe?

Okay DH and I think the tech was totally wrong. First of all, he was looking at the water heater vent on the roof because this is the furnace vent:


Photobucket - Video and Image Hosting


I measured the pipe myself and it is a 5" diameter pipe. Here is where it comes out of the furnace and terminates into the attic ceiling:


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And this is what it says on the pipe itself:

"TYPE B GAS VENT 1 IN. MIN. CLEARANCE TO COMBUSTIBLES . . ."


Photobucket - Video and Image Hosting


The furnace is a Plus 80V model 315AAV and on the Bryant website it says it's capacity is 66k - 154k BTUH and doesn't say anything about BTU.

So, what do you think?


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RE: Correct size for furnace flue pipe?

I've never seen a roof termination for a furnace like that in all my years....I see you are in TX but I still can't imagine that is manufacturer approved....the vent against the wood in your second picture is against any building code. B vent needs 1" from combustibles.


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RE: Correct size for furnace flue pipe?

Isn't that just great.

Since this is a new construction house, I'm calling the head city inspector tomorrow and ask how his office managed to pass the HVAC with this pipe strapped to a combustible.

Thanks for noticing Tinmantu!


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RE: Correct size for furnace flue pipe?

Are you sure the first pic isn't an attic vent?....I just can't believe that someone would use that as a furnace vent. It wouldn't cut it where I live because a 6 inch snowfall would bury it. Curious to hear comments from hvac guys in the south. Never seen such a thing.


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RE: Correct size for furnace flue pipe?

No not an attic vent, we have ridge vents for attic venting. I looked at my neighbors roofs and they all have the same setup. The last time I saw a snowfall accumulation around here was 24 years ago.


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RE: Correct size for furnace flue pipe?

I agree with all the posters. That is the worse garbage
install i've ever seen. I guess your code boys are just
brain dead. Could be you have no codes. Your vent on the roof will NEVER draft properly in a million years, eat the shingles because of acid fumes,and could be a fire hazard. Even your gas pipe install is totally garbage and illegel. Tinmantu is dead on. No way in hell is that vent supposed to touch any combustable material. I guess you live in a no code zone and anything goes until 3 people are killed.
If i did that install up here i'd be out of business, my licence pulled, sued, and probably in jail. That includes
the gas piping. I feel for you. We were told in school that
the U.S watches our codes. We set most of your gas and electric codes. Just takes you guys years to catch up and i think you need three code related deaths before there is a change, might be five. Can't remember. Bottom line is if
your home was in Canada with that install the house would
be locked off untill all code infractions were repaired and
the installers were jailed or out of business. Well not
jailed but out of business or forced back to school with a
major fine. Here is how it works up here. I make one mistake with venting or gas i pay $150.00 for a new inspection. Make another mistake on the same home i pay $500.00 for the inspection. I make a third mistake on the same home i lose my licence and have to go back to school
and pay a $1000.00 fine. As i said before, i feel for you.
You want to know what a gas code or electic code is, come up here. I can't believe most of you people are still alive. Is this a conspiracy on their part ? Canada is an
elected dictatorship but our codes protect people. Can't
say the same for the entier U.S. Some states have it right on. Some haven't a clue. Just one man's opinion.


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