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Major Water Condensation in Furnace Flue Vent in Attic - Drywall Damage

23291 Views 11 Replies 7 Participants Last post by  AlphaAirInc
Hello everyone,

Before I begin, I would like to state that I have contacted two different HVAC professionals who have visited my home and provided two different solutions regarding water condensation building up inside the furnace flue vent in the attic.

Below is a brief description of my setup:

I live in a 2300 sq. ft. two story colonial, with a natural gas furnace in the basement rated at 135,000 BTUs. The house is 20 years old and has the original furnace. The water heater and furnace share the same flue vent which extends from the basement, up two stories, where it reaches the attic. In the attic the vent bends at a 45 degree angle towards the back of the house, where it bends another 45 degrees up and out the roof. (I am assuming they angled the vent towards the back of the house so the vent stack would not be visible in the front of the home). At this last bend where the flue vent exits the roof is where water condensation is building up inside the vent and dripping onto the drywall below.

The one HVAC professional explained that the long stretch of vent pipe in the attic from the front of the house to the rear of the house was allowing the furnace exhaust to cool prematurely inside the vent, which in turn was causing the water to condensate and drip from the elbow. The solution he suggested was to wrap the entire vent pipe in the attic with fiberglass insulation to stop the premature cooling of the exhaust and allow the warm air to continue until it exits through the vent stack.

The second HVAC professional stated that this was a normal issue and suggested that due to the age of the furnace and the amount of work it would take to install/repair the vent piping that the best solution would be to install a high efficiency furnace which uses a PVC exhaust pipe through the basement rim board.

Can anyone provide any further insight?

I will eventually replace my furnace with a high efficiency furnace, but both the HVAC pros stated I still have a few years left in my existing furnace. I would hate to throw the baby out with the bath water so to speak, replacing everything because of water condensation in the attic.

Any suggestions or feedback would be greatly appreciated.
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Thanks for the vote on number 1. I was thinking I could attempt this first as long as there are no possible negative consequences. I have been reading some documentation regarding venting pipe installation and noticed there are quite a few warnings and safety guidelines to follow. The venting pipe in my attic has stickers which clearly state there needs to be a minimum of 1" clearance of all combustible materials, but fiberglass insulation is not combustible at the temperatures provided.

I guess it would be great if someone could let me know if this is safe?
FYI this is a contractors only site. My advice would be to call a qualified contractor to asses your situation and fix it following your local codes. Best of luck.
Thanks and I realize the purpose of this site. I used to be a general contractor and now do light residential remodeling - however I have no expertise in HVAC and would like to know not only for my personal situation, but also going forward in case I ever run into anything like this with future clients.

I also paid two HVAC professionals to visit my home to assess this situation, and was somewhat alarmed with the two varying responses.
Sounds like sound advise in both cases, since the first suggestion is the cheaper option it cant hurt to try.
Thanks for the feedback.

After seeking advice from a third HVAC pro I've decided to simply replace the furnace.

I guess there is a number of reasons which could lead to the condensation, from inadequate ventilation sizing (pipes being too large and too long), to the furnace running continuous short cycles which don't provide for adequate drafting of the furnace exhaust.

I figure the furnace needs replaced anyways, so might as well get it out of the way as opposed to putting a band-aid on it.

make sure they do a load calc. Too often a contractor will just replace with teh same size that is already there. And when going from a 80% to a 90% plus, it causes even more short cycling. And can cause lots of no heat problems.
The high efficiency boilers with the PVC pipe put out water vapor in the winter. My inlaws had one put in and the steam goes up outside the kitchen window. A bit annoying, it would have been fine just a few feet over. Nobody thought of it when they sited the new boiler.
By the way, they are warmer and saving lots of money from when they had their old oil furnace replaced. And now the cellar is cold, not hot like it used to be.
If you got high utility bills, and budget allows, I would follow advise the second guy gave you.
You will get a high efficiency unit, and eliminate vent in the attic all together... Everything will be vented on the side of the house, including the power vent WH. :thumbsup:
Thanks for the feedback everyone.

@beenthere - Thanks for the tip. The second HVAC pro stated the furnace I currently have is already too large for the house, and he would need to perform calculations based on square footage, room sizes, and duct work to determine the most appropriate furnace size.

@pdmig - My parents had the exact same issue with their older home before they renovated - though they use their fireplace insert quite a bit now.

@greg24k - Yes, replacing the furnace at this point in time seems to be the most prudent choice, and I have received quotes ranging from $2000 - $3000 for single stage and two stage furnaces, which seem very reasonable.

I guess now I just need to determine if the two stage variable speed furnace is worth the extra investment.

Thanks again for the feedback.
The two stage shouldn't run you too much more money. Around $100-$400 more. It offers a small increase in comfort in your home. If your house is newer and reasonably well sealed I would go for it.

It goes without saying that I vote option 2. Especially for $2-$3k. That is a great deal. Just make sure the venting is installed properly (according to the manual) and if you need to add a basement return air now is a good time to address that.
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