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Thread: Furnace Condensation Problem 80% AFUE 4" Flue

  1. #1
    I&C Engineer (mostly WWTP) Lakee911's Avatar
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    Default Furnace Condensation Problem 80% AFUE 4" Flue

    I have a relatively new furnace--it's about 2 years old. I'm getting water in the furnace. It's running down the flue and into the furnace causing it to rust.

    My masonary chimney has a 4" stainless liner in it and a "martian hat" vent on the roof and it is watertight too. The problem is, I'm getting a lot of condensation in the flue before it gets to the chimney/liner. I have an approx. 12 feet horizontal x 3.5 feet verticle run in the basement.

    I run a 20 degree temp swing 2x daily in the house in the winter, so that's a lot of opportunity for the water vapor to condense.

    I'm not sure what to do to solve. Temporarily, I installed a verticle tee right off an elbow from the furnance. I wanted to create a dripleg cleanout to keep the water from running down into the furnace. This won't solve the problem long term and the flue will continue to rust and rot out.

    Someone suggessted using double wall or B-pipe, but I can not find what I need at bLowes: it's 6" stainless, pricey and I didnt find elbows either.

    So, what to do?

    Please help

    Thanks
    Jason

  2. #2
    Engineer Furd's Avatar
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    It's strange that you can't find the B-vent. Both big blue and big orange carry 6 inch galvanized B-vent in my area.

    Do you have a two-stage furnace? If yes, does it run from a two-stage thermostat? Flue condensation in 80% AFUE furnaces is often due to short cycling. The furnace doesn't run long enough to actually heat up the exhaust vent and as a result there is condensation.

    How close of a differential do you have your thermostat set? What is the run time-off time-run time of your furnace under "normal" operation?

  3. #3
    I&C Engineer (mostly WWTP) Lakee911's Avatar
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    I really need the B-Vent in 4", but I couldn't find anything galvanized there either. Nor any elbows.

    I'm pretty sure its a single stage furnace. How do I prevent short cycling? I don't think that's the case. Typically I run something similiar to the following schedule:

    4:30AM 73degrees
    7:30AM 54degrees
    4:00PM 69degrees
    10:30PM 57degrees

    I think the condensation would occur when its at the colder temps, but in the case of heating up at the next setpoint, it couldn't be short cycling then, could it?

    I have a programmable t-stat, so I dunno the differential time. Maybe it can be programmed? I'll look at it tonight.

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    Plumber jimbo's Avatar
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    The short cycling question refers to this: At some point, the thermostat calls for heat. It comes on. How long does it run until it is at temp and turns off? Then, how long does it stay off, before it calls for heat again, and then how many minutes does it run? This is the cycle asked about. If the "ON" times are short compared to the "OFF" times, this is called short cycling, and is not desirable either in heat or cooling mode. It can be caused by an oversized unit/.

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    I&C Engineer (mostly WWTP) Lakee911's Avatar
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    I checked the t-stat and its set right for an 80% natural gas furnance. I don't know how much cycling it does until it gets colder. It's not been on yet this fall.

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    Retired Defense Industry Engineer jadnashua's Avatar
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    The one non-condensing boiler I've owned used a double-walled flue since it was closed combustion. the instructions said to ensure that above about 2' (?) the pipe must slope down to where it exits the sidewall. The thought was that the gasses would be too hot right at the exhaust to condense, but that they could as they continued on. Having it slope down, away from the boiler ensured that any moisture would drain outside, verses back into the boiler.

    You might be able to hook up the drain from a condensing boiler to your pipe, (well, probably not since it is usually pvc) if it was constructed out of materials appropriate for the temperature and was trapped properly to prevent outgassing. This is probably not an approved solution, and would be unsafe unless properly engineered (which a typical homeowner wouldn't be qualified to do).

    Have you discussed this with the manufacturer to see if they have any suggestions? That is often the best place to start.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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    Engineer Furd's Avatar
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    Jimbo was correct in his explanation of short cycling.

    If your furnace starts and runs for a sufficient time before shutting down the condensation issue will be minimal. However if it starts, runs two minutes, shuts down and four minutes later it starts up and repeats this cycle you will have problems.

    Short cycling is caused by several things including a furnace that is significantly larger than necessary, a thermostat with a minimal differential (on-off) setting, a thermostat that is incorrectly positioned in the house and outside temperatures that are such to only require minimal heat input to maintain comfort.

    Two-stage furnaces can help considerably in reducing short cyclinig.

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    I&C Engineer (mostly WWTP) Lakee911's Avatar
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    Unless the stat is broken, then I could only imagine that improper placement would be a problem. There is a register about 6 feet away on the floor, but its mostly closed. I'll have to see how long it runs when it does get colder out. I'll move it if is short cycling and then try.

    How long should it run to be considered not short cycling?

    I wish I had a datalogger that I could hook up to it to log the time each time it goes on and off....

    Jason

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