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  1. #1
    Join Date
    May 2010
    Location
    West Chester, PA
    Posts
    177

    Condensation in furnaces

    I'm trying to understand the issue of condensation in gas furnaces (furnai?). My understanding is represented by this simplified image and description:



    Non-Condensing Furnace: Cold air from intake vent is fed to the blue burner which produces hot gases that travel through heat exchanger tubes and exit at high temperature from the exhaust vent. Although not clearly shown, there is no place for the acidic condensation to drain and thus it is imperative to minimize/eliminate it. Condensation is minimized due to higher temperatures throughout the path. The heat exchanger shape/pitch is such that should droplets of condensation form inside the tube, they would slide back towards the hotter burner region to be evaporated.

    Condensing Furnace: Cold air from intake vent is fed to the blue burner which produces hot gases that travel through heat exchanger tubes and exit at warm temperature from the exhaust vent. There is a drain for condensation, and condensation is always expected in the final segments. The heat exchanger(s) shape/pitch is such that droplets of condensation slide away from the burner and towards the condensate drain.

    I don't know if any non-condensing furnaces have "primary" and "secondary" heat exchangers, but I've noticed that condensing furnaces do (with the hotter early segments comprising the primary, and the final segments comprising the often physically different, larger surface area secondary).

    Based on this it is much easier to imagine a condensation problem in low-firing non-condensing furnaces. In the condensing furnace, should condensation form it has a path through the "absolutely must be condensation resistant" secondary and out through the drain. The only potential points of concern, that I can think of, would be a) improperly shaped or pitched heat exchanger tubes, b) bad heat exchanger tube junctions which allow the condensation to collect, and c) surface condensation which isn't sufficient to form droplets/streams that move towards the drain. I don't know that any of those actually pose a real world problem though (?).

    One thing that came to me as I was preparing this: IF I have a reasonably correct grasp of this subject (I certainly may not), then at least until the furnace condensate drain trap fills, there is a path for hazardous exhaust gases to go right out the condensate drain line. I have seen several instances where the furnace condensate drain line and evaporator coil drain line were tied together (directly, or indirectly through a condensate pump). Thus it appears to me that in certain, perhaps code-breaking, installs there is a [temporary] threat of furnace exhaust making its way up the evaporator coil drain line and out the open evaporator coil pan into the air stream blowing into the home. If there is something I'm overlooking... something besides a potentially dry trap... that would prevent said exhaust from taking that route I'd be interested to know what it is.

    Comments on the above welcome and much appreciated!

  2. #2
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    67,778
    Condensing furnaces have 2 heat exchangers.
    NO condensate is suppose to be in the primary HE, only the secondary HE can condensate.
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  3. #3
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    67,778
    When teh A/C and furnace drain lines are piped together. the A/c line is suppose to have its own trap to prevent exhaust from entering the air stream.
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  4. #4
    Join Date
    May 2010
    Location
    West Chester, PA
    Posts
    177
    Quote Originally Posted by beenthere View Post
    When teh A/C and furnace drain lines are piped together. the A/c line is suppose to have its own trap to prevent exhaust from entering the air stream.
    Uh... what keeps that evaporator coil condensate trap full of water during the months when the air conditioner isn't running? In every install I've looked at, said trap was up at evaporator coil level and thus up hill from where the furnace line and ac line met.

  5. #5
    Join Date
    Jan 2004
    Location
    Lancaster PA
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    By the time the trap is empty. The furnace trap has water in it, and can't exhaust vapors into the drain line.
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  6. #6
    Join Date
    May 2010
    Location
    West Chester, PA
    Posts
    177
    Quote Originally Posted by beenthere View Post
    By the time the trap is empty. The furnace trap has water in it, and can't exhaust vapors into the drain line.
    Sounds like a bad assumption in there... that the air conditioner had been run recently enough that the evaporator drain trap still had water in it. I know two people in our general neck of the woods who have gone some summers without ever using their AC.

    Just how quickly does a condensing furnace trap fill up when dry?

    Is the furnace trap usually internal or external? I ask because I *think* in the case of one of the installs I looked at there was no external trap on the furnace line. IIRC, it just came out the side of the furnace and ran straight down into a condensate pump (how far from the bottom I don't know).

  7. #7
    Join Date
    Oct 2010
    Location
    SW Michigan, near Battle Creek
    Posts
    921
    exhaust gasses can get out till the furnace trap has water in it. every condensating manual I have read says to fill the trap before firing the furnace. In fact it takes like 5 minutes for the trap to fill if you just fire the furnace with a dry trap.

  8. #8
    Join Date
    Oct 2010
    Location
    SW Michigan, near Battle Creek
    Posts
    921
    most brands the 90% primary heat exchanger is the same as the 80% heat exchanger and the secondary heat exchanger is added betwean it and the inducer in the 90%

  9. #9
    Join Date
    Oct 2010
    Location
    SW Michigan, near Battle Creek
    Posts
    921
    and some funaces will not make the pressure switch untill the trap is full

  10. #10
    Join Date
    Jul 2010
    Location
    georgia
    Posts
    562
    from what I have gathered, the two condensate systems shouldnt share a common drain.

  11. #11
    Join Date
    Jan 2004
    Location
    Lancaster PA
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    67,778
    The furnace trap fills the first time it runs.
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  12. #12
    Join Date
    Mar 2008
    Location
    Shelby Twp MI
    Posts
    820
    Alot of condensing furnaces have an internal trap and it states in the IO manual not to put an external trap on it. It's amazing how long water can stand in internal traps. I just checked one last week and it was still full from last season.
    Jim

  13. #13
    Join Date
    Oct 2010
    Location
    SW Michigan, near Battle Creek
    Posts
    921
    duhhhhh
    almost all furnace traps are on the low pressure (sucky) side of the inducer. before they fill up the furnace sucks in room air not blow out vent gasses. old goodman 90's and lennox pulses notwithstanding. that is kinda why we use inducers and not combustion blowers. boy do i feel DUMB!!!!!!!

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