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My HVAC system is forced hot air, with natural gas. The flue vent is making a weird noise when it's on, kind of like water sloshing around.

It's hard to describe the noise, so here's a video (it's a bit louder in real life than it appears in the video):

https://photos.app.goo.gl/mRQeXErE6qDFY9r76

What could this be--what should I be checking?

Edit: adding some pictures of the inside tubing. I do hear what seems like water moving around there. flue vent low point

Edit 2: I pushed the tubing upwards, so that the water would flow back into the furnace and the drainage tubing. Unfortunately, this has caused the furnace to stop heating. I don't see any flame when the thermostat calls for heat. Air is ventilated, but not heated.

Here's a picture of the furnace and the flue vent. The flue vent part that is inside the furnace is on the top left (black). In its connection, there are two drain tubes, that I think feed a left and right drain respectively. My furnace has its drain on the right, so would expect the left tube to lead nowhere (I can't really see or feel where it goes). The right tube leads to a little black box in the back, which has a connection to the external drain piping (which goes to a chamber with condensate neutralizer, and then a pump to pump it out into the sump pump hole). That all seems fine.

So what could've caused the furnace to stop heating? What if water came down the flue vent and it didn't all go into the drain tubes, but some of it went into the other connection? That seems to be the motor, right? But why would that have an effect on the flame/heating?

flue vent on top left

Edit 3: what caused the furnace to stop heating is that the inducer motor was flooded. When the thermostat called for heat, the motor would turn on, but it would have to move a bunch of water around, so it drew more current. Eventually the control board would kill it and output error E294 (on my Lennox) for "Combustion air inducer motor amp draw too high". The solution was to take out the motor and drain it.

I will also elevate the flue vent pipes to prevent the low point from pooling more water in the future. I also saw that the condensate traps need to be primed with water, to prevent gases from escaping that way. I didn't quite understand the mechanism, but I'll probably be doing that as well.

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  • Sounds like there's a piece of fabric-like material (maybe insulation or sheet plastic) flopping around in there.
    – Hot Licks
    Nov 4, 2022 at 21:37
  • Turns out it's just water
    – Rodrigo
    Nov 15, 2022 at 21:05

2 Answers 2

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Does the indoor portion of the vent pipe have a low spot? Perhaps it has some bends to get around an obstruction? If so, you may have an accumulation of water that has condensed from combustion gases. If deep enough, flue gases may be intermittently blowing through the standing water.

The cure would be to eliminate the low spot by rebuilding the vent pipe without one.

I've seen this happen in flexible dryer vents that had a long horizontal run through an unheated space and had sagged from the weight of condensed water.

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  • Thanks for the suggestion. I'll look for a low spot in the pipes and also post a picture to see if there are any other clues
    – Rodrigo
    Nov 6, 2022 at 1:56
  • OK I added a picture. It doesn't seem like a low spot at first glance, but it might be (I'll have to take a level and figure it out). Regardless, there appears to be water in there, because I can hear it in that corner. I'm not sure if the water is there because of condensation (as you suggested), or because some water got in through the exhaust after some heavy rain... Rebuilding the vent pipe seems expensive. Would you advise against just using a shop vac to get that water out? (Sure, it might return in the future, but that future may be years).
    – Rodrigo
    Nov 8, 2022 at 22:38
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    @Rodrigo Condensing boilers and furnaces sometimes have air pressure sensors on the air intake and/or combustion gas pipe to detect a blockage. A shop vac could overwhelm such a sensor and damage it or knock it out of calibration. If you know someone with a laser level that projects a level line, that will be the quickest way to check if there's a low spot. To drain water as a temporary fix, you can drill a small hole at the low spot in the PVC pipe, let the water run out, then plug the hole with a self tapping stainless steel screw through a blob of silicone caulk on the hole.
    – MTA
    Nov 8, 2022 at 22:55
  • I'm marking this as the accepted solution, even though it is not what I tried. I went with Greg's solution below, because it was easier. But it resulted in a flooded inducer motor, which I then had to take out and drain.
    – Rodrigo
    Nov 16, 2022 at 0:09
  • @Rodrigo Great that you got it working again. Now if you could support the pipe in the raised position with some pipe strapping or a length of strong wire such as picture hanging wire, that should prevent a new accumulation of water whether it originally came from condensation or from rain.
    – MTA
    Nov 16, 2022 at 2:26
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Because that furnace flue is plastic pipe we know that your furnace is a 90% (or higher) efficiency unit. In some installations the flue length may be very short such that water vapor never does condense inside the pipe, but in general, it is expected that condensation will occur. The flue must be installed with a slope so that condensed water drains. Usually the slope should cause the condensate to drain toward the furnace to avoid buildup of ice at the outdoor end of the flue.

Try lifting the flue. Begin as near to the outdoors end as you can, raise the pipe an inch or two, and work your way toward the furnace raising the pipe as you go. Listen and feel for that pool of water to begin flowing toward the furnace. An assistant could watch for water exiting the drainage tube at the furnace as well.

If you're not able to identify and correct a low spot where water had collected, then you may need to cut the pipe open and inspect it. Yours appears to be 2" size and PVC material. Both DWV (drain/waste/vent) and SCH40 grades are appropriate. You can cut the pipe open in a convenient location, look inside to inspect it, and re-assemble with an ordinary 2" PVC coupler and cement -- the same stuff one would use for irrigation.

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  • Hi Greg. I followed your advice to lift the pipe a bit from the end that's closest to the outdoors. Indeed, I could hear the water flowing back towards the furnace. The bad part is that now the furnace won't turn on. Well, the air handler turns on, but it seems that the flame is not turning on, so only unheated air is coming out. What do you suggest? My guess is that something that shouldn't be wet is wet. Do you think just waiting a few hours will let the water evaporate? What does the "drainage tube" that you mentioned look like?
    – Rodrigo
    Nov 15, 2022 at 21:02
  • I probably should've drilled a hole like @MTA suggested. I was just a bit uneasy about being able to seal it back correctly. But now I'm afraid I just turned a small problem into a bigger problem :(
    – Rodrigo
    Nov 15, 2022 at 21:06
  • I added an update to the question with a picture of the furnace. Let me know if you have any ideas--any help is much appreciated.
    – Rodrigo
    Nov 15, 2022 at 22:08
  • After watching some YouTube, my guess is that the condensate trap is a bit clogged, and it's causing the pressure switch to turn off the inducer motor. And maybe the furnace is smart enough to not turn on the heat because it knows the inducer motor is off.
    – Rodrigo
    Nov 15, 2022 at 22:34
  • OK never mind the pressure switch. The inducer motor was flooded with water (the water that came from the flue vent). When trying to start, it was pulling too much current and being killed by the circuit board (I saw error E294, which was "Combustion air inducer motor amp draw too high". The solution was to take out the motor completely, drain it, and put it back in. Now everything is functioning properly.
    – Rodrigo
    Nov 16, 2022 at 0:07

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