HVAC flue vent making weird noise

Question

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.

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?

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.

Answer 1

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.

Answer 2

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.