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Rhythmic Knocking Noise From Vent Pipe

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Feb 01, 2014, 09:20 PM
Rhythmic Knocking Noise From Vent Pipe
Hello everyone,

I have a bit of a qunadry I was hoping someone could help me out with. Recently, we had a major problem with our high efficiency HVAC unit. We discovered that its condensate line was draining into a gutter. Well, we have had an awful winter here and this became a problem because the gutter would soon freeze up and then so would the condensate line. This happened one night and we got water everywhere when the pipe burst.

So, we brought someone in and they reroute the condensate line to a pump which then pumps the water to the vent pipe. This has all worked fine and the line isn't freezing anymore, but now we have another problem.

The pump will come on, the water will start to go into the vent pipe, and for about 5-8 seconds afterwards we will get what sounds like a rhythmic knocking on our wall. It sounds like someone is in the bathroom next to our bedroom and wants to come in. I've gone up on the roof and looked down the vent pipe and the water comes on, followed by the noise, and then what appeared to be some gas/vapor rose out of the pipe.

Does anyone have any suggestions as to why this is doing it or how to get it to stop? It is obviously coldest at night, which causes the furnace to be on, which causes water to be made, which causes the pump to come on, which causes the knocking and it is definitely loud and long enough to wake my wife and I up.

Thanks for any advice.
Feb 01, 2014, 10:56 PM
My guess is dripping over a downward elbow in the drain line. The sound you hear is the drips of water hitting the dry lower end of the fall - amplified by the empty pipe. Once it gets more water in at the bottom then the dripping noise goes away.

Is there anywhere they can reroute the drain line from the pump so that it is emptying into a more constantly wet portion of the drain system - a real drain line vs using the dry vent section?

Feb 02, 2014, 01:42 AM
hmmmm......maybe jaybee is correct...
i have another idea, when a pump kicks on, the drain line can "surge" or "jump" i think the noise your hearing is the condensate drain pipe moving/hitting where it ties into the vent line.
ck, where the pipes tie into togather and secure the line from moving
Feb 02, 2014, 04:50 PM
pump line run up under the joists, perhaps, and when it's running water it is knocking? if that's the case, put a little foam pipe insulation between them. you don't have to pay through the nose for the correct size, just get a bundle of 1 to 2 inch foam insulation sleeves, zip your finger down the split-seam on it, put a piece between the sump drain and the joists, and keep going until it's all protected from hitting. two to three bucks.

in a new install, those plastic J-hangers are the bee's knees, the sump pipe snaps in and you nail or screw the end of the J to the joist. run the pipe a couple of inches below, use a level to set your drainage at 1/4 inch per running foot, and life will be good. reasonably decent sound insulation, too.

sig: if this is a new economy, how come they still want my old-fashioned money?
Feb 05, 2014, 10:52 PM
I guess we have another 1 hit poster, oh well, let's party..
You went away and left long time ago
Now your knocking on my door
I hear you knocking
But you can't come in
I hear you knocking
Go back where you been........

Oh my darlin
knock three times on the ceiling if you want me
twice on the pipe if the answer is no
oh my sweetness
means you'll meet me in the hallway
twice on the pipe means you ain't gonna show
Feb 06, 2014, 08:43 AM
Hello everyone,

Sorry, I haven't responded back yet. I've been looking into what you all suggested. I've been doing various investigations and experiments and unfortunately haven't yet discovered the reason it is knocking. It isn't clanging around where the vent meets the pipe and I don't think it is where it is hitting the bottom elbow.

My thoughts now are either that the air from the pump is causing some sort of knocking in the drain pipe (could that even be possible?) or that the pvc is expanding/contracting upon the water hitting it and it is causing noise at the floor joist where it goes through?

I have no idea really.
Feb 06, 2014, 04:28 PM
oh yes we are going to find it. try this,
get a pitcher of water, go up in the attic, pour the water into the pan of the unit...
and watch, listen, be columbo....find that bump!

i have actually craweled inside a unit, shut the door and sat their looking for noices and leaks..gotta do what ya gotta do