Negative Pressure in Drain, Backwater Valve, Zero Slope

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takeman

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Our basement had 9 inches of sewage flooding in 2013.

We finally had a plumber install a backwater valve - Mainline Fullport 4" Normally Open. As installed, the valve body measured at the lid has zero slope.

While nothing was flowing through the backwater valve, out of curiosity, I lifted the gate by hand. When the gate was almost vertical, with about 2" gap at the top of gate, it pulled itself shut! It's as if there is negative air pressure in the lateral connected to the inlet of the valve.

Is it possible that there is negative air pressure in the lateral? Why or how? Is this a problematic condition?

With the valve gate open, when I flush a toilet, the water shows up trickling through the valve within seconds.
But, with the valve gate closed, the gate doesn't open when I flush a toilet. It opens only 3-5 minutes after I let a faucet run continuously. The gate seems to open only after a significant amount of water "backs up" upstream. This happens with nothing in the drain downstream of the valve, with the cleanout closed or opened.

I wonder if this behavior - the valve gate not opening immediately - will lead to problems during normal usage. Will the valve gate shut too often, with minimal rainfall - leading to basement flooding not with city sewage, but with effluent from our own house?

Your comments please.
 

Reach4

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While nothing was flowing through the backwater valve, out of curiosity, I lifted the gate by hand. When the gate was almost vertical, with about 2" gap at the top of gate, it pulled itself shut! It's as if there is negative air pressure in the lateral connected to the inlet of the valve.

Is it possible that there is negative air pressure in the lateral? Why or how? Is this a problematic condition?
I presume by "lateral" you mean the street side, which is to the right in the diagram. A negative pressure would tend to pull the flap open. Are my presumptions wrong?

diagram.jpg
 

takeman

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Hi Reach4

I never had to be concerned with plumbing terms before. I meant by "lateral" the (almost) horizontal drain pipe under the floor leading to the inlet of the backwater valve. Maybe I should've just said "drain".

In the mean time I looked up vent stack, and a symptom of a clogged vent stack is negative pressure. But this negative pressure seems to be there all the time, not just when fluid is trying to flow out the drain. Could the negative pressure be there all the time because warmed air is flowing out the roof through the vent? - in which case it's not clogged?
 

Reach4

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Could the negative pressure be there all the time because warmed air is flowing out the roof through the vent?
I would think so, plus any wind blowing across the opening will lower the pressure too.
 

hj

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Normally, the only way to get significant "negative pressure would be to put a pump on the sewer line. Right now, given your description, I am not sure how it could be happening, unless the rubber hinge has gotten so aged that it no longer flexes properly, which would be a common problem with that type of gate whether normally open or closed. A clogged vent stack, IF it creates a negative pressure, only does it between the fixture flow causing it and the clog, which would have absolutely nothing to do with the back water valve.
 

Kreemoweet

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It's very common to find a rather strong flow of air from the building sewer upwards and out through the vent piping.
This is the same "stack effect" that makes a chimney work. While the laws of physics demand that there is, indeed, a
"negative pressure" to make the air flow, it is very slight. The effect depends on the vents being clear, so you have
nothing to worry about there.
 
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