Sewage pump noise mystery
The lowest floor of my house which is below street level uses a sewage pit serviced by a Zoeller pump. The 3" pipe goes horizontal for 10', up for another 10' and then what looks like 15' to connect to the sewage line. Along the first horizontal 10' are a cast-iron check valve and shutoff valve.
My problem is this double bang (water hammer effect?) that this system seems to be having whenever the pump shuts off. The system always have a single small thud that we can hear on the floor above but this double slam is definitely something no one misses.
The biggest issue is that this double slam does not happen all the time. However it does seem to do it only when the pump hasn't come on for a little while. (It comes on at least a few times a day) Someone else commented in this forum on another thread about possible air coming back down the discharge pipe if the pump hasn't been used for a while.
Early this year I had to replace the mechanical float so I switched to a electronic float (the name escapes me). A few months later, the clamp that joins the two vertical pipes in the pit worked loose which I then messily fixed. Retrospectively, is this new slam/bang resulting in the clamp working loose?
So last week I brought in a plumber to look at it. He thought it was because the pit cover and subsequently screws were loose thus causing the noise. Despite my protests that they have been loose for years without a problem, I let him tightened them down. It didn't make any difference. His supervisor then took a look and suggested that the pipe be tied down to the joists so that the pipe do not move.
The next day, I worked with 2x4s, brackets and metal tape to connect the discharge pipe to the joists. As commented in a Sewage Noise thread in this forum, this made it WORST as the noise is now transferred to the floor above. Now it's like someones knocking at the door when the pump stops. This weekend, I'm going to untie the connections !
This is what I've done so far that hasn't worked:
- Tighten down the screws, pit cover, vent pipes on the pit.
- Repositioned the pump to more vertical align the discharge pipe.
- Added more pipe hangers with metal tape.
- Locked the pipe to the floor joists above. Worst now.
This are my available options for next steps:
- Leave more liquid in the pit to allow for more noise dampening and weight and higher run frequency? (The old mechanical pump had a more limited range so the pump was operating more often)
- Wrap rubber mats around all hangers
- Replace the check valve with a compression type fitting for greater isolation ?
- Add another check valve (3" valves seems to be rare and costly) ?
- Re-loosen up the pit cover fittings to allow for free movement ?
- Cut and add clamp with rubber fittings for noise isolation ?
- If it is air coming back down the discharge pipe, what is the fix for that ?
Plumbing experts, please help.
Thanks in advance.
Exhausted.