Bang when pump turns on

I am on a system which has a 35 gallon pressure tank, maybe 4 years old, the pressure switch is a 30-50 switch. When the pump first kicks on, like maybe after 4 or 5 hours of non-usage, it does so with a fairly loud noise that can be heard throughout the house, much like it is rattling the pipes. After the first cycle through, if I leave the water on long enough, the next time the pump turns on it sounds normal. It happens only after it is non-used for a few hours. The tank is pressurized to 28 psi. Any ideas?

thanks
frank

Your check valve on the pump is leaking back. The extra check valve you have on the surface is causing the loud water hammer thump. Remove the above ground check valve, and the thump will go away. But then check to see if you are losing pressure while no one is using water, which means you will have to fix the check valve that is down in the well.

I agree with valveman's prognosis. A leaky lower check valve can create a partial vacuum at the top-side check valve, depending on where the static water table sits. The in-rush of water closes the vacuum gap and slams against the held pressure at the top-side check valve.

Check valves hold better if they have pressure pushing them closed. When you install a second check valve top-side, it removes the pressure from the lower one.

Thank you gentlemen. I do have a check valve at the pressure tank on the pump side. I guess the one you speak of is down in the well or on the pump itself. In either case, it sounds like I need to get the well guys out here because I surely couldn't pull it myself.

thanks again
frank

First remove the check valve at the pressure tank. Then and only if you see the pressure dropping while no one is using water do you need to have the well pulled.

OK, I removed the check valve after the well guy said that he didnt think the problem was toward the well. He determined this by pulling the 1 inch plastic pipe from the well where it comes into the basement. He put his hand over the end and didn't feel any suction. So after he left I pulled the check valve and reconnected everything. When I last used any water last evening, the pressure was at 35 psi. This morning it was at 35 psi. When I ran water long enough to turn the pump on, no noise. So not sure what it all means.

frank

What it means simply is that the lower valve held because it now has pressure against it to hold the seal. Did you not understand my explanation of the vacuum in the water column?

So if the thing has been fine for years with the check valve in there at the top side, are you saying that there is a leak now at the bottom side and the leak is not evident because of the added pressure now that the top valve is gone?

thanks
frank

Yes, that is what I'm saying.

OK, many thanks. Thinking I'll just ignore it unless I notice the pressure dropping.

frank

frank in ohio said:

OK, many thanks. Thinking I'll just ignore it unless I notice the pressure dropping.

frank

Click to expand...

That is a good idea. You don’t have a leak or the pressure would have been lower than 35 the next morning. Removing the second check valve solved the bang on start up problem didn’t it? The check valve down below only had to leak about a thimble full of water to cause the bang on start up against a second check valve. As LL said, removing the above ground check valve puts all the pressure on the below ground check valve, and makes it seal better. It is apparently not even leaking a thimble of water any more.

What it means is that your well man doesn’t understand this basic principle. You were right to research this on your own. Most of the old well men who knew this kind of stuff are no longer with us. If the younger generation can’t solve your problem by punching buttons on a computer, you are out of luck. There are no electronic controls and no “app” that will solve water system problems. It is simply a matter of experience in knowing what works and what doesn’t. Even if you think your pump or well man has experience, you are right to go on the Internet and check out everything yourself. An uneducated consumer is ripe for the picking.