Water pressure doubles overnight.

[alphamabox]

Hi all... I'm new here. I found this forum by Googling plumbing help so hopefully someone could point me in the right direction to my issue.

My house is on city water and has an electric hot water heater that holds 74 gallons. I installed an expansion tank recently because my pipes started to hammer. My static water pressure is 40 lbs in the house and I've set the expansion tank to 40 as well.

If no water is being used for an hour or more, my pressure will rise to about 50 lbs according to my gauge that is hooked up to a cold water line. I thought that having an expansion tank would equalize the pressure to keep it at 40 but doesn't seem to be working like that for me. A 10 lbs rise in pressure doesn't seem that bad but overnight that sucker will rise to 80 lbs.

The expansion tank I installed is an Amtrol ST-12 and my water temperature is about 115-120 degrees.

What am I missing here? I don't notice any hammering but the rise in pressure does concern me. Unfortunately, I didn't measure overnight pressures before installing the expansion tank.

Thanks in advance for any suggestions you guys might have.

 

[SHEPLMBR]

Are you on municipal water?

 

[alphamabox]

Municipal water I'm assuming is water provided by the city and not a well. If so then yes.

I think I figured out my issue. I have about 3 feet of piping in between my hot water heater and expansion tank. The expansion tank pressure was set without any water in and not installed at 40 lbs of pressure at room temperature. When the hot water expands it carries heat with it which in turn increase the air pressure in the tank.

I checked my pressure this morning and it was 80 lbs again so I checked the temperatures and sure enough pipe temperatures in between the heater and expansion tank were higher than the supply side. I have adjusted the expansion tank pressure (with water off and water pressure off the top of the tank) back down to 40 lbs since it went up to 55 lbs due to the water temp.

I'll check it tomorrow again but I'm almost certain this has to be the issue.

 

[SHEPLMBR]

Do you have a pressure reducing valve?

 

[alphamabox]

Do you have a pressure reducing valve?

Yes, I do have a pressure reducing valve where the main supply line comes into the house.

 

[alphamabox]

The problem is solved. It was the water temperature that was raising the bladder air pressure overnight that doubled my water pressure. I have adjusted the air pressure first thing in the morning when the temperature was the highest to 40 lbs and now my water pressure is 48 lbs at peak overnight.