# Pressure spike: PRV or thermal expansion?

Discussion in 'Plumbing Forum, Professional & DIY Advice' started by 70runner, Jun 6, 2012.

1. ### 70runnerNew Member

Joined:
May 25, 2012
Location:
Fallbrook, CA
City water, 3/4 supply line to Watts N45B PRV, then to twin softener system, to pressure gauge on outbound side of softener. Perhaps once every day or 2 I'll notice the pressure gauge at ~ 90psi. Opening a nearby faucet will bring the pressure down to the regulated 60psi level. I have two water heaters in the home, no expansion tanks. Is there a procedure to determine if the 90psi spike is caused by the city supply/defective PRV or thermal expansion in one/both of the water heaters?

2. ### jadnashuaRetired Defense Industry Engineer xxx

Joined:
Sep 2, 2004
Occupation:
Retired Systems engineer for defense industry.
Location:
New England
Almost certainly thermal expansion...whenever you have a closed system, you need an expansion tank. Go to www.watts.com and use their potable water expansion tank calculator to determine the proper size, then install one and your problem should go away.

4. ### 70runnerNew Member

Joined:
May 25, 2012
Location:
Fallbrook, CA
Thx...would I need a single tank or one on both heaters?

5. ### jadnashuaRetired Defense Industry Engineer xxx

Joined:
Sep 2, 2004
Occupation:
Retired Systems engineer for defense industry.
Location:
New England
The expansion tank (single) goes on the cold side (i.e., the inlet). Approximating the piping volume, make sure to include all the pipe you can think of and see that is hot. The volume of a pipe is 3.14*r^2*length. There are 231cu in in one US gallon. Keep the units the same, or account for it when converting when calculating the volume (don't mix feet and inches!).

6. ### 70runnerNew Member

Joined:
May 25, 2012
Location:
Fallbrook, CA
Jim, thx for help. Good point on approximating the piping volume. I used the Amtrol expansion tank sizing calculator (assume they use a nominal pipe volume.?.). Using 80gal (twin 40gal tanks) water heater, 60psi and 140degF puts me in their TF-8 rig (3.2gal).

7. ### hjModerator & Master PlumberStaff Member

Joined:
Aug 31, 2004
Occupation:
Plumber
Location:
Cave Creek, Arizona
As a practical matter, almost any expansion tank will control the average residential system. Expansion ONLY occurs in the water heater so it is the only volume you have to worry about. If the pressure does NOT increase when the heater is not operating, then it is expansion pressure build up. A bad PRV will "continually" increase the pressure no matter how often you drain it down. 90 psi is NOT a serious problem, however.

8. ### 70runnerNew Member

Joined:
May 25, 2012
Location:
Fallbrook, CA
Here's another question...assuming thermal expansion is spiking the water pressure, would it be better to install an expansion tank in proximity to both water heaters? For example if I install a single expansion tank at water heater A (whA) then the extra warm/hot water produced by the heating cycle at whB will move down the whB cold water supply line toward the expansion tank, perhaps causing warm/hot water at faucets in proximity to whB even on the "cold" faucet valve position.???.

9. ### hjModerator & Master PlumberStaff Member

Joined:
Aug 31, 2004
Occupation:
Plumber
Location:
Cave Creek, Arizona
The warm/hot water does NOT move ANYWHERE. Pressure in the system is constant everywhere expect for loss due to elevation. Therefore, an expansion tank ANYWHERE in the system will do the same job. The reason for installing it at the water heater, after the shutoff valve, is to minimize pressure buildup in the tank if the valve is turned off, but the heater continues to operate.