My rainshower leaks every day ONLY between the hours of about 10am to 4pm. I took off the pancake so I'm only seeing water come out the pipe, not water that is stored up in the pancake. I have not used the shower for about 2 months, so this is new water, not old water left in the pipe from the last shower. About a tablespoon of water comes out every day in spurts.
I've replaced the Watermark volume cartridges twice (two new ones), so I really don't think that's the issue. I don't think its the valve seat, because the water only leaks during certain times of the day.
There is no correlation whatsoever with the water heater running. I don't have an overflow tank on the gas-fired water heater.
Our house has a Zurn 70XL PRV, and I have confirmed that it is set at 58 PSI. I have monitored the pressure at a hose bibb during the day when the shower is leaking, and the pressure never rises much, perhaps to 70 PSI occassionally for an instant. My pressure gauge has a red needle that marks the high-pressure that occurred.
I called the City. The water pressure at the street is bout 70 +/- 7 PSI. The location of our municipal water tank supply apparently is inconsitent with ever being able to get more than 70 PSI from the street. The City said there are no spikes over this amount ever.
Our house does not have a check valve with the street (we have an open system), and the Zurn has an integral bypass, so the pressure in my pipes should never really exceed 77 PSI, which isn't enough to cause a leak, IMO. I have an identical Watermark shower valve located nearby that never leaks.
At this point, I'm thinking that the sun is heating the pipes and causing thermal pressure build up. however, I'm confused as to why the Zurn bypass doesn't bleed the pressure back to the street, or why my pressure gauge at the hose bibb never reads higher than about 70 PSI, yet I'm still getting a leak ONLY during the day. This has been happening for weeks and weeks on end, and never have I seen a drop after 4pm or before about 9am.