Pressure swings on hot water system

Marc_19

I am experiencing pressure swings on my hot water system. On coldest days the pressure is rising and the pressure release valve is discharging water.

My system:
Munchkin 199k BTU with Vision 1, 4 years old
Also making domestic hot water
Old four BR home with radiators
Sytem worked with no problems for 3 years
System set for max water temp of 170 on coldest days

Last year system began discharging water during coldest days. Bad PRV? Replaced it--still discharged. Then checked Watts-30 expansion tank. Pressure low, some moisture released. Adjusted pressure in tank--then still discharging from PRV. Bad tank? Replaced last month with Extrol 30, sized for my system volume and temps, according to their charts. Adjusted tank pressure to 12 psi.

Now: I am watching manifold temperature and pressure like a hawk. No water releases yet--no extreme temperatures yet.

System off: Temp 70, pressure 15 psi
Cold morning:Temp 140, pressure 25 psi, expansion tank 17 psi. (Outdoor temp 20.)

It seems that with my pressure this high on a 20 degree day, the pressure is going to blow the PRV (over 30 psi) again on coldest winter days. Can you help me with what is going on? Does it have anything to do with the system fill valve (Watts 1156F--tag reads "set 12-15 PSI, range 10-25 PSI)? Last winter as an experiment I shut off the water supply to the system and within a couple of days the pressure dropped until the system shut down.

I appreciate your help. All you pros really gave me some great advice in helping my choose my system 4 years ago.

Marc