I have a 50 Gallon State water heater attached to hydronic heating system in my home attic. I had the water heater professionally installed by a plumber about 5 years ago. The unit had a problem with the 150 PSI tp valve blowing immediately after the installation. The plumber was frustrated and confused at why it was blowing. He added an expansion tank just to be safe. He also installed another new tp valve and its worked fine for 5 years. About six months ago I noticed the tp valve blowing again. I've had a couple of plumbers come out and they can't figure out what the problem is. They checked the expansion tank and it isn't full of water and appears to be OK. I performed my own investigation. Water pressure at a hose bib outside of the house is a constant 60-65 PSI. I placed a pressure gauge on the water heater at the drain bib as well. I discovered that pressure at the water heater bib would reach 140 PSI towards the end of the water being heated after someone took a shower right before the burner would kick down. The 140 PSI slowly drops after the burner turns down. The PSI at the outside hose bib remained a constant 60-65 PSI. The tp valve starts leaking a small, but steady stream at 140 PSI. It's obvious to me that the tp valve is bad because it shouldn't blow until 150 PSI, but I'm being told the internal pressure of the water heater should never get that high. I called the Tech line at State Water Heaters and told them of my problem. The temperature setting on the water heater isn't to hot and is set at 120 degrees. It was at 125 degrees. State says the internal pressure of the water heater shouldn't get over 80 PSI. The pressure gets to 140 PSI without the hydronic house heater being on. The weird thing is the water heater pressure recently stopped jumping to 140 PSI and stayed at 65-70 PSI through the heating cycle. It was fine for a about 3 weeks and the problem has started up again. Should the internal pressure of the water heater get up to 140 PSI? The manufacturer says no. Does anyone have any idea what could be going on here?