I have an in-fllor basement hydronic heating system, using a standard, dedicated hot water heater. The system's worked great for the 8 years we've been in the house, which is 14 years old. Earlier this year, the hot water heater's pressure relief valve was leaking water through the overflow tube. When heating season came back aroundthis fall, I drained the water heater, replaced the valve & re-filled the heater. Unknowingly, I replaced a 30 psi presssure relief valve with a 150 psi temperature and pressure relief valve. Fearing that would create a problem, I replaced that with the proper 30psi pressure relief valve, exactly the same as the original that I had. I did that without draining the tank fully, and with hot water still in the tank. When I attempt to re-fill the tank, the pressure valve opens and freel gushes water. Any advice as to what I've done wrong, or what I have neglected to do at all? Could I have damaged some other part of the system when the 150 psi valve was in use?

I don't know much about these things, so any hel will be greatly appreciated.