This is a natural gas boiler, but that doesn't really matter. A couple weeks back I drained down teh system to install a small water-air Modine heater in my garage. My house and garage are slab heat. I put in the Modine to help with warming up the garage faster. Anyway, I refilled the system, purged the air and all seemed ok. A few days latter I check on everything and my boiler pressure was pushing close to 30psi! Normally it sits around 15psi. I bled some water off and seemed to be ok. Also turned the regulator down a couple turns. Few hours later go check on it and the regulator is pissing out water. I thought maybe it failed, but know I have it turned off and the pressure is going from 0psi when the boiler is cold to 20psi when it's warm. I have never had that kind of change before. The only other thing I did was add more air to the expansion tank. It only had about 5psi in it. The info I was told is it should be system pressure + 2-3psi. I filled it to 18psi. Any ideas on what is going on? I'm going to replace the regulator because it's leaking, but the pressure issue has me confused. Should I let air out of the expansion tank?