I have a four-unit building with an older ack-o-matic hot water boiler, 4 taco zone valves, 1 B&G 100 series pump. A month ago, I noticed when the boiler was fired up, the pressure would get up to 25-30 psi and the pressure relief valve would start to drip quite a bit. When the burners turn off, the pressure comes back to 12-16 psi and the relief valve stops dripping. This happened no matter what heat temp the boiler was set at (150 - 180). Fortunately, all 4 zoned units were getting good flow and heat. I first drained the expansion tank (closed the valve to it first and then opened it when I was done). That would make the pressure stabilze for a few days, and then the same thing would happen. There is no hole or drip in the expansion tank. I called an HVAC pro, who did what I did and said it looked good and left, but of course a few days later the problem was the same. I did the same thing 3-4 times, same outcome and eventually even after the expansion tank drainage, the pressure would still get too high a few minutes later. I then drained each unit separately and there was no gurgling and the flow was good. Then I checked the feeder and it would always fill up to 12 psi when I drained some water out of the system. The HVAC pro returned and he tried draining separately from the expansion tank, relief valve, and the bottom side of the boiler itself and in each case no air but the high pressure remained when the burners come on. We also tried bleeding the baseboards in all four units (each room) and there was no air. He kind of gave up and left. Since then, it has been about the same but now there is a small knocking sound and with that I can see the guage on the pressure meter jump every few seconds. I also tried to let the boiler cool, drain some water, let the feeder get it up to 12 psi, close of the fresh water supply before the feeder and let the boiler run. It would still go up to 18-20 but then when the burners were off the pressure would come back down to 6-8 psi. Right now, I left it the way it is but I am very nervous about the high pressure and I am sure it will destroy the boiler eventually. I've has a few HVAC pros and they don't know why it is happening and are looking to replace any and all parts. Anybody have any ideas about what might be wrong or what else to try? Thanks...