I've had this pressured wood boiler system for 4 years now and it has been working great. This year when I started it up I had a problem with the system not circulating the heated water. The wood boiler showed 185 degrees, but the line 3 feet from the boiler and in my house were cold. I found my air bleed was plugged and I had air in the system. Now it heats up to 185 degrees and circulates the heated water fine, but my system pressure is 15psi at 185 degrees (just where I want to be), but drops to only about 1psi at 170 degrees. This is the temp that my aquastat opens the damper and starts the blower fan. Previous years the system would only have a couple psi change from 185 to 170 degree. I'm wondering if my expansion tank is not working properly? Why else would the pressure drop with such a small temperature change?