I have a Watts thermostatic mixing valve in my radiant heat system that seems to be partly blocked. I have emailed Watts about how to clean it but did not receive a reply. Back flushing through the valve did not seem to help. If I remove the valve, do I just mechanically clean it with a brush or should it be soaked in some sort of cleaner? Any suggestions would be appreciated.

I recently had to replace my expansion tank and now I can't regulate the pressure in the boiler. It goes from 0 psi when cold to 30 psi when hot. I have added pressure to the expansion tank and then reduced it. The last time I messed with it, I added water until the relief valve popped but the pressure reading was still 0 PSI. But, when the boiler warmed up, it went back up to 30 PSI and only discharged a small amount from the relief valve. If I test the pressure in the Expansion tank with a tire guage it reads about 18 when cold and 28 when hot. Would a faulty presure guage behave this way? If I replace the combination temperature/pressure guage do I have to drain the boiler again? The boiler is a Peerless and I'm using it in a two story house with radient floor heat. I can't figure out what I'm doing wrong and would appreciate any advice. Thanks.