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One zone on boiler overheats and blows pressure relief valve
I have a Dunkirk boiler with two zones heating a two story house. The second floor zone has been functioning all along. I had a bad Grundfos pump on the 1st floor. It moved freely with a screwdriver but would not move when calling for heat. I replaced the pump. The new pump comes on and off just fine. While trying to refill and bleed the system I noticed the really old pressure reducing-Auto fill valve was so plugged with debris that it was not working so I replaced that along with installing a new ball valve before it. If I fire up the boiler with only the second story zone running it works fine. Pressure on the boiler is 15 when cold. The cold pressure on the one year old expansion tank reads 15psi cold as well. When I run the second floor zone it runs perfectly. It shuts of at 180 and runs at about 20psi on the boiler. This zone has been functioning fine all week. When I turn on the first floor zone the pump turns on but I get no circulation in the zone unless I manually open the flow control valve. Once open I can hear the water flowing through. Within minutes of doing this the temp runs away and blows the pressure relief valve. I have tried this with the water fill supply both off and on. I am stumped. Any ideas out there?