I have a great Peerless (WBV-03) oil fired boiler that's about 6-7 years old. It's got a Beckett (R7184B) controller, a single Taco pump and three zones with Honeywell V8043E valves. I live in a 1,500 sq ft ranch with a finished basement and hotwater baseboard heat. The basement zone has a simple analog manual t-stat, the main floor zone has a digital manual Honeywell RTH110B t-stat and the third zone goes to my Amtrol hot water maker, which I love.

Yesterday I ran "heat test". The downstairs t-stat worked perfectly. I raised the temp, the zone opened, the boiler fired, and the pipes got hot. The Amtrol zone has been working fine all summer, so no need to test that. Then, I tested the main floor zone and it didn't work right. When I raised the temp on the main floor t-stat, I heard the t-stat click, as if to call for heat, but the boiler never fired. The corresponding zone valve DID open automatically, but I did not detect the pump running (the pipes didn't vibrate like they do with the other zones), and the pipes didn't get hot. At the time I ran the test, all the pipes were cold as no heat was called for elsewhere in the house. So, it seems the main floor t-stat is failing to ignite the boiler when heat is called for.

Any ideas why? Is it the t-stat or zone valve or mybe something else? I am a relative layman, but I have a multimeter and I'd like to fix this DIY-style if I can. Thanks!