I have three heat zones in my home, two upstairs and one in the basement. I recently upgraded my old mercury Honeywell thermostats upstairs to a digital type CT8775A. They are connected to Honeywell motorized valves at the boiler, type V8043E1012.

When I installed the thermostats I was careful to map the wiring and everything seemed fine, the digital display came up, the room temp was monitored and the backlight was working fine. However, now that I am starting to need the heat the problem occurs. When the room is above the temp of the thermostat setting everything is fine and the display works. When I raise the temp setting on the thermostat to turn the heating on, and after the furnace has fired up, the display goes blank, the backlight doesn't work but the furnace continues to burn. When the furnace switches off afet a couple of minutes the display reappears, only to disappear again.

I have tested the wiring connections at the boiler and behind the thermostat and they are all good. I have manually overridden the valve and tested the motor which appears to spin up OK. I don't know where to look next. Has anyone else had the same problem or can anyone suggest what the problem might be?