This thermostat doesn't require a common, just the two wires. I would look
to the wattage of the transformer that's powering the three zone valves


"Achattaway" wrote in message
...

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?


--
Achattaway