Honeywell Thermostat Problem

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?
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Achattaway

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This is Turtle.
I can't see it from here but it sounds like a grounding of the thermostat problem. Manual thermostats don't need these grounds but digital does need these grounds to work properly. check out the paperwork on the thermostat and see about the grounding of the thermostat and which wire will do this.
TURTLE
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TURTLE wrote:

Doesn't that depend on whether or not they are battery operated?
check out the

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CBHVAC wrote:

This is Turtle.
I see you corrected me on the working of ground on the thermostat as a common and not a ground. Well slick the wire that I called a Ground leaves the thermostat and goes to the common side of the 24 volt transformer and then RIGHT OVER TO THE FRAME OF THE FURENACE TO A GROUND LUG. So now slick what is it a Ground or a Common.
Now after all this correcting of myself you did nothing for the poster but tell him a bunch of bullshit to think about. He did not want to hear all this bullshit from you but what was wrong with it to get it right and you did not answer anything for him but corrected me out of a flame war over at alt.hvac.
Leave the Flame CORRECTING back at alt.hvac !
TURTLE
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Not a chance...So you gonna start calling a R line a hot? Call it what it is, and act like what you claim to be...
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On Sat, 3 Dec 2005 18:04:09 +0000, Achattaway

Not being able to see what you have is a problem. what type of controls do you have? how many transformers and what VA are they? +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ spam protection measure, Please remove the 33 to send e-mail
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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

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Thanks for the replies so far. I have checked out the following as suggested: The wiring instructions for the thermostat do not show a requirement for a ground at the terminals, and it does not require a battery. An identical thermostat unit installed on the bedroom zone is working correctly. The transformer is rated at 24V so that rating should be fine with these thermostats. I have been monitoring excatly what seems to be happening: The display is on and showing the current room temperature. When I turn the dial to set the heat temp I want for the room the flame symbol appears and a few moments later the furnace fires up. After a short period of time the furnace cuts off, and after another short period the valve closes. When the valve closes, that is when the display shuts off. Several minutes later the display comes back on the valve has reopened, the room is not yet at the requested temperature, and the furnace may fire up again. After a short period of time again when the valve closes the display disappears again. This continues until either I switch the required setting to below the current room temperature, or when the room reaches the heat setting. I tried swapping the front panel from the bedroom zone, and the same thing happens. I am trying to determine whether the fault lies with the valve which is a few years old (not sure quite how old), or whether there is a fault with the thermostat before I go to the expense of replacing one or the other. Your suggestions are most welcome.
RBM Wrote:

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On Sun, 4 Dec 2005 16:51:09 +0000, Achattaway

24 volts is normal but I really need to know how many transformers and the VA rating of each before I can even take a guess. +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ spam protection measure, Please remove the 33 to send e-mail
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If you remove the stat and touch the two wires together, does the valve open and circulator run?. The boiler won't necessarily start as it may already be at high limit. If things run properly by touching the wires together, and since you say you've already swapped the thermostat with another, I would be looking for a weak transformer powering the zone valve(s). There may be one or several. You need to find them and look for the wattage (VA) Standard heating transformers are 40 VA, but sometimes zone valves are powered by the transformer in a switching relay or aquastat relay.

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