replace 3 wire thermostat

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kylelee

kylelee

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hello. I am replacing a 3 wire thermostat in an old apartment. It controls a motor/valve which installed on the hot water pipe for the radiator. I just brought the honeywell CT87K to replace it. Can i use this thermostat to replace it? and the most important part is the old wiring is only label 4,5,6 on the motor. but my new thermostat is R.W.Y. So How do i figure out the wiring?
please check the pic
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While most zone valves are power open spring closed, yours looks like a less common power open/power closed. This means you need a SPDT stat which I think the one you bought is. You should have 3 wires at the old stat-red (power), black (heat close), white (heat). The new stat will be W heat, R power, Y heat close.
 
texie said:
While most zone valves are power open spring closed, yours looks like a less common power open/power closed. This means you need a SPDT stat which I think the one you bought is. You should have 3 wires at the old stat-red (power), black (heat close), white (heat). The new stat will be W heat, R power, Y heat close.
Click to expand...
but the problem is how can i figure out which number stands for which wiring. The old stat only shows 4,5,6

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I think he answered that

You should have 3 wires at the old stat-red (power), black (heat close), white (heat). The new stat will be W heat, R power, Y heat close.
Click to expand...

You new stat has W, R, Y so put the white wire from the T-wire to the W on the new stat. The red wire from the old cable goes to R on the new stat and Black wire goes to Y
 
Your valve isn't by chance a variable flow type and you actually need a potentiometer type thermostat to drive it? Makes sense with five terminals that it might be this type of control, two leads are input volts, three leads to the potentiometer, which has variable set point and also varies with ambient temp. How far the valve opens basically depends on how off balance the resistance of the circuit is. At set point it is balanced/near balanced.

R, W, Y designations on HVAC controls is usually R- L1, W- heat, Y- cool.

If terminals 4-5-6 are not for a potentiometer then they are likely Common, NO and NC aux contact terminals and not a thermostat terminal.
 
If that thermostat has a Heat-cool switch it will not work.

You could make it work by having the thermostat operate a rib relay with a 24 volt coil and spdt contacts. A call for heat powers the rib relay coil through the stat and the relay contacts (NO) open the valve. When the call for heat ends the stat opens drops out the relay and the relays (NC) contacts drive the valve closed

In the old days Honeywell had a T26 stat that would be used for this. That's probably a discontinued model. Don't know what has replaced it
 
synchro said:
That appears to be a Flair zone valve, and according to pg. 7 of the document linked below the terminals on the valve are:
4 - open
5 - common
6 - close

As texie mentioned: "The new stat will be W heat, R power, Y heat close."
So the wiring between the valve and thermostat terminals should be:
4 -> W
5 -> R
6 -> Y

Click to expand...
That probably is what he has, I don't know what his replacement thermostat function is but it basically would need to be SPDT design and as mentioned if it has a heat-cool switch on it it wouldn't work in this application.
 
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