Wiring issues with Honeywell CT87K


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Old 10-30-16, 01:51 PM
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Wiring issues with Honeywell CT87K

Recently my old Honeywell mercury switch thermostat (CT87A) broke, it was used to control a gas fireplace. On taking the old thermostat off, I found it was hooked to 2 wires (green and white) with a third wire (red) not used. I then went out and bought a new Honeywell CT87K because we like the simplistic style. However, I can't seem to get the wiring correct to make the thermostat operate like the old model. There are three posts to connect to R,Y and W. R and W appeared to be the pattern for the old style thermostat, but nothing happened when I wired the new model up the same way. Oddly enough, R and Y immediately turned the fireplace on but with no control over on and off. Anybody have any suggestions?
 
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Old 10-30-16, 02:49 PM
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Welcome to the forums.

R and W are the terminals you should be using.
Where did you have the system switches set ?
I'd try 1 and 2 both off.
 
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Old 10-30-16, 04:07 PM
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Yes, both switches were left at the factory specs. Both off. Thank you for the information. Two wires, two terminals, at least it narrows options. The frustrating part is that I tried these options and could not get the fireplace to activate once I toggled the thermostat higher then the stated room temperature. What are the odds that it's just the thermostat? A new thermostat might not mean a working thermostat.
 
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Old 11-02-16, 01:30 PM
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Wiring puzzle with CT87K Honeywell Thermostat

I posted this problem a few days ago, followed the advice given and I am still being stumped. Went out and got a new thermostat (same model) and I'm still getting the same results. Nothing. Swapping the green and white wires between the R and W terminals is still giving me nothing for the operation of my gas fireplace. However, once again green and white in either of the R and G terminals immediately starts my fireplace with no on or off control. The factory settings are still in the off positions. I'd really hate to call in an electrician or heating specialist for something that seemed fairly straightforward and simple.
 
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Old 11-02-16, 05:25 PM
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Did everything work before you replaced the Stat? what color wires are connected to the fireplace?
 
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Old 11-02-16, 08:14 PM
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I combined your two threads. It's extremely confusing to have two running on the same topic.

I'm a little confused. You mention R and W and R and G.
The 87K only has R, W and Y terminals.
 

Last edited by PJmax; 11-02-16 at 09:18 PM.
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Old 11-03-16, 12:23 PM
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The old mercury switch thermostat was on its way out when we bought the house. We were told that the thermostat "Would stick sometimes and needed to be tapped on to start the fireplace." Years of this process took its toll on the old thermostat until it rarely worked. The wires going to the fireplace unit are black and white. Somewhere in the walls it changes to the old wiring of green, white and red. Only the green and white wires were being used with the old thermostat. My bad on the terminal lettering, it's as you posted, R,W and Y. When I connect the green and white wires to the R and Y terminals I get an immediate fireplace start with no control. Sorry about the posts, I'm still getting used to this site.
 
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Old 11-03-16, 03:18 PM
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No apologies necessary.

I'm still trying to figure out the issue with your stat. On a call for heat.... W and R are supposed to close. They used to put a heat anticipator on thermostats that was adjustable. Your old round Honeywell had it. Your new one shouldn't.

If you have the installation instructions that came with your stat.... post the publication number.
 
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Old 11-03-16, 04:06 PM
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The green and white from the fireplace should be connected to R&W the Y terminal should not be used.
 
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Old 11-03-16, 06:48 PM
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I hope that this is the publication number.
69-1959EFS-03
 
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Old 11-03-16, 08:49 PM
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Geo.... they've tried that on two different stats.

That publication is the same one I was using... thanks.

I think I know what the problem is. The old round stat used a mercury switch on a temperature activated swivel. So when that stat called for heat it created a dead short on the R and W terminals.

The new replacement stat uses the 24vac line to activate its circuitry. The problem is that your fireplace is not 24vac... it's called a millivolt system.

You will need to purchase a stat that uses batteries. Something in the RTH110 line would be ok.

Sorry I didn't think of this sooner.
 
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Old 11-03-16, 10:04 PM
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Thank you for all the help. I have called the Honeywell service line twice and never came as close to a relevant answer as what you have provided me.
 
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Old 11-04-16, 05:30 AM
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Sorry about that Pete,that was a good catch on your part,I use an Aube TH135-1 stat on my gas log (power pile) and it works great,I think Aube is a Honeywell product.
Geo
 
 

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