Replacing Electromagetic Thermostat with Electronic Honeywell Thermostat

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  #1  
Old 01-13-08, 02:27 PM
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Replacing Electromagetic Thermostat with Electronic Honeywell Thermostat

I have an old T8085A Honeywell thermostat driving my Bryant 396J Gas Furnace. I want to replace the thermostat
a Honeywell RTH230B. I do not have air conditioner installed. I tried following the instructions on the
manual of RTH230B, but am confused when I opened up the existing thermostat box.

The old one has the wire configuration as shown in the attached picture. The Yellow is not connected to yellow,
but to a "C". There is one more "C" on the old thermostat which is left empty. A Blue wire from wall is not connected
to anything.

Picture:
http://img161.imagevenue.com/img.php..._122_950lo.JPG


Now, my new t-stat has only Rh, Rc, G, W Y points to connect. I am unsure about the Yellow wire. What is the "C"
equivalent in the new RTH230B ?
 

Last edited by diywoman; 01-13-08 at 03:25 PM. Reason: Adding Link to picture
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Old 01-13-08, 03:34 PM
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Thermostat

You need to look at where the thermostat wires connect to the furnace. Make note of which wire connectes to which terminal on the furnace circuit board.
 
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Old 01-13-08, 05:01 PM
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Originally Posted by Grady View Post
You need to look at where the thermostat wires connect to the furnace. Make note of which wire connectes to which terminal on the furnace circuit board.
I checked the circuit board on the furnace. It has "C" "Y" "GC" "GH" "R" "W". The wires coming in are connected like this

The wire connected to "C" on the old thermostat (T8085A)( Yellow Wire) goes to the "C" terminal on the board. The "Y" terminal on the furnace board is left open.

Now, On the new thermostat (RH230B), there is no "C" terminal which I can connect to. Any clue on how i can go about it ?
 
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Old 01-13-08, 05:07 PM
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Thermostat

I would have to presume, lacking a 'C' terminal, the new thermostat does not need a common BUT that number is totally unfamiliar to me. Darned Honeywell anyway for putting different numbers on thermostats they sell at retail than the same stat sold to tradespeople.
 
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Old 01-13-08, 05:07 PM
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t8085a - demystifying its wiring

The T8085A has a clock (timer) that runs on 24V, so it needs power from the transformer. That is the reason for its subbase having that "C" terminal. That clock gets its juice from the "C" and "R" terminals on the subbase.

Having said that, and having read that you do not have cooling in your house, that yellow wire is sure to be wired down in your furnace to the "C" terminal on the furnace's terminal board.

Granted, the "Y" terminal on a thermostat's subbase should be the one having a yellow wire connected, but in your case, since you do not have central cooling, the service tech that installed this t-stat decided to use whichever wire he pleased, and it was quite allright to use the yellow.

The most popular thermostat cables have either "6" or "8" wires. One uses what one needs and leaves the others there, not connected to anything. In your case, your thermostat needed wires connected to terminals R, C, W and G. So there's nothing for the Blue wire to connect to, and is just sitting there doing nothing; and I can almost bet that if you remove the subbase you'll see one more unused wire, likely a brown or a black.

Hope this helps. :mask:
 
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Old 01-13-08, 05:24 PM
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Originally Posted by pflor View Post
The T8085A has a clock (timer) that runs on 24V, so it needs power from the transformer. That is the reason for its subbase having that "C" terminal. That clock gets its juice from the "C" and "R" terminals on the subbase.

...
Hope this helps. :mask:
Thanks pflor. This is the first time I am educating myself with this , so just confirming my interpretation of your reply.

So, in essence, I should be able to connect the new RTH230B with all the wires as earlier, except for one difference - that is leave open (& seal off) the Yellow wire connected to the "C" on the T8085A thermostat.

Is there anything other than that I would need to do before connecting this ? Just trying to avoid doing (or missing out on doing) anything which could damage the Furnace Control board.

Thanks a lot folks.
 
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Old 01-13-08, 05:33 PM
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RTH-230B demystifying its wiring

The rth-230b has 5 terminals: Rc, Rh, Y, W and G

On the thermostat side:
Keep the factory installed jumper between Rc and Rh
Connect the t-stat's red wire to terminal Rh
Do not connect anything whatsoever to the Y terminal
Connect the t-stat's white wire to the W terminal
Connect the t-stat's green wire to the G terminal

On the furnace side:
Disconnect the yellow t-stat wire from the C terminal
Keep the red t-stat wire connected to the R terminal
Keep the white t-stat wire connected to the W terminal

Keep the green t-stat wire connected to whichever one of the G terminals on the terminal board it is connected.

You will no longer have that yellow t-stat wire connected to anything. Both, the blue and the yellow are now laid off.


Your thermostat is energized by batteries only, that is why it does not have a C terminal. Remember to change the batts once a year or so

Hope this helps too :mask:
 
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Old 01-13-08, 07:03 PM
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It Works!

Thanks folks for all the help. I could install the new thermostat there. Its feeling good to see a little thermostat on the wall compared to the suit case like electromagnetic one which was pulling the wall down.
 
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Old 01-13-08, 07:32 PM
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I want to add my 1, If you find that that t-stat cycles the furnace on and off too often, change the CPH (Cycles per Hour) to 3.
 
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Old 01-14-08, 12:39 AM
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Congrats!
As Jay says, IF you find that the unit is cycling too often, the CPH could be changed.

Factory set CPH value is "4" [as per t-stat manual].

Not a difficult thing to do to change the CPH. BUT if you're quite comfortable with the way the t-stat and your furnace are working together, just leave it alone
 
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