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Old 08-16-2012, 10:25 PM   #1
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Wiring RTH 8500 from T874/Q674C


I have old Honeywell thermostat T874/Q674C with heat pump and baseboard heaters trying to wire it to RTH8500.

On Q674C base are wires

W1 - white
W2 - black
G - green
Rc,Rh - connected together red
B not connected
O not connected
Y2 not connected
Y1 - yellow

The manual for RTH8500 saying
If your old thermostat had Y1, W1 and W2 wires, stop now and contact a qualified contractor for help.

Can anybody suggest the wiring?
Thank you
Paul

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Old 08-18-2012, 07:01 AM   #2
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Wiring RTH 8500 from T874/Q674C


I would wire it the with w1- O and w2 - Aux and the rest is the same.
on pg 10 is the wiring diagram, if anyone else wants to look at it.
http://customer.honeywell.com/techli.../69-1902EF.pdf

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Last edited by Acomfort; 08-18-2012 at 07:03 AM.
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Old 08-18-2012, 07:34 AM   #3
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Wiring RTH 8500 from T874/Q674C


The C indicates that your thermostat is non heat pump. Your current wiring indicates you have non heat pump. What is the model number of your heat pump? what type of baseboards do you have? Electric, or hydronic?

http://customer.honeywell.com/techli...0s/60-2485.pdf
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Old 08-18-2012, 09:35 AM   #4
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Wiring RTH 8500 from T874/Q674C


Thank you for you reply.

The baseboard heaters are electric.
This is condominium unit, on the heat pump are stickers which Honeywell and McQuay Model CC260F 5
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Old 08-18-2012, 01:47 PM   #5
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Wiring RTH 8500 from T874/Q674C


well, I can verify that it is a water source heat pump and am still looking for a wiring schematic for your system. Does the current t874 control the electric baseboard? If it does you are going to have to do some investigating or get a pro out there to look at your setup. Typically a 24V control will not control a 240V baseboard heater.
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Old 08-18-2012, 02:00 PM   #6
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Wiring RTH 8500 from T874/Q674C


I would think it somehow does control the baseboard heaters - I do not have any other thermostat.

Thank you
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Old 08-18-2012, 02:09 PM   #7
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Wiring RTH 8500 from T874/Q674C


are you sure that there is no thermostat located directly on the heaters? Maybe a knob.
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Old 08-18-2012, 02:15 PM   #8
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Wiring RTH 8500 from T874/Q674C


No I just replaced them with newer smaller ones - Dimplex brand but there were no knobs on the old ones.
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Old 08-18-2012, 02:18 PM   #9
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Wiring RTH 8500 from T874/Q674C


so where does their power come from? It must leave the electrical panel and either go to a thermostat or through a relay/contactor. You can test to see if your thermostat controls them by switch it to heat and turning it all the way up. My guess is that it does not control them.
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Old 08-18-2012, 02:21 PM   #10
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Wiring RTH 8500 from T874/Q674C


OK thanks
I will check later, I know there is separate breaker for the baseboard heaters
Maybe it will be best for me to call some pro to check it.

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