Replace two thermostats with a single unit


  #1  
Old 07-01-17, 07:08 AM
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Replace two thermostats with a single unit

Hello,

I have a separate heating (natural gas boiler with radiators) and air conditioning (forced air heat pump) unit which are not connected at all. Each unit has a separate thermostat (see below, round unit is for heat and square unit is for air conditioning).

I would like to modernize these thermostats and replace with a single unit. The air conditioning thermostat has three wires (red, yellow, green) which are not labeled. The heating thermostat has two wires (red, white) which are also not labeled.

Note: The A/C thermostat has a heat option but it is not in use.

I think the model numbers for the thermostats are as follows:
Heat: Honeywell T87F 2873 3 9744
A/C: Carrier T87F 2543 2 HH01AD040 BX4

Unfortunately I cannot find any manuals or wiring diagrams for these old thermostats.

I'd like to know:
(1) is it possible to replace these with a single unit?
(2) if yes, do you have any advice? I'd like it to be programmable, but wifi is not necessary.
(3) does anyone have an idea about this non-labeled wiring? Or have a link for them?

Thanks!

Front view of each thermostat, A/C to left and heat to right:
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NOTE: The following photos should be rotated 90 degrees, I don't know how to do it in this forum...

Cover of A/C removed showing model number:
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Wiring of the A/C:
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Close up of A/C wiring:
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Cover of Heat removed showing model number:
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Wiring of the Heat:
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  #2  
Old 07-01-17, 07:46 AM
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Those are old Honeywell rounds, good stats. Shouldn't be a problem using 1 stat for both. Use the cable from cooling stat. It has a white and blue spares.
 
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Old 07-01-17, 12:00 PM
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I recommend a Honeywell TH8321R thermostat.
C is optional but it looks like you have an extra blue wire fore C.
Connect green to G yellow to Y and red to Rc.

Connect the heater red wire to R and the heater white wire to W.
No jumper between R and Rc on the new stat.
 
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Old 07-02-17, 07:18 AM
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Great, this actually seems really easy. I think the most difficult part will be fishing the wire from one side to the other. Thanks for your help and advice.
 
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Old 07-02-17, 08:54 AM
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Houston204, I have one additional question. You said "C is optional". Maybe I'm wrong, but I understand without a power source the thermostat is drawing power only from the batteries. I'd prefer to have a wired power source so that the heat doesn't go out in the winter time when the battery dies.

Because I have two separate units, will connecting the C wire cause a problem? I was worried that the thermostat might create a circuit between R and C to power itself, and in my case R is going to the gas boiler and C is going to the A/C unit.
 
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Old 07-02-17, 12:05 PM
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Running from the batteries is something that I would avoid if possible as well. I would not install the AA batteries at all. Leaking batteries the most common reason that I replace digital thermostats.

The thermostat draws power from RC and C.
Connect the blue wire to C at the air handler and at the new thermostat.

The button cell battery is all that is required if you have a common wire, and I believe it is already installed, but you must pull a small piece of tape to allow it to work.
 

Last edited by Houston204; 07-02-17 at 01:41 PM.
 

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