thermostat wiring: heat pump + gas furnace (2H/1C dual fuel) with lockout

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  #1  
Old 01-16-12, 11:52 AM
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thermostat wiring: heat pump + gas furnace (2H/1C dual fuel) with lockout

I just purchased a house with a single-stage American Standard heat pump and gas furnace (aka 2H/1C dual fuel). The current system, controlled by a Honeywell FocusPro TH6220, does not lockout the heat pump when the gas furnace runs. I plan to replace the current FocusPro with a Honeywell Prestige YTHX9321 and will use the accompanying outdoor temperature sensor to control an automatic compressor lockout.

I would like to confirm thermostat wiring. Most of the wiring is straightforward, but I am uncertain of one connection with respect to ensuring that the compressor and furnace do not run concurrently. (I have consulted the heat pump wiring diagram, but that didn't lead to clarity.)

Here's a summary of how I plan to wire the new thermostat:

Heat Pump_________Gas Furnace_____Thermostat
common [blue->black]_____C__________C (common)
power [red]______________R__________R (power)
changeover [orange]______(wirenut)__O/B (changeover)
X/W heat [black->white]__W__________AUX (aux heat)
compressor [yellow]______Y__________Y (compressor)
(none)___________________G__________G (fan)

(where _ is used for space formatting since this system ignores spaces)

Observe that "thermostat AUX" is connected to "furnace W" and "heat pump X/W". (The optional heat pump outdoor temperature sensor between X and W is not installed.) Even though it might seem this wiring could lead to the heat pump running when the furnace is on, I believe it is correct for the following reason.

When the thermostat is in "2H/1C dual fuel lockout" mode and the thermostat turns on heat, it will do one of the following things. Either it will (1) energize Y, G and O/B to turn on the heat pump or (2) energize AUX to turn on the furnace.

In the second case there are two things to note. First, even though "heat pump X/W" will be energized through "thermostat AUX", the heat pump will not run because neither the compressor terminal (Y) nor changeover (O/B) will be energized. Second, the furnace controls the fan (G).

Is my understanding correct?

Thanks in advance!
 

Last edited by dukenesini; 01-16-12 at 12:05 PM. Reason: table formatting ignores spaces/tabs!
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  #2  
Old 01-16-12, 02:00 PM
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Upon further reflection, I can see one problem in my thinking.

If the heat pump can heat with thermostat Y, G and O/B energized, then "heat pump X/W" is superfluous not only for furnace heating, but also for heat pump heating.

It would seem that the connection between "heat pump X/W" and "furnace W" -- a connection that currently exists -- should be removed, resulting in the following diagram:

Heat Pump................Gas Furnace.....Thermostat
common [blue->black]_________C_____________C (common)
power [red]__________________R_____________R (power)
changeover [orange]______(wirenut)_________O/B (changeover)
X/W heat [black->white]......W_____________AUX (aux heat)
compressor [yellow]__________Y_____________Y (compressor)
.............................G_____________G (fan)

(where _ is a wire and . is a space)


But, I now I haven't explained what to do with "heat pump X/W".
 
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Old 01-16-12, 02:16 PM
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You had it right the first time.
The outdoor unit will energize W when it goes into defrost.
 
  #4  
Old 01-16-12, 02:24 PM
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Ahh, now the connection between "furnace W" and "heat pump X/W" makes sense!

Thanks!
 
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