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[pwdiver]

Ive had a 2 wire thermostat piggybacked in to my Honeywell thermostat so in the winter I have the heat pump set to 67 degree's and use the 2 wire thermostat to control the heat in the house by just cutting the air handler fan on and off, I want to do away with the 2 wire thermostat to the honeywell and just use it. Right now I have it working that way but I had to disconnect the control wires for the heat pump. Every thing is working OK but I had wanted to have the heat pump act as my backup in case the boiler went out just couldnt keep the compressor from trying to come on. Ill try to post a picture later this morning.

[RSI]

You want the Honeywell RTH8500D thermostat to control heat from the OWB and also run the heatpump without any additional controls? If so, I am not sure if that is possible. It looks like it is a 2 stage thermostat but doesn't appear to have the 2nd stage run a heat pump.

Do you just need to run the fan for the OWB heat or do you also need to control something else. You are probably best off using a strap on aquastat to keep the heat pump from running when the OWB lines are hot. The thermostat is most likely already energizing the G wire when the heatpump is calling for heat so you would just need to run the Y wire through the aquastat to break the connection when the pipe is hot. If you have an O connection for the reversing valve then you should be good with that. If you have a B connection then you may want to get a 2 pole aquastat and also break that wire too. If you don't, it will just cycle the changeover valve every time the thermostat calls for heat.

[pwdiver]

Thanks for the reply, do you know of a thermostat that might be able to do what i want, Just control the fan for main heat then have the heat pump as the backup

[RSI]

You may want to look through some old posts here about using a 2 stage thermostat. The biggest issue is backup heat coming on when you don't need it.

You could make that thermostat run the heatpump on 2nd stage with a relay but it probably wouldn't have the compressor protection delays.

[pwdiver]

This is my current wiring that is controlling the fan for heat with the outdoor boiler but I have the heat pump and emergency strips disconnected. It works great this way but want to have heat pump as back up

[RSI]

Is there a reason that a strap on aquastat won't work? You will probably screw up the A/C if you make it do what you want.