I need help. I have a gas Amana furnace that I have hooked to one thermostat (4 wires) and then a second thermostat (2 wires) hooked for only the fan to come on with use of my outdoor wood furnace. When the 2 wire thermostat call for the fan to come on, the outside compressor will come on. It seems the power will “bleed” to the Yellow wire. It doesn’t do this if I just run the fan on the furnace without the 2nd thermostat. I have turn off the compressor, but don't think it good to run it like that.

Wiring on HVAC Control Board to Thermostats
R = Red 24 VAC
W = Heating
Y = Cooling
G = Fan

2nd Thermostat hooked up to the Red for power and Green for the fan.
Compressor is attached to Y & C terminals on HVAC Board.

I have read and been told I need a relay to keep the Green and Yellow apart. What relay would I need and how to fix this problem. Thanks so much for your help.

Matt

Before you do some damage stop. You need to have the thermostat for the wood furnace totally separate from the thermostat for the regular furnace.

Hook up the wood furnace with a two wire thermostat BUT use a separate 24 volt transformer and a relay. The relay will be used to jump the fan connections for the gas furnace. This will isolate the thermostats from each other with separate power supplies. It will also return your gas furnace thermostat to its original configuration and stop the interference.

There are many other ways of doing this but I figured the simplest would be best.


Simple relay wiring diagrams are available by doing a Google search

HC1000, Thank you so much. I was headed to get parts when my HVAC called and said they were on the way. I had decided to do it myself when they want two service calls to fix thermostat and service furnace. He said your advise was dead on. I hadn't hurt anything because fan was set for same speed on cool and heat. I should have known I needed to get on the high side and not to mess with low voltage. Thanks again.

Matt