when I wire a two stat system one for wood boiler and another stat for furnace and central air my central air is getting back fed power, how do I resolve without putting in a winter summer switch?
The exact setup is dependent on several elements to include make and model of thermostats, make and model of the central air system, make and model of the boiler, and the type of wiring used to connect the systems. Without knowing the specifics, nobody can provide you with an adequate answer. It is possible that your system setup would require the use of a isolation switch when operating the central air.
The easiest thing is to get a 2 stage thermostat. One stage for your central heat and the 2nd stage wired for your boiler and fan.
I assume you are trying to run the furnace fan only to circulate heat from the outdoor boiler while retaining the ability to use the furnace and central A/C normally. A separate control transformer should be used for the outdoor boiler thermostat then you can use a simple isolation relay to parallel the fan contacts on the furnace control board. If the furnace fan is wired for two speeds then your isolation relay should be a single throw double pole relay so you can isolate the speed that you will not be using with the wood boiler to prevent the possibility of both speeds being simultaneously energized. Trying to jump the existing (G) circuit will result in a backfeed that will bring on the outdoor unit especially if the existing thermostat is a mercury bulb type.
