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[hondaracer2oo4]

RSI, yes I have come to the same conclusion that running a 24 volt transformer from the fan limit switch and running the 24 volt out to a relay on the owb would be easiest and best to do. Not sure where you are concerned about backfeeding power though? The OWB thermostat only powers the FHA blower by way of a Honeywell RA89A switching relay. The line voltage from the RA89A is pigtailed together with the line voltage from the fan limit switch, then the line voltage heads to the FHA blower.

[RSI]

Then power will get to the transformer you are adding when the OWB thermostat calls for heat.
You can test it easy enough, just wire everything up and turn up the OWB thermostat. If the pump shuts off you will either need to move the transformer to the oil burner control output or add a 120v relay.

[RSI]

Just another thought, you might be better off replacing the Honeywell RA89A with a DPDT fan center relay. Then you could use it's internal transformer and not need to add another and not have the issues of the OWB thermostat also killing the pump's power. (would require a slight rewire on the blower)

[slimjim]

Maybe i'm not understanding? I'm thinking the goal is to kill the circ @ the wood boiler whenever the oil burner does fire, would you not be better served to run the circ off an aqua stat at the boiler only allowing the circ to run when the wood boiler temp is up to set point and another aqua-stat at the rad to kill the oil burner when water temp is up @ the rad, this could all be done locally @ wood boiler and Oil burner with line voltage

[hondaracer2oo4]

Slim, the goal is to allow the owb circ to circulate when I am using the oil burner to prevent freezing of the unit but cut the power to the circ when the oil burner or fha fan is running in the house. If I wired a low limit aquastat to the line at the owb to kill the power when it dropped below a certain temp it wouldn't be able to circ water to prevent freezing.
RSI, I do see what you are saying now. When the RA89A powers the blower it backfeeds line voltage to one side of the fan limit. The side that it backfeeds to is the side that the 24 volt transformer is on which in turns powers it and cuts the circ pump.