For the record, anyone looking to hook up a coal boiler to an existing forced air furnace, this is the fan control (Honeywell L4064) you need. I had a hard time figuring out what to do to make the fan run properly. The problem is that my forced air furnace is a newer model which uses a timer instead of a thermostatic switch to run the fan. The sequence is as follows: thermostat calls for heat, the exhaust fan comes on, then the burner lights. 90 seconds later the fan comes on and runs until the thermostat on the wall is satisfied OR one of the safety switches opens. Once the house is up to temp the burner shuts off and the fan continues to blow for 120 seconds to expel and heat left in the plenum. That is what makes it 90% efficient. I have 2 thermostats now, one turns on the coal boiler and starts the circulator pump, and once the plenum gets above 140 degrees the fan comes on and warms the rooms. Once the thermostat is satisfied the stoker shuts down but the fan keeps going until the plenum is below 100 degrees. Cold air never comes out of the ducts. During this weather where it's 30 at night and 60 during the day I leave gate vales to the heat exchanger open so the heat exchanger stays hot and gravity flow warms the house without noticeably drawing down the boiler water temp. I put an on/off switch on the fan line to prevent the fan from coming on when doing this, otherwise the fan will run and cause the boiler to cool down and waste coal.