Wood boiler with gas furnace with ecm blower

It depends on the model and brand of the furnace. The problem with simply breaking the Y signal on this type of furnace is the logic board and how it controls the ECM blower motor.

The general consensus is that, on a furnace with HEAT, COOL, and FAN ON functions, the control board sends different commands to the ECM motor depending on the thermostat inputs. A call on W from the thermostat engages the fan timer and operates the blower at the programmed heating speed. A call on Y and G tells the board to operate the blower at the cooling speed. However, a call on G only is often interpreted as a continuous circulation or ventilation mode, causing the blower to run at a very low speed—sometimes only 10% to 15% of full airflow.

This is because many modern furnaces are programmed to run the blower as slowly as possible during continuous fan operation in order to improve air filtration efficiency and reduce electrical consumption.

When you disconnect Y, as you might on an older PSC blower motor furnace, the blower may no longer operate at either the proper heating or cooling airflow. Instead, it may run so slowly that the heat from the duct coil is not delivered to the rooms with enough CFM to perform properly.

As a result, you may need to use a series of relays or other control logic to “fool” the furnace control board into operating the blower at the desired speed.

Tell me the model and brand of your furnace, and I will look up the control system and see if I can make a diagram showing how to get the blower to operate the way you want. Also what thermostat are you using? is it the legacy R, W, G, Y, C wiring or a communicating system where there may be 2 wires for DC voltage and one or 2 wires that communicate like ABCD or 1,2,3,4. That is important to know.