WHenever we update our A/C system we will have a constant running fan and then we can have a humidifier and air filter system. Because of our hot water heat, the forced air part of the heating system doesn't run until the baseboard fin tubes can't keep up. (usually when the outside temp get's below zero or the wind chill is sub zero) There is a large heat exchanger in the ductwork that is fed by a zone pump from the boiler.