I already drained the system and expansion tank...there is an automatic-feed valve already on the system.....a professional told me the thermostat must be set to high and therefor calling for heat all the time......he said the furnace can`t keep up with the demand and thus the water is turning to steam....is this possible?....maybe i need a new expansion tank since it is +30 years old.....maybe i should have said before that this woodfurnace is an add-on to an oil furance...thanks again