Hello this is Scott,
I have had this happen before, but it only seems to happen with units above the water heater, My 2 thoughts are the a failed checkvalve in the pump or a failing zone if the air handler has one. In the problem units I had I installed hard shutoff ball valves at a location convienient to the homeowner.
Thanks Scott - the Apollo unit is equipped with factory installed shut off valves which I have closed while I determine how to resolve the problem. This is not a zone system but the unit is located above the water heater. Could it be relevant that prior to this past heating season the homeowner installed a new high end hydro / potable specific water heater ? (Never had a problem before). Also, If I am unable to come up with another resolution and to promote simplified customer operation of the unit, would installing a low voltage n/c solenoid valve in the supply water side of the coil be advisable. Should the valve be activated by the thermostat system switch (leaving the valve open while in the heat mode) or opening on a call for heat? Mike (SOSHEATCOOL@GMAIL or m.Customer@gmail)