Burnham furnace will not fire for one zone of two zone system

Grimption

Hello and thanks in advance for reading my post. Here is a brief description of my forced hot air heating setup:

- 6-7 year old Burnham gas fired hydronic system

- 2 zones, one up and one down



Problem: When zone 1 (main floor) reaches the trigger point, the air handler fan turns on but the furnace does not. The symptom is cool air blowing out of the floor ducts.



Actions that do not fix the problem:

- Turning the thermostat off and then back on

- Changing the setpoint for the thermostat

- Changing the differential for the thermostat



Action that does fix the problem: If I go upstairs and change the thermostat setting, forcing the heating system to turn on for zone 2, both the air handler fan and the furnace will turn on. If I then shut off the upstairs thermostat, the furnace will stay lit and the symptom (cool air through the main floor vents) will go away.



I will be calling a heating contractor tomorrow, as the Burnham needs a cleaning anyway. However, I'd like to get some idea of what might cause the behavior described. Any suggestions are appreciated.



Thanks,



-chris