On my furnace there are two blowers. One is the main blower for circulating air throughout the house. The other is a smaller blower that pumps the hot combustion air to the flue vent/exhaust duct (and then up the chimney). I was referring to the latter (which I call the “combustion blower”).
Due to obvious safety concerns (carbon monoxide and all that stuff), the furnace should not be allowed to operate if the flue vent is clogged, and/or if the combustion blower is not working. So furnace manufacturers measure the flow of the combustion air immediately after the combustion blower. If there is no airflow, the furnace’s control system will assume the flue vent is clogged, and/or the combustion blower is not working, and it will shut down the furnace.
To measure the airflow in the combustion exhaust duct, a differential pressure sensor measures the difference in air pressure between two points in the air path in the blower housing. To do this, the blower housing has two small ports about 1 inch apart. Flexible hoses connect each port to a differential pressure sensor located about 10 inches away. When air is flowing, the sensor will “see” a difference in pressure, and will close an electrical switch located inside the sensor. This will send a signal to the controller that says, “I see airflow in the combustion exhaust duct. Continue operating.” If the controller does not get this signal, the furnace will kick on, not see flow, kick off, kick on, not see flow, kick off, etc. etc.
The flow sense system is easy to troubleshoot. Start by simply disconnecting both wires from the differential pressure sensor and connecting them together. Does the furnace operate correctly? If so, then the problem is with the flow sense system; if not, the problem lies elsewhere.
If the problem is with the flow sense system, start by disconnecting one of the hoses from the blower housing and suck or blow on it until you hear the switch inside the differential pressure sensor actuate. If you can’t get the switch inside the differential pressure sensor to switch by sucking/blowing on one of the hoses going to the sensor, then the differential pressure sensor is likely bad. If you can get it to switch, then it’s probably one of the following:
- The flue vent is clogged.
- The exhaust blower isn’t working.
- One or both of the small air ports are clogged.
With my furnace, I established that #1 #2 were not the case. That only left #3.