My upstairs air handler (Lennox Signature CBX32MV) is not increasing the blower speed when the thermostat calls for heat. I have two air handlers fed by hot water from an oil furnace that also provides hot water. The downstairs air handler and hot water are fine and the upstairs was fine until a week ago (further down I have a possible intervening issue)

When heat is called upstairs, I hear the coil filling with water (and the circulator pump next to the oil furnace is working). The blower fan starts and runs at what I believe is 490 cubic feet/minute - a very gentle air flow (A-490 is from the air handler's circuit board's 7 segment LED). After several minutes I get an error message on the Lennox Harmony III Zone Control of 'Insufficient Heating' (0101).

When I test the blower in idle mode using the built in test on the circuit board in the air handler, I get over 2000 cubic feet a minute. Nor error messages (thus far) on this circuit board. Configurations are correct matching board with unit.

I am thinking the sensor is not identifying the increase in heat as air passes over it from the coil as the latter gets hotter. Therefore it doesn't call for the blower to increase speed. But.......

The sensor was recently replaced (within last two years). And the system was working fine a week ago. Last Tuesday I had the annual check-up of the oil furnace done. After the technician left, we had no heat - either upstairs or downstairs - and no hot water. They came back two more times and still didn't fix the issue until a third return this past Saturday identified the electrodes to be too high. They also replaced a sensor in the combustion unit and told me the expansion tank was shot. Since then hot water and downstairs heat has been fine.

Could the continual call for heat from upstairs, when the boiler wasn't firing, have done something? Is it too much of a coincidence for it to have been working then not working after the service call (but coincidences happen!). Or is it a faulty temperature sensor downstream of the coil? Or is it the shot expansion tank; I have just ordered the replacement? Or something else?

Thanks in advance for any suggestions.