About four months ago we replaced our existing heat pump system with a Carrier Infinity heat pump system (25HNB636 outdoor, FE4ANF005T indoor, SYSTXCCUID01-V Infinity Control) that incorporates an Aprilaire 5000 electronic air cleaner. We run this system with the indoor fan speed set at "Low" to ensure air circulates in the house even when there is no call for heating or cooling (i.e., the temperature in the house is at the temperature set in the Infinity Control.)

After two months of trouble-free operation, we noticed one day, in a period of no call for heating/cooling, that the indoor fan had stopped, even though in the controller it was set to run at low speed. In this situation, the fan would not run at any speed setting (low, medium, or high.) When a subsequent call arose for heating/cooling, the fan would however come on at a speed chosen by the Controller. When the call for heating/cooling was satisfied, the indoor fan shortly thereafter would stop as noted above, even though set to run at low speed.

We requested the installer to return and he determined that the main circuit board in the air handler was defective and replaced it. That action seemed to fix the problem, and the indoor fan then ran, as desired and set, at low speed in between calls for heating/cooling.

But now, after two additonal months of operation, the problem of the indoor fan cutting off has recurred, albeit in a somewhat different form. The problem is intermitant; now in periods of no call for cooling, sometimes the indoor fan will run at low speed as desired; sometimes it's off, and when it's off the fan will not run at any speed that we set in the controller (low, medium, or high.)

What is causing this problem?

Any assistance will be appreciated!