I have changed fan relay switch 2 times. Works OK for days then the fan won't shut off even when thermostat switch is put to off until you shut off the breaker.

My air handler is AquaTherm Model # 24MBQ3 and thermostat is White Rodgers Model 352. This is an addition to my previous question that the air handler fan won't shut off sometimes even when you shut off thermostat ,only when the breaker is turned off.I have replaced the thermostat once and the fan relay switch 2x with same result. Sometimes it works OK for days. Confused, any ideas?

This answer may be a bit far fetched, but I have run into it twice in my career. Some units can act strangely (mostly with blower function) with certain thermostats. The units that I have seen act this way actually stated in the install manual that they were not compatible with "power robbing" thermostats. Read through your air handler literature from front to back, and see if stat, or stat wiring requires special attention. Short of that, call in a pro, or call AquaTherm's tech support line.