I have a Ducane heat pump. When running in AC Mode, the outdoor fan and compressor kick off just fine. The blower fan seems to kick off as well, though it's not putting out very much air. I can feel cool air blowing out of the vent but it's a small trickle, and it's not enough to keep the house very cool. I've also noticed that the pipes are freezing over inside my outdoor unit (which I have been told is a side-effect of the blower fan not pulling its weight). I've cycled the breakers and verified that the filter is clean / new, with no improvements. One of the breakers on the box that controls the outside unit seems very flimsy, and it has been suggested that this should be changed as an electrical issue there may be preventing the blower fan from running in 'high' mode as is usually the case with AC. Is there any merit to that suggestion (i.e. an issue with the breaker could be causing the fan to run slowly) or is there a more likely problem that would cause this type of symptom? Any help / suggestions would be appreciated.

1 Answer 1


There are a multitude of blowers out there. The outside breaker you mention likely has no relation to this problem. The pipes are indeed freezing due to too little airflow. Here are the most likely causes and solutions for this:

  1. Assuming you have one of the most common PSC types (and not some other type like an ECM, slip induction, etc), then a weak run capacitor for the blower motor can cause this problem. The run capacitor for the blower should be checked with a capacitance meter and replaced if it is below 90% of the rated value. Or, if you happen to know how to use an ESR meter, then check the capacitor that way.

  2. It is possible that it is a multi-speed motor with the wrong lead energized in the "cool" mode. Typically Red=Low, Blu=Med, Blk=High. The black lead is usually the one needed for a/c operation.

  3. Sometimes the motors wear out by having the lubricant in the bearings go dry. In this case the blower wheel will not turn effortlessly and smoothly by hand when the unit is off. If the bearings are dry, then you need a new blower motor.

  4. (unusual) Since you have a heat pump, if it is a higher performance model it may have an electronic blower speed control that is supposed to automatically slow the blower when it is very cold outside to ensure an adequate warm air temp inside. Typically, this would be shunted to high speed blower mode for a/c operation, but if a control relay has failed or appropriate wire become lose, or other fan speed control failure has occurred, then the speed control could be causing the blower to run very slowly (as it would in cold weather heating operation). If you have this then you will have to determine if it has failed by temporarily bypassing it to see if the fan runs on high speed without the speed control unit inline.

Verify main power for the unit is off before proceeding with any of this work. The main power for the air handler is usually separate and in addition to the main power for the outdoor unit.

**Edit: I see that Ducane air handlers offer options including both electronic blower speed controls as well as constant torque (ECM) motors. If your unit has either of these then the low speed problem may have other causes. For example, ECM motors have an electronic digital driver that is set using DIP switches or other means. An incorrect low torque setting yields low blower speed. I recommend you post the exact model number of your air handler as well as a photo of the interior showing the back end of the blower motor and the control board. A close up of any DIP switches as well as a legible photo of the schematic on the inside panel would be nice too if you want any additional comments.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.