HVAC blower fan testing & diagnosis FAQs:
Questions & answers on diagnosing and repairing HVAC blower fans in the air handler.
This article series explains how to inspect and test a heating or air conditioning indoor air handler blower fan that is not working. We also discuss fan fan squeaks and noises. The blower fan assembly is the green component in the page top illustration from Carson Dunlop Associates The Illustrated Home.
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The questions and answers discussed below were posted originally
at BLOWER FAN OPERATION & TESTING - please be sure to read that collection of diagnosis and repair steps.
The blower fan is located inside a horizontal air conditioning unit in many home air conditioning systems, especially when the air handler is located in an attic or crawl area.
I cannot find the fan limit switch.
The plenum is above the blower and does not have an access panel. I assume it would be located in a position to gain access.
My air handling unit has the blower motor and control board/transformer above the evaporator and the plenum is above that level. On 2017-05-11 by Dan C
by (mod)
See our article series FAN LIMIT SWITCH to see locations and sppearance
by Dan C
I am very sure this system has no fan limit switch. The schematic does not show a limit switch.
The fan connects directly into the control board. I believe it is controlled by a P&B relay on the control board. The system will work properly most of the time after power down. I have also removed the G wire from the thermostat at the control board and it continues to run.
by (mod) diagnosing an air handler that does not use a fan limit switch
Dan C:
Air Handlers that use a gas or oil fired heat source would use a fan limit control switch.
An Electric heating system used as backup for heat pump would not require that control.
If that's your situation then I suspect there is a corroded electrical connection or a bad control board or relay in your air handler.
My fan in the attic runs continuously as it is for the heat. If I turn the breaker to off will it hurt the AC unit? On 2016-03-28 by Anonymous
by (mod) - probably not
If your same attic blower assembly is used for cooling, distributing cool air to the building interior, the A/C won't work with the blower turned off.
And if you called for cooling, it's possible that the cooling coil in the air handler would quickly ice-up as no air would move across it.
I recently had my blower motor replaced and my AC is working great but now when I just want to turn on my fan to circulate air, it blows ice cold AC air.
Did wires get crossed on my furnace contol board? Any suggestions on a fix? On 2015-09-23 by Pjespo
by (mod)
The thermostat's FAN ON switch should only turn on the fan so long as the thermostat temperature setting (in cooling mode) is above the room temperature.
If that's not happening I suspect a wiring or control board or relay defect.
Air movement speed from grilles is not stable-pulsation. What is possible reason? 2015-09-14 by Garri
by (mod)
Garri
First take a look at the blower fan operation for
- voltage variation
- loose wiring
- a defective control that is cycling the fan on and off intermittently
- loose belt on a belt driven fan
- loose pulley on a belt driven fan
- failing blower motor or blower assembly bearing
LS 3845 said: it was a good read i can use the tips every day (Apr 28, 2011)
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