I have a carrier furnace that has a low speed fan blower motor that will not shut off no matter how long I leave it running. I checked that removal of the G wire at the control board does not change anything. This should rule out the thermostat being an issue. I checked the furnace limit switch and it seems fine with good continuity at the two switch parts (the button part and the actual part that sticks into the hot zone). The heater works fine and will shut itself off when it reaches temp but this only changes the high speed fan. I checked continuity over the fusible link off of the limit switch and it has good continuity. Anyway since the heater works I read that it’s not a problem with the furnace high limit switch. I pulled the low speed fan wire off the control board and that successfully shuts off the low speed fan but then I hear a small humming sound from what might be the HFR or heating fan relay. So is the remaining issue pretty much just the control board or am I missing something I can check?
Low speed furnace fan blower motor will not shut off
blowercarrierfansfurnace
Related Solutions
Your furnace is shutting down on limit. There can be a number of causes for this.
Start by making sure you have a clean filter in the furnace. A dirty or restrictive filter can cause this issue. Alternatively you could also have a dirty or plugged up A/C evaporator coil which is located above your furnace. This can also cause a furnace to cycle off on high limit, however is uncommon unless you have run your furnace without a filter for prolonged periods of time. Also if you have a high efficiency furnace, there is more of a chance to have the secondary heat exchanger plug up rather than the A/C coil.
You can also have a blower motor issue. The motor can be faulty or seizing up causing it to not spin fast enough to move enough air through the furnace. Another cause may be that you have a faulty limit that is opening prematurely.
The major issue that can cause this problem is a heat exchanger issue. If your furnace is cycling off on high limit, and everything checks out, you may have an internal blockage of the heat exchanger. If this is the case either the heat exchanger or the furnace will need to be replaced. A combustion analysis of the furnace is required to prove this is the issue.
If you happen to have a 2 stage high efficiency furnace you could have a bad ventor motor gasket which is allowing air to be drawn in behind the motor. Big cracks in the condensate collector pan can also cause this but is very uncommon. You can also have the a two stage gas valve high fire solenoid stuck open, but this is also very uncommon and can only happen on 2 stage furnaces. Also if you have a bryant, payne, or carrier high efficiency, you could have a cold spot baffle leakage that is allowing air to be drawn in between the primary and secondary heat exchanger however this is also very uncommon.
If you think your blower is not spinning fast enough you may also have a bad blower motor, or if an ecm motor, the control board may be bad. If its a fixed speed motor and your getting power to it but its not spinning fast enough it could possibly be a bad blower motor. If you decide to change out the blower motor, make sure to get the appropriate replacement capacitor to go with it. I actually did have the same issue on an older carrier high efficiency furnace a few weeks back. It ended up being a bad blower motor, however the motor spun freely and had no indications of seizing or hard starting.
According to the manufacturer's documentation, your furnace should have a 3 or 4 speed blower motor. The problem is, the furnace control module doesn't use a separate speed when the unit is running in continuous FAN mode.
When the R-G circuit is made, the blower motor operates on heating speed.
The control module only uses two speeds, one for COOL, and one for HEAT/FAN.
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You could run the blower at a slower speed during both HEAT and FAN calls. But you'd have to insure that the blower is maintaining adequate flow during heating, otherwise you will damage the system. This is not recommended, as you could damage the system, and void the warranty.
You might be able to swap out the control module, but you'd have to contact the manufacturer to determine if there's a compatible replacement.
The other option is to build your own control circuitry, but that's beyond the scope of this site. And will likely void the warranty on the unit.
Best Answer
Dangerous! Don't pull LO FAN plug, it dissipate the heat.
Sounds like LO FAN relay stuck, better replace the whole (blower control) board.