After replacing the motor of my home furnace due to dried lubricant. I noticed a new strange behavior. After the furnace turns off, the fan just starts 10 seconds later for 1-3 minutes and turns off. I don't understand why wouldn't the fan continue to run until the furnace cool down? why does it stop and restart?
I want to play with the fan limit control and the upper limit control is fixed at 150degrees, the lower limit control tab is moveable.
thanks.
calvin.

 

Welcome to the forums.

Must be a fairly old furnace to have a mechanical fan limit control.
Both cut in and cut out should be adjustable. The high limit may not be.

You problem is more common with timed shutoff where the replacement blower is running slower so that when the timeout occurs..... the furnace hasn't fully cooled.

 

Typical blower motor control of this type has two contacts wired in parallel to provide power to the blower motor. One is a normally open relay contact whose coil is controlled by the thermostat. When there is a call for heat, the relay is energized closing the contact and the motor is powered. The other contact providing power is on the fan/limit control. This contact is closed when the temperature rises above the fan/limit control fan-on setpoint (adjustable) and open when the temperature falls below the fan/limit control fan-off setpoint (adjustable).
In regular heating cycle, the thermostat controlled relay is the first to apply power to the blower motor. The fan/limit fan-on setpoint is selected to close the fan contact to maintain power to the blower motor before the thermostat is satisfied and power is removed at the thermostat controlled relay. With the thermostat satisfied, the blower motor continues to run until the fan/limit fan-off setpoint is reached, opening the fan contact and the second source of power is removed from the blower motor. If the fan-off setpoint is too low, the blower ends up blowing cold air. If the fan-off setpoint is too high, the residual heat causes the fan-on setpoint to be exceeded and the blower is turned back on without a call for heat by the thermostat.
Your problem is the fan-off setpoint is set too high. Setting it correctly is a trial and error approach. Good luck.

 

Not entirely correct,

On older gas furnaces.... the blower relay is not used for the heat mode at all. It's strictly the job of the fan limit control to turn the blower on and off based solely on plenum temperature.

On an older furnace that blower relay is set up for a faster blower speed for FAN ON and A.C mode.

 

I guess at this point: check blower rotatation and speed and blower wheel for dirt buildup.
Sounds like it has a Honeywell L-4064 or equivalent fan/limit control. I'm sure you have already inspected the filter

 

The following is for PJmax
The type of blower control I am discussing is found on my early 80’s Heil Quaker model NULK080AG01, as an example. This may/may not be your definition of old and the relay labels can be misleading.
I would consider the design of a furnace blower control that turns on the blower motor based on plenum heat as highly inefficient. It takes time after the furnace ignites before the limit/fan sensor starts responding to the added heat. The higher the fan-on setpoint, the more fuel is being burned and wasted before the blower comes on. The lower the fan-on setpoint, the longer the blower will continue to run after the thermostat is satisfied because the blower –off setpoint must be at least the hysteresis value below the fan-on setpoint and you don’t want the blower restarting due to latent heat.

 

Inefficient as it may be..... that's the way it is.

The L4064B has been controlling blower fans for years.....all by itself.

 

Thanks all for the replies.
I replaced the new motor with the same spec so the air flow is pretty much the same. I also get warm air through the vents so the direction is good .
I made a mistake that when I took out the squirrel cage blower wheel for cleaning, I didn't mark the location of the small metal clips for balancing. I feel the furnace vibrates a bit when running. I am not sure it's due to the balancing, the new motor shaft issue. Like I said, the old motor has hard time to start and stops spinning as soon as the power cut off so it could be running as slower RPM.
I am not sure how to figure out where is the source.

 

Most motors are multi-speed. Possibly the speeds are different ?

When you replaced the motor.... did you also install a new capacitor ?
The cap needs to be the correct size for the new motor. The value will be right on the motor.

Those clips are usually factory installed for balancing. It would pretty difficult to balance a wheel from scratch.

Possible you can see the marks on the wheel where the clips were and put them back on ?
There should be teeth marks where the clips were.

 

I replaced with a new motor that has the same spec: RPM, AMP, 4 wires configuration and correct capacitor. I don't notice any reduce in the air flow from the vents. Unfortunately, when i realized that I have moved the clips, there is no mark.