furnace blower problems

[yonauste]

:mad: No matter what I do or what I replace, the furnace blower runs constantly. The ony way to stop it is to turn off the electrical power to the furnace. It is a gas furnace over 30 years old, with a belt driven motor. Is it possible that all of these years, I have been using the wrong fan/limit switch? When the last two stopped working, they were replaced with another one of the same make and model?

[Wgoodrich]

I suspect you may have a fan switch with wires crossed, or a plugged furnace having flames lick out the front causing heat damage to your fan switch. Both are common problems.

Check the black wire going from power through the fan switch to the fan. The hot wire is to be disconnected by the fan switch between power source and fan motor by that fan switch.

If flames are licking out the front of your furnace the venting inside the furnace is plugged or partially plugged causing a back flash of flame. Just a few minutes of that flame licking the bottom of that fan switch and that fan switch is done no longer any good.

Let us know what you find.

Good luck

Wg

[mdshunk]

If your furnace is 30 years old, you likely have a Honeywell L4064 fan/limit switch, or one nearly identical to it by another manufacturer.

With the cover removed from the fan/limit control, there will be a dial in the top center of the control. The two terminals on the left side of the control are for the fan. The two terminals on the right side of the control are for the limit switch. You should have a wire on the upper left terminal and a wire on the upper right terminal. The bottom two terminals may have one wire on one terminal OR it may have a wire on each lower terminal with the jumper between the two broken out.

It is the left side terminals that we are concerned with if you're having fan problems. If you have a wire on the lower left terminal and no wire on the upper left terminal, this is your problem. Move the wire from the lower left terminal to the upper left terminal. If you have a wire on BOTH left side terminals, try switching them. Nothing bad will happen, just try this and see if your problem goes away. Could be that the wires got switched.

What are the stop settings on the dial? Should be (rule of thumb)"fan on" at 130-140, "fan off" at 90-100, and "limit" at 190-200.

If you pull the wire off the upper left terminal (with insulated pliers) while the fan is running (and it's not supposed to be) and the fan stops... you have a bad fan limit switch OR the dial settings are wrong.

Some fan limit switches have a white or black post sticking out of the middle that you can "pull out" to make the fan run on constant. If yours has a plastic post in the middle, try pressing it "in".

Manufacturer and part number of the fan/limit swtich helpful for more help.