My (50 yr old) furnace fan is suddenly always on.

The thermostat fan switch is set to "auto", of course.

And the main burner properly goes on and off as it should - but the fan
just keeps on going and going...

So I'm thinking it's the Honeywell combo fan/limit switch.

I've tried changing its "Off" setting to as low as it goes, but with no
effect.

But how to be sure?

How do you test it?

Bob (but not THAT Bob) wrote:

My (50 yr old) furnace fan is suddenly always on.

The thermostat fan switch is set to "auto", of course.

And the main burner properly goes on and off as it should - but the fan
just keeps on going and going...

So I'm thinking it's the Honeywell combo fan/limit switch.

I've tried changing its "Off" setting to as low as it goes, but with no
effect.

But how to be sure?

How do you test it?

First, don't you want to set the "Off" setting _higher_ rather than
lower?

Also, could it not also be the fan switch on the thermostat? You could
try disconnecting the (green?) wire that controls the fan and see if it
turns off.


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CJT wrote:

Bob (but not THAT Bob) wrote:

My (50 yr old) furnace fan is suddenly always on.

The thermostat fan switch is set to "auto", of course.

And the main burner properly goes on and off as it should - but the fan
just keeps on going and going...

So I'm thinking it's the Honeywell combo fan/limit switch.

I've tried changing its "Off" setting to as low as it goes, but with no
effect.

But how to be sure?

How do you test it?

First, don't you want to set the "Off" setting _higher_ rather than
lower?

Also, could it not also be the fan switch on the thermostat? You could
try disconnecting the (green?) wire that controls the fan and see if it
turns off.


After blowing cool air continuously for about a day, it has gone back to
its proper cycling with the main burner.

Sure wish I knew what caused it to stick in the first place.

CJT is correct, you need to RAISE the off setting a little... it is too
close to 70 deg

Mark

If the house air temperature is higher than what is normal for the heating
season then the circulating air temperature may not drop low enough to cut
the fan off. Slightly raising the switch setting may be needed.
Don Young
"Bob (but not THAT Bob)" wrote in message
...
CJT wrote:

Bob (but not THAT Bob) wrote:

My (50 yr old) furnace fan is suddenly always on.

The thermostat fan switch is set to "auto", of course.

And the main burner properly goes on and off as it should - but the fan
just keeps on going and going...

So I'm thinking it's the Honeywell combo fan/limit switch.

I've tried changing its "Off" setting to as low as it goes, but with no
effect.

But how to be sure?

How do you test it?

First, don't you want to set the "Off" setting _higher_ rather than
lower?

Also, could it not also be the fan switch on the thermostat? You could
try disconnecting the (green?) wire that controls the fan and see if it
turns off.


After blowing cool air continuously for about a day, it has gone back to
its proper cycling with the main burner.

Sure wish I knew what caused it to stick in the first place.

Bob (but not THAT Bob) wrote:
CJT wrote:

Bob (but not THAT Bob) wrote:

My (50 yr old) furnace fan is suddenly always on.

The thermostat fan switch is set to "auto", of course.

And the main burner properly goes on and off as it should - but the fan
just keeps on going and going...

So I'm thinking it's the Honeywell combo fan/limit switch.

I've tried changing its "Off" setting to as low as it goes, but with no
effect.

But how to be sure?

How do you test it?

First, don't you want to set the "Off" setting _higher_ rather than
lower?

Also, could it not also be the fan switch on the thermostat? You could
try disconnecting the (green?) wire that controls the fan and see if it
turns off.


After blowing cool air continuously for about a day, it has gone back to
its proper cycling with the main burner.

Sure wish I knew what caused it to stick in the first place.


this is Turtle

Well let me tell you what it is.

The fan/limit is stuck in the fire wall of the firechamber and sences
the heat of the air side of the fire chamber. When it get to a certain
degree temp. it will turn on the fan. then when the fan sences the
chamber has cooled off it will turn off the fan. When it is working
goods it will work good but you have from time to time rust that falls
off the inside of the fire chamber and lands on the rod of the
fan/Limit switch and will keep it running because of the rust will jam
the rotor of the rod on the fan/limit switch and keep it running all
the time. So every now and then you need to pull the fan/limit switch
out and clean the rust out of it. Now UNPLUG it before doing this.

Now short cut to cleaning the fan/limit switch is to hit the wall near
the fan/limit switch with a hammer or screw driver to just knock the
rust loose when it sticks. 90% of the time this works.

TURTLE

Mark wrote:

CJT is correct, you need to RAISE the off setting a little... it is too
close to 70 deg

Mark

I tried raising it too, but it was just plain stuck that day.

TURTLE wrote:

Bob (but not THAT Bob) wrote:
CJT wrote:

Bob (but not THAT Bob) wrote:

My (50 yr old) furnace fan is suddenly always on.

The thermostat fan switch is set to "auto", of course.

And the main burner properly goes on and off as it should - but the fan
just keeps on going and going...

So I'm thinking it's the Honeywell combo fan/limit switch.

I've tried changing its "Off" setting to as low as it goes, but with no
effect.

But how to be sure?

How do you test it?

First, don't you want to set the "Off" setting _higher_ rather than
lower?

Also, could it not also be the fan switch on the thermostat? You could
try disconnecting the (green?) wire that controls the fan and see if it
turns off.


After blowing cool air continuously for about a day, it has gone back to
its proper cycling with the main burner.

Sure wish I knew what caused it to stick in the first place.

this is Turtle

Well let me tell you what it is.

The fan/limit is stuck in the fire wall of the firechamber and sences
the heat of the air side of the fire chamber. When it get to a certain
degree temp. it will turn on the fan. then when the fan sences the
chamber has cooled off it will turn off the fan. When it is working
goods it will work good but you have from time to time rust that falls
off the inside of the fire chamber and lands on the rod of the
fan/Limit switch and will keep it running because of the rust will jam
the rotor of the rod on the fan/limit switch and keep it running all
the time. So every now and then you need to pull the fan/limit switch
out and clean the rust out of it. Now UNPLUG it before doing this.

Now short cut to cleaning the fan/limit switch is to hit the wall near
the fan/limit switch with a hammer or screw driver to just knock the
rust loose when it sticks. 90% of the time this works.

TURTLE

Thanks - that sounds like the likely cause of the problem.