How do you test a fan/limit switch?

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?
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Bob (but not THAT Bob) wrote:

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.
--
The e-mail address in our reply-to line is reversed in an attempt to
minimize spam. Our true address is of the form snipped-for-privacy@prodigy.net.
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
CJT wrote:

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.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
CJT is correct, you need to RAISE the off setting a little... it is too close to 70 deg
Mark
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Mark wrote:

I tried raising it too, but it was just plain stuck that day.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
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

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Bob (but not THAT Bob) wrote:

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
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
TURTLE wrote:

Thanks - that sounds like the likely cause of the problem.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

HomeOwnersHub.com is a website for homeowners and building and maintenance pros. It is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.