Checking High Limit Switch


Old 12-02-15, 10:06 AM
Thread Starter
Join Date: Jun 2002
Location: Northern Minnesota
Posts: 1,612
Checking High Limit Switch

I would like to check the limit switch (L150-20F) to see if it is opening too soon, mostly curiosity.

Can I put this in water and check continuity using an ohm meter and digital meat thermometer or would dunking it in water be a bad idea? New one will be here in a few days.

Sponsored Links
Old 12-02-15, 10:15 AM
PJmax's Avatar
Group Moderator
Join Date: Oct 2012
Location: Northern NJ - USA
Posts: 51,393
Upvotes Received: 210
Dunking it in water is a bad idea.
Old 12-02-15, 10:27 AM
Thread Starter
Join Date: Jun 2002
Location: Northern Minnesota
Posts: 1,612
I was just about to do it then thought being electrical it could be bad, I'll wait for the new one then do it.

Old 12-02-15, 11:25 AM
Geochurchi's Avatar
Join Date: Nov 2012
Location: USA
Posts: 3,841
Upvotes Received: 8
You might try a heat gun if you are that curious.
Old 12-02-15, 01:56 PM
Thread Starter
Join Date: Jun 2002
Location: Northern Minnesota
Posts: 1,612
That's okay, I'll wait till the new one arrives.
Old 12-02-15, 02:14 PM
SeattlePioneer's Avatar
Banned. Rule And/Or Policy Violation
Join Date: Jul 2007
Location: Seattle, Wa
Posts: 5,503
You can do a fairly good job of checking on the limit switch by drilling a small hole in the warm air plenum and inserting a thermometer to measure the temperature when the fan circulating air through the furnace is running.

The warm air temperature allowed is governed by the rated heat rise for the furnace, which is given on the rating plate in the burner compartment of the furnace.

The heat rise is the outgoing air temperature from the furnace minus the incoming temperature.

Measure the heat rise in your furnace by drilling a second hole in the return air duct near where it enters the furnace.

Lets suppose that incoming air temperature is 60 degrees F and the outgoing air temperature is 150 degrees. The heat rise would then be 150-60= 90 degrees.

A typical furnace might have a rated heat rise of 60-80 degrees. The 90 degree heat rise would be excessive and require that the furnace and ductwork be inspected to remove defects and obstructions that are causing the excessive heat rise. An overfired burner, plugged filter, too low fan speed and lots of other causes could cause that kind of issue.

Ideally you'd be aiming for a heat rise somewhere near the middle of the rated heat rise.

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off

Thread Tools
Search this Thread
Display Modes