Forced Hot air limit switch

[JOHN F]

I have a forced hot air system and I have had to replace the limit switch 4 times in last 11 months. Part number HH 12zB and it has 200 degree rating. All power feeds measure correctly. This is in town house with 3 floors of ductwork. System works fine between limit switch replacements. I have lined out limit switch for short durations ( middle of night and heat needed), but I consider this a fire hazard over long term, plus unit runs continually in that mode. With defective limit switch, fan runs continuously when themostat calls for heat, but naturally the unit doesn't actuate the pilot, so get cold air only.

Parts are fairly cheap..like less than $10, but more an irritation and concern when traveling.

Thanks

[Eric in North TX]

First, I'm no HVAC tech, but I do maintain mine. Just a thought; I wonder what would happen if you put 2 in parallel.

[Tony Hwang]

Hmmm, Not a good idea. They won't work in unison and defeat the purpose. Can you increase blower speed?

[DK]

So what is happening to them? Did you smash one to bits and look for clues with the parts? You do know there are different mfgs of the things, don't you?

[scott21230]

"I have lined out limit switch for short durations ( middle of night and heat needed), but I consider this a fire hazard over long term, plus unit runs continually in that mode."

Can you use the fan switch in your thermostat instead of doing this?

And where are you getting $10 limit switches from? I thought they cost more like $40? Link?

I had a problem with mine once, and it turned out to be the crappy push in terminals on the switch. Replacement models have both the crappy push in terminals and regular screw terminals (like the back of an electrical outlet).

[barry]

First, you're confusing "limit switch" with "fan switch", both of which often occupy the same enclosure.

J

[tnom]

I had the same problem or similar. The push in connectors were different on the new switch compared to the original.The furnaces wiring would go into the push in connector but didn't appear to be very secure, so I soldered on larger gauge pigtails to the original wiring. I then plugged the proper sized wire into the connectors.

I wonder if his connectors are arcing and failing?

[Todd H.]

Hrmm.

Wise!

I'm curious about the root cause of these recurring failures.

It is possible that the limit switches are doing their job and that there is a real temperature rise issue in your furnace that needs to be addressed? Clean filters, fan running normal speed, etc?

Best Regards,

-- Todd H.

[CJT]

The "limit" part of a limit switch is a safety device which should never be exercised in a proper installation, because the furnace should never reach the temperature to which the limit switch is set.

If yours is activating, you have another problem. As others have suggested, it could be a fan set to the wrong speed, an incorrect limit temperature, a clogged filter, and/or incorrectly sized (or blocked) ducts.