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Discussion Starter · #1 ·
I have a Heil Gas Furnace Ultimate II that had a problem with a faulty high limit switch. I've replaced the switch with a new replacement (L300F Supco Manual Reset Limit Switch SRL300) and it worked fine for a while. But recently I've noticed that the high limit switch will occasionally get triggered, resulting in cold air being blown out until we reset the switch. I've also noticed that the piece holding the high limit switch gets very hot from the flame, but I'm not sure if this is by design or not and whether this is somehow triggering the high limit switch. I'm not really sure what is causing this and would appreciate any suggestions or assistance. Thanks in advance.
 

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Could be a cracked heat exchanger. Watch the flames when the blower comes on, if they start jumping around. Heat exchanger is probably cracked or burned through.
 

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Discussion Starter · #4 ·
Took us a while of constant checking, but it doesn't seem like the flames are jumping around. As a matter of fact, we noticed that the high limit switch is going off even when the heater/flames are not on. The piece that's holding the high limit switch remains very hot though, since it's right on top of the ignition flame, and that never turns off. This happened with our old high limit switch too, which we replaced not too long ago because we thought it was faulty. We are stumped as to why this is happening, and can't really figure it out since we haven't changed anything on the heater and can't remember if the piece holding the high limit switch has always been this hot in the past.
 

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Blockage in the flue/chimney. Any changes made to the duct system recently?
 

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Discussion Starter · #6 ·
Nope. We were told by a friend that a blockage in the chimney might be causing the problem. But we've already looked into that and there were no blockage issues. No changes made to the duct system recently either.
 

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Do you also have a gas water heater connected to the same chimney.
 

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What kind and size chimney do you have. A motor chimney may have been damaged by the water heater or furnace. And now is leaking air in at motor joints.
 

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Need a chimney guy.
 
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