Quote Originally Posted by Mr Bill View Post
What brand/age of unit are you talking about that the "high limit" kills the power to the stat? all the high limit does on any newer "board type" unit is break the circuit back to the board, the lugs on the board receive direct power from the thermostat.
Most of the ones we see this problem on are the newer Carrier/Bryant/Payne 80% units. Not sure of model numbers, but they are under 10 years old. It's the style that uses a thumbscrew to hold the large main door on, and the blower/board are behind a sub panel. Diagnostic code of "33" when it shuts off the burners on high limit and kills the 24V to the thermostat. Blower continues to run while on high limit. Once the furnace cools back down power is restored to the thermostat. This is not the only furnace that does this, just the most common one we see for high limit shutoff. When testing, the discharge air from the registers gets to about 130 degrees before the furnace kicks off on high limit.

I've seen a few old pilot light furnaces cut the power to the primary side of the transformer when out on high limit. Not very common, I think most do like MrBill says and just kill the power to the gas valve. Not sure how many just kill power to the gas valve because we get very few calls on them. We see more of he induced draft style furnaces when it comes to high limit shutoffs, I'll start paying more attention to which brands/models I'm seeing.