What is it

[Richard Roshak]

Found this today on a 9 year old Trane 80, which replaced a natural draft oil fired system. Can seem to find anything on the internet.

[Matt Fellman]

Is it Normal for furnace to reach high-limit switch often? - Page 4 - HVAC-Talk: Heating, Air & Refrigeration Discussion

The only thing online I could find is in post #45 of the above thread on an hvac board.

It looks like just an old limit switch from a past appliance. I'm not sure as to why it would still be there. I'd think the newer furnace has its own limit switches.

[Jerry Peck]

Is it Normal for furnace to reach high-limit switch often? - Page 4 - HVAC-Talk: Heating, Air & Refrigeration Discussion

The only thing online I could find is in post #45 of the above thread on an hvac board.

It looks like just an old limit switch from a past appliance. I'm not sure as to why it would still be there. I'd think the newer furnace has its own limit switches.

And this one: Trane XR90 - diagnostic LED flashing 4 times - http://www.hvacmechanic.com/ Forums 5th post up from the bottom (7th post down from top).

Does not add anything useful, though.

[Markus Keller]

Looks like a flue damper with a limit setting. I would say this might work on an old style regular venting unit to keep heat from going out the flue or cold coming down the flue.
The furnace is probably short cycling because the limit is messing with the pressure switch. Your furnace is power vented, see what look to be two orange rubber tubes? Those go to the pressure switch. Try jumping the pressure switch for a couple cycles and see if it still short cycles. You'll probably have to un-jump between cycles, some units will lock out otherwise. Don't remember Trane sequence, don't use a lot around here.

[Jerry Shipman]

It's a masonry chimney adapter. They are often used to meet the requirements for alternate vent design according to the NFGC. They allow a constant draft to be achieved. It has nothing to do with a power venter. The two orange wires are connected to a limit switch that wires into the limit circuit of the furnace. It is there to detect backdraft or a blocked vent.

[Bob Harper]

Jerry nailed it.
Here is the Carrier version: http://www.xpedio.carrier.com/idc/gr...k-kgaca-01.pdf

It is basically a draft hood with spill switch designed to minimize flue gas condensation.
HTH,
Bob