What is this?

[Richard Roshak]

This device was installed on a mid efficient heating system vent connector. Why would the installer install a device with opening on a system under positive pressure?

[Gary Burnett]

It's a resettable limit switch. Will turn off unit if flue gasses or heat exchanger gets too hot. I would bet the unit is not under positive pressure and has a air inducer fan that starts up but the vent is under negative pressure.


My guess again.