[turbo6bar]

Gas furnaces have a limit switch in the plenum. It turns off the gas valve when temps exceed 180 degrees, if I am not mistaken. Most limit switches are pre-set during manufacturing, but some older switches are adjustable with small tabs on a rotary dial. This would be easy to install inline with a gas valve. The switch is closed during normal operation and breaks to open when temp hits the limit. You would also need a relay that would break and hold open when triggered by the switch. This would keep the gas valve from turning off AFTER a potential fire is out and temps fall below the limit.