Please see the attached picture and pay attention to the "temperature dial" in the inset

With the current settings the gas burner cycles like this

-limit temperature 200F
-at that temperature the burner shuts off and the furnace is cooling off to around 150F in about 35 seconds
From there the burner starts again and it the temperature goes up in about 1min

The temperatures in the circled area are the highest in the area but stable. I am not sure how hot these should normally be. I just fixed a problem with the control module and in the process I think I played too much with the fan/load control switch. I am trying to guess if everything is back to normal

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2 Answers 2


If you're tripping the high limit, then I'd say it's not back to normal. Sounds like you're not pushing enough cool air through the heat exchanger, which means it can't dissipate the heat fast enough.

With a conventional gas furnace, you should be at about 140°F-170°F at the supply plenum. A high efficiency furnace will be lower, around 110°F-140°F. However, the temperature at the outlet of the furnace, is dependent on the temperature of the incoming air. If you have the documentation for the furnace, it should give you an idea of the temperature rise you should expect.

The expected temperature rise, as well as the max outlet temperature, might also be listed on the nameplate of the furnace.

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  • Today I have measured the temperature using one of those laser temperature readers. It read an average of 75 Celsius degrees on the bottom spot and around 45-50C on the top area. Over the weekend I am planning to read the temperatures as you indicated to see what is the temperature rise and if I am within the limits. I have added my own furnace label above
    – MiniMe
    Nov 28, 2014 at 4:05

For maximum efficiency you should have the blower kick on as soon as possible and run till the furthest register drops to 90 degrees. And you NEVER want your max limit to be above 150 degrees or severe burns can result. So for lowest gas consumption, optimum safety and heat exchanger life 100-130-150 should be the settings.

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