Thermostat is reading crazy high temps when it is really 67. Furnace won't deliver heat. Fan works. Is it the thermostat (batteries replaced already) or the furnace?

  • Check the vents in the thermostat, air needs to get to the sensor. If not that, it needs to be replaced
    – Ack
    Apr 11, 2020 at 21:16
  • It is a White Rodgers type 1F86-344 and no lint or dirt visible. So I'm thinking of installing an old mechanical thermostat that I have. Do these modern digital ones often fail. It is only 12 years old.
    – Fats
    Apr 11, 2020 at 21:24
  • I would think that probably depends on the quality of it, it's not that it's digital, it's all about how well it's made.
    – Ack
    Apr 11, 2020 at 21:27
  • Made in China if thats any help? More to the point; will the old mechanical Tstat work on a cerca 2003 Heil 80% furnace?
    – Fats
    Apr 11, 2020 at 22:15
  • Still curious to find out if a nice old Lennox with double mercury switches will work on the Heil. I did find the trouble with not starting. Exhaust vacuum tube was plugged. I can't understand how that happens. There is a lot of white crumbly stuff lying on the burner bar. Same stuff seems to be clogging the vacuum tube. How does it get in there? The rubber tube to the switch doesn't have any holes.
    – Fats
    Apr 11, 2020 at 23:59

1 Answer 1


Yes thermostats do go bad. The old double mercury thermostats are great for simplicity and no batteries needed.

Yes digital thermostats do fail. Just like all electronics they have a life and if in an electrical noisy area the life will be shortened. I have only seen a few thermostats that were actually integrated as part of the heating / cooling system.

Depending on the features your system has you will need a common wire a call for heat and a call for cooling, fan only, different fan speeds / heating levels . You can install the old thermostat and although it won’t cost anything today loosing some of the features of a modern digital may be worth the expense. Most only have cooling/ heat a fan only setting and off.

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