Thermostat works sometimes, other times I have to shake the switch to get the air conditioner working. Is this a problem with the thermostat or the central air system? Thermostat is about 15 years old, air conditioner the same age.

  • 2
    "shake the switch" - meaning...? You didn't say "shake the thermostat" - if you mean that, it's the thermostat. If you mean some other switch, you should describe it; pretend we're third graders and need it explained, since we can't see what you mean...
    – Ecnerwal
    Commented Jul 13, 2015 at 23:50
  • There is a switch on the thermostat to turn it on and off, like a light switch. Sometimes it turns on the unit, sometimes not. There is another switch that regulates or sets the desired temperature, example I want it to be 78 degree cool. I have to shake that switch to get the air conditioner to turn on. Is it the thermostat or the air conditioner that needs repair? Commented Jul 15, 2015 at 4:21
  • Since the switch is on the thermostat, it's the thermostat, as already stated.
    – Ecnerwal
    Commented Jul 15, 2015 at 11:40

1 Answer 1


Since the AC is kicking on, the problem isn't there. Thermostat is just a simple signalling circuit. Since you have to shake seems to suggest something is wrong with the thermostat.

Some of the old thermostats have a glass bulb with a bead of mercury mounted on the end of a bi-metal string. As the metals expand, the bulb will tilt allowing the mercury to slide down and complete a connection at the base of the bulb to signal AC to kick on. By shaking the thermostat your probably causing to mercury to make contact.

I think these mercury switches were probably used on system older than 15 years. Your thermostat could be just a simple bi-metal spring with contacts make a connection to another set of contacts on a slider. Again shaking will also cause the spring wobble and bend enough to make the necessary contact.

Anyways, problem is likely with that bi-metal spring. Since you have been shaking the thermostat the bi-metal string has either dislodged or way out of calibration. There might not even by any tension on the spring anymore. Has anyone recently bumped or knocked it off the wall at some point in the past?

I think your best bet is to replace the whole thermostat with a newer digital one. You should be able to find one at any of the big box hardware stores.

  • Thank you Harry. I will replace the thermostat. "A" for the day! Commented Jul 15, 2015 at 20:59
  • It might be the contacts of the toggle switch. Either way, replace it.
    – Mazura
    Commented Aug 14, 2015 at 15:52

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