Are thermostat mercury switch tubes supposed to be exactly aligned?

Old 12-25-22, 03:37 PM
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Are thermostat mercury switch tubes supposed to be exactly aligned?

Merry Christmas!

I have an older Trane system with a mechanical thermostat that uses two mercury switches mounted to the bi-metal temp-sensing coil.

One of the mercury tubes is just a hair off from parallel with the other so what will happen is when it heats to a certain point the heating element turns off but the air handler will keep running because there's still contact with the mercury switch.

Are they supposed to be like this or are both mercury tubes supposed to be exactly aligned so they always complete or break the circuit at the same time?
Old 12-25-22, 03:54 PM
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Merry Christmas to you! I can't answer your question regarding the thermostat alignment. However, I wanted to mention that it's normal for the air handler/blower to run for a period of time after the thermostat has reached its set point. It would be very inefficient if it didn't. When the thermostat stops calling for heat, there is still a lot of heat left in the heat exchanger. The blower runs until the heat exchanger temperature drops to a low level. Many thermostats have an "anticipator" setting. This slightly heats the thermostat so that it turns off slightly before the set point is reached. When properly set, the anticipator prevents the heating system from overshooting the set point. The anticipator essentially gives you fine control over the cycle length.
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Old 12-25-22, 05:46 PM
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It sounds like you have a 2 stage stat. Stage 2 mercury bulb alignment would be slightly different.

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Old 12-27-22, 02:04 PM
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Hi, does the space maintain the correct thermostat set point?
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