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Mercury Switch Problems

What is a mercury switch?

A mercury switch is a sealed tube with two electrode and mercury in it. When the liquid mercury is at opposite ends of the tube, the electrodes are not connected. When the tube is tilted, the mercury pools causing the circuit to be complete and allowing current to flow. These are often found on older model thermostats where a heat sensitive coil will respond to ambient temperature changes and tilt the switch to start and stop the flow of electricity to the heating or cooling unit. Read below where Experts have answered questions regarding mercury switches and mercury switch replacement.

How do I connect a new Honeywell RTH221B unit in place of an old mercury switch thermostat?

Case Details: The old one has just one black and one white wire. The new one a Y and G terminal on the left and RH. RC and W on the right, with a jumper between RH and RC. 

Typically a two wire mercury switch thermostat should have a red and white wire. It could be that the red wire was replaced in the past with the black one. Connect the back wire to one of the R terminals and the white one to W. If that does not work switch them around with the back to W and the white to R.

How do I connect my new Honeywell RTH7600D thermostat to my Trane model TWE036C140B0?

Case Details: My old unit has X2, O, G, Y, T, R, W (with circuit board trace jumper to U), and B terminals

The new unit you mention should be a good replacement. The T terminal is no longer needed so you can tape the wire and tuck it away into the wall. The B on the Trane unit is an old designation. It is now C (common). R to either R or RC, it doesn't matter which one as they are connected with a jumper. O, G, Y will remain the same. The O/B terminal is already configured to O so you don’t need to touch that. W will go into AUX and you will not be using the L terminal.

Do I need a mercury switch replacement in this case?

Case Details: A natural gas furnace has an old mercury switch thermostat fitted on the wall. The pilot light is on but adjusting the slide on the thermostat procures not result. 

There is a simple way to check if your thermostat is bad. Remove the thermostat from the wall and disconnect the two wires that go into it. Now touch them together. If the furnace starts, your thermostat is bad and may need replacement. If the problem persists it could be a generator or gas valve issue.

Can you help me in replacing my old Lennox round dial mercury switch thermostat with a Honeywell Series 2000 digital thermostat?

There should be two cables going into the old thermostat. The red wires from each cable should be connected to H. Cable 1 white wire will go to W, the cable 2 black wire will go to G and the orange one to Y. Disconnect the old thermostat and connect both the red wires to either RH or RC on the new thermostat, which should have a jumper between them. If it is not there, place one. Black will now go to G, white to W and Orange to Y. There will be a C or Common wire that will go to the X terminal. There may be 2 dip switches on the back of the new thermostat. Switch one locks the keypad and switch 2 extends the fan operation. There is no need to adjust either of these switches.

Because of the hazardous nature or mercury, mercury switches are not in common use today. When dealing with mercury switch replacement and troubleshooting, follow the instruction in your owner’s manual or those which come with the new thermostat. If this does not help, getting the help of an Expert is advisable to prevent any damage and because you are dealing with a hazardous material.

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