How Do I Check a Honeywell Mercury Thermostat to See If it Is Working Properly?

The Honeywell mercury thermostat has been installed in homes and businesses for decades, though it is used less often since the digital programmable thermostat became popular. Mercury thermostats are extremely durable and long lasting but do wear out and develop problems occasionally. Troubleshooting a mercury thermostat is quite easy due to the simplicity of the unit and its limited function.

Man adjusting thermostat
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How a Mercury Thermostat Works

A thermostat is a temperature-controlled switch. In a mercury thermostat there is a small glass tube with a drop of mercury sealed inside. Two wires protrude into the tube at one end. The tube is held in place by a bimetallic coil that rotates the tube as the coil heats up or cools off. When the tube rotates enough, the drop of mercury makes contact between the two wire contacts, allowing electricity to flow between them. This electricity powers a relay that turns on your furnace or air conditioner. The thermostat has a switch to select operation as either a heating or cooling thermostat. This switch must be moved manually at the start of the heating or cooling season.. To work accurately, a mercury thermostat must be mounted vertically and level. The mounting base has marks to align the thermostat on a level line drawn on the mounting surface.

Troubleshooting

Remove the thermostat cover and look at the mercury bulb while turning the dial; the bulb should rotate as the dial is turned. The drop of mercury should move and make contact with the wire contacts as the temperature selector is turned up. A second set of contacts within the tube controls the cooling cycle and make contact when the thermostat is turned down to the desired temperature. Remove the wires from the contact screws and check for continuity across the contact screws. There should be an open circuit when the mercury is not contacting the wire contacts and a closed connection when it is. If the thermostat fails either of these simple checks, it is defective. The easiest way to check thermostat function is to jump across the terminal screws with a jumper wire. Usually the power wire is red and the heating wire is white, so making a connection between those two screws should turn on your furnace. If it does come on, do the same test with the cooling circuit by connecting between the red and yellow wire. This test bypasses the thermostat, sending power directly to the furnace or air conditioner. If both circuits work when the thermostat is bypassed, the thermostat has failed and needs to be replaced. If nothing happens, check the voltage to the thermostat. Locate the low-power transformer and use a multimeter to check for output voltage. If none is found, check the input circuit for power. Be careful; this is line voltage and can seriously hurt or kill you. If input voltage exists but no output voltage, the problem is the transformer; replace that. If the transformer is functioning properly, check the low-voltage wires to the thermostat. Disconnect both ends of the wires from the transformer and check for continuity after turning the thermostat all the way to its highest temperature setting. If your tester doesn't light up, the wiring has a problem, and repair or replacement is indicated.