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Troubleshooting a Thermostat

Pop the cover from a standard thermostat and you’ll discover that your heating system’s “brain” has remarkably few components. That’s because it amounts to nothing more than a temperature-sensitive on/off switching device. A programmable thermostat, on the other hand, adds high-tech circuitry. The sensing is done by a coil, or a strip, of two metals that expand and contract at different rates. As room temperature drops below the setting you’ve selected, this bimetal coil or strip closes a set of electrical contacts, sending a low-voltage signal to a transformer that turns on the furnace or boiler. When air warms above the thermostat setting, the bimetal opens the contacts again, shutting off the heat. Switching to the cooling mode simply reverses these cycles.

The switch is typically a sealed tube filled with mercury. When the coil tips the tube, the mercury tips over to complete an electrical circuit. A thermostat for a piped system should have a heat anticipator. This shuts off the boiler a little early since pipes will continue to heat after the boiler has turned off.

Thermostats are as reliable as any other switch. When a heating or cooling problem comes up, first make sure the temperature setting is at the right level—sometimes turning the dial up or down a few degrees will get things going again. Troubleshoot the system’s other components, as shown on the preceding pages. If they’re all in working order, shut off the main power switch, return to the thermostat, and try the procedures shown below.

The wires that lead to a thermostat carry very low voltage, so there is no need to shut off power before working on one. A thermostat should be positioned to measure the temperature of an average airspace in your home. If it is near a heat source or in the path of drafts, it may not provide comfortable heat levels for the rest of the house. Keep lamps and other heat-producing appliances away from the thermostat.

Tools: Screwdriver, paper or a dollar bill, fine brush, level, thermometer.

What’s under the cover? Remove the outside cover by simply pulling it off. To gain further access to parts underneath, remove two or more screws and detach the dial. The mercury-filled contact for the switch does not need to be cleaned. If your system turns on and off too often for your liking, turn the heat anticipator toward a longer setting. If you experience wide temperature swings, turn the anticipator to a lower setting.

Cleaning the contacts. In older units, clean dirt from exposed contacts by drawing a new dollar bill or other piece of paper between them. Or you can use a cotton swab dipped in alcohol.

Dust the bimetal. Dust on the bimetal will impair a thermostat’s efficiency. With a small brush, clean the bimetal and any place you find dust. Or blow out dust using “canned air” designed for cleaning electronic devices.

Check for level. Thermostats with mercury switches must be level. To reposition a thermostat, first remove the cover. Loosen the mounting screws, level the thermostat, and retighten the screws. If screws were driven into drywall only, they may come loose. Install plastic anchors. To level a round thermostat, hold a level or a weighted piece of string up to the vertical alignment lines.

Test for accuracy. To check the accuracy of a thermostat, tape a thermometer to the wall. If your thermostat delivers the wrong level of heat, you could simply scratch a mark on the cover indicating the desired temperature. Or replace the thermostat.

Replacing a Thermostat - Installing a new thermostat is an easy project, but you must work methodically. Shut off power—the voltage is not dangerous, but crossing live wires could cause damage to the controls at the heater. Remove the mounting screws, and gently pull the old thermostat out. Loosen screws to remove the wires, and label each wire with a marked piece of tape so there will be no confusion. Wrap the wires around a pencil or a small stick, making certain they cannot slip back through the hole and into the wall cavity. To ensure good contacts, cut the bare wire ends and restrip the insulation. Bend the wire ends to wrap around the terminals of the new thermostat, and screw them down tight. Check for level as you anchor the new unit.

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