Troubleshooting The Thermostat

May 28, 2004
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It's a standing joke that some homeowners think the brand of their HVAC system is a White-Rodgers or Honeywell. This month's Tech Tip looks at the top five thermostat problems and their possible diagnoses.

Some of these troubleshooting tips may be used over the phone by your front person, to help screen for problems that the homeowners themselves might be able to correct. Their use should be discretionary, depending on the level of mechanical aptitude your customer seems to have. It's a judgment call.

The information can also be given directly to select customers for their own reference, again on a discretionary basis, or given to less-experienced employees performing seasonal system maintenance (i.e., clean and checks) to possibly solve customers' problems on the spot.

We have listed these top-five thermostat problems in reverse order (with a nod to David Letterman).

Our Top-Five Countdown

Problem #5. The thermostat setting and thermostat thermometer disagree.

Solution. In this case, the thermostat setting probably requires adjustment:

  • On older mercury or snap-action thermostats, the temperature indicator can be adjusted using a small screwdriver.

  • On most digital thermostats, the thermometer can be adjusted ±5 degrees. (Check the manufacturer's literature on the proper procedure.)

    Problem #4. The furnace or air conditioner cycles too fast or too slow (there are narrow or wide temperature swings).

    Solution. This cycling rate problem could be influenced by the location of the thermostat and/or the size of the system's equipment. Therefore:

  • Ensure that the thermostat in not in an area where full sunshine or open doors to the outside can affect the temperature the thermostat "sees." If it is, move the thermostat to a less susceptible area.

  • Keep in mind that digital thermostats normally provide precise temperature control and may cycle faster than some older mechanical models. Check the manufacturer's literature to adjust the cycle time faster or slower.

    Problem #3. "Heat," "Cool," or "Fan" runs constantly.

    Solution. In this case you may be looking at a mechanical error or operator error:

  • Is there a short in the wiring, thermostat, or Heat/Cool/Fan system? (Note: Turn the power off to the furnace and air conditioner before inspecting any wires.) Check each connection to verify that they are not shorted or touching together. No bare wire should be showing from under the terminal screw or terminal block.

  • Is the Fan switch set to "Fan On"?

    In the Fan On mode the fan will run continuously. To have the fan run only when the system is heating or cooling, switch to Fan Auto. If the fan continues to operate in the Fan Auto mode and the equipment is not calling for heating or cooling, contact your local service person.

    Problem #2. Customer says there is no cooling.

    Solutions. Of course, there are many reasons why a cooling system doesn't seem to be working. Check for the following thermostat-related fixes:

  • Is the system switch set for "Cool"? Make sure the "System" switch is set in the Cool mode, at a lower temperature than the room temperature. Within a few seconds, you should hear a click. This sound usually indicates that the thermostat is operating properly, and the cooling system should begin operation shortly.

    Note: Some heat pumps have a five-minute time delay built in as a safety feature. If the cooling system does not begin operation after five minutes, a service call is required.

  • Is there a loose connection to the thermostat or system? (Again, turn power off to the furnace and air conditioner before inspecting any wires.) Verify that thermostat and system wires are securely attached.

    Problem #1. Customer says there is no heat.

    Solutions. Again, there could be numerous system problems behind the no-heat call. The following may be diagnosed quickly:

  • Is the pilot light lit? If not, relight the pilot.

  • Is the System switch set for Heat? Make sure the System switch is set to the Heat mode and raise the thermostat setting so that it is greater than the room temperature. Most thermostats, both mechanical and electronic, will click within a few seconds. Some heat pumps have a five-minute time delay built in as a safety feature. If the heating system does not begin operation after five minutes, a service call may be required.

    The heating system should then begin operation shortly after hearing this click. If the heating system does not begin operation, contact your local service person for assistance.

  • Are there loose connections to the thermostat or system? Verify that thermostat and system wires are securely attached. Again, you must turn power off to the furnace and air conditioner before inspecting any wires.

  • Perhaps the furnace is in a lockout condition. (This may also cause intermittent heat.) Many furnaces have safety devices that shut down when a lockout condition occurs. Try turning off the electrical power to the unit for several minutes, then turning it back on. If the heat does not turn on or works intermittently, a service call may be required.

    For more information, click on the Emerson Climate Technologies logo above.

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