Checking a Thermostat's Calibration

A thermostat is a highly sensitive control instrument that responds to even the slightest changes in temperature. While it has fewer parts to malfunction than the other components of your heating and cooling system, it can be a source of problems.

What You'll Need
You'll want to have these tools on hand to check a thermostat's calibration:
  • Glass tube thermometer
  • Tape
  • Paper towel
  • New dollar bill or soft brush
  • Level
  • Screwdriver

A thermostat cover that's improperly installed or inadvertently bumped can cause the heater or air conditioner to fail to start. Or the thermostat base may slip out of level, causing it to operate incorrectly. A far more common problem, however, is dirt. Dirt can affect the thermostat's calibration and interfere with its operation. If a thermostat set for 70 degrees Farenheit, for example, is really maintaining the temperature at 73 degrees Farenheit, the additional energy used can increase your fuel bill by as much as 7 percent. To prevent this, check your thermostat for accuracy every year before the heating season begins.

Other problems with a thermostat can often be traced to switches on the base and wires near the bimetallic element that loosen and become corroded. Tighten loose connections with a screwdriver, and use a cotton swab to clean away corrosion.

Checking Thermostat's Calibration

Here's how to check a thermostat's accuracy and clean it if necessary:

Step 1: Tape glass tube thermometer to wall a few inches away from thermostat. Pad thermometer with paper towel to prevent it from touching wall. Make sure neither thermometer nor thermostat is affected by any outside temperature influences. In some homes, hole in wall behind thermostat through which wires enter is too large, allowing cold air to reach thermostat and affect its reading.

Step 2:  Wait about 15 minutes for mercury to stabilize. Then compare reading on thermometer with reading of thermostat needle.

Step 3:  If variation is more than a degree, check to see if thermostat is dirty. To examine thermostat, remove faceplate, usually held by a snap or friction catch. Blow away any dust inside it. Do not use a vacuum cleaner; its suction is too great. If thermostat has accessible contact points, rub new dollar bill between them to clean these spots. Do not use sandpaper or emery cloth. If element is coiled, use soft brush for cleaning.

Step 4: If thermostat has mercury vial inside, use level to make sure unit is straight. If thermostat is not straight, loosen mounting screws and adjust thermostat until it is level. Then retighten screws.

Step 5: After cleaning thermostat, check it again with glass thermometer, as detailed in Steps 1 and 2. If thermostat is still not calibrated properly, it should be replaced as detailed below.

Check out the next section for tips and techniques on how to replace a thermostat.

For more information on energy efficiency in the home, try these links.

Replacing a Thermostat

There are several crucial steps to consider when replacing your thermostat. Replace a faulty thermostat with a new one of the same voltage. The thermostat must be compatible with the heating system. Here's how to replace a thermostat:

What You'll Need
You'll want to have these tools on hand to replace a thermostat:
  • Screwdriver
  • Utility knife
  • Replacement thermostat
  • Tape
  • Level

Step 1: Remove old thermostat. Take faceplate off old unit, and look for mounting screws. Remove screws to release thermostat from wall. Remove wires from back of old thermostat by turning connection screws counterclockwise. Be careful not to let loose wires fall down between walls.

Step 2: Clean exposed wires by scraping them with utility knife until wire ends shine. Attach wires to new thermostat. New thermostat must have the same electrical rating as old one.

Step 3: Once wires are attached to replacement thermostat, push wires back into wall, and tape up opening to prevent cold air inside walls from affecting thermostat.

Step 4: Install mounting screws to secure new thermostat to wall. If thermostat has mercury tube, set unit against level during installation; mercury tube thermostats must be exactly level.

Step 5: Snap faceplate back into place. Make sure new thermostat turns heating/cooling system on and off when temperature setting is adjusted.

Following the easy guidelines in this article will ensure that you can successfully maintain and replace your thermostat when the time comes.


For more information on energy efficiency in the home, try these links.
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