-Troubleshooting your Gas Thermostat
Your gas thermostat controls the temperature of your house by giving feedback to the furnace controls – telling it when to burn and circulate the warm air and when to rest and shut off the burner & fan.
Without a correctly functioning thermostat, the furnace will not work properly. You must also ensure that the thermostat you have is correctly matched to your climate control system. The simplest of thermostats have only 2 wires running to them. More complicated thermostats (those that control both heating and cooling systems can have several wires and terminals running to them and can be very complicated. Do not try to change your thermostat without having a furnace technician help you.
How a basic Thermostat works:
Bimetallic Strip / Coil
Non digital thermostats work when something moves as a result of changes in temperature. This is accomplished by a special coil of flat wire called the bimetallic strip. Bimetallic simply means there are two different metals joined into one strip of flat wire. What is special about this is that the two different metals expand and contract at different rates in response to temperature changes. This means the metal strip curves one way or the other, depending on the temperature. Knowing the exact rates of expansion of the metals gives you a very precise temperature response ratio.
Switch or Contact
The switch is what turns the heat on or off. As the bimetallic coil moves, it turns the switch on or off. Many thermostats use a mercury switch (called an ‘ampoule’. Mercury is placed inside a sealed glass container with wires inside. As the coil moves, it tilts the glass container one way or the other. As the mercury moves back and forth, it either turns the heat on, or turns it off (it works the same way for the cooling system).
Heat Anticipator
The heat anticipator is a wire mounted in the thermostat that is connected to the bimetallic strip. This wire has a certain resistance to electrical flow and you to fine tune when thermostat turns the furnace burner on and off.
Troubleshooting a thermostat:
Mechanical Thermostats have a coil of flat wire (the bi-metallic strip) inside them which expands and contracts depending on the temperature. This coil is what tells the controls to turn on (or turn off) the heat. If there is dust or grime caught in the coils of the wire, it can give faulty readings. Remove the cover on the thermostat and with a clean dry brush (a paint brush will work) clean the coil and anything else under the cover. Be sure to get all the dust out of the coils of the wire and from under the cover of the thermostat.
When you have the cover off your thermostat, check to be sure there are no loose wires.
Caution:
The bimetallic strip is fragile. Do not bend or break it or damage any of the contacts. If you have a mercury switch, be very careful not to damage the ampoule that contains the mercury.
When you replace the cover, DO NOT press down too hard. The cover can interfere with the thermostat if it is jammed to tight against the coil. At a minimum, clean your thermostat once a year (before heating season). If it also controls your cooling system, clean it twice a year.
If your furnace is cycling too often, you may have a broken or damaged heat anticipator wire. Check to be sure the connection is tight. If it is not loose, you may have to replace the wire or the thermostat.
Digital Thermostats:
Digital thermostats are usually used to control both heating and cooling systems and often also control humidifiers. Usually, if a digital thermostat isn’t working, it is because the batteries have died. Replace the batteries and reset the controls. If the unit is still not working, call a furnace technician to replace the thermostat.
