What Is the Purpose of a Limit Control Switch?

Identification

Home heating and air-conditioning systems may be out of sight and out of mind, but they go a long way toward making our home a comfortable environment. A wall thermostat is about all there is to managing how they perform--until there's a problem. The limit control switch, another thermostat-type device, plays a key role in establishing whatever temperature is set on the thermostat. This device is located on the furnace or air-conditioning unit. In most cases, it works in conjunction with your thermostat's functions, provided the thermostat is set correctly. In addition to numeric settings, most modern thermostats show an "on" and "auto" setting, or a "manual" and "auto" setting. These settings determine when the blower fan comes on and how long it should stay on. And while these settings may not directly affect a room's temperature, they do affect how your furnace circulates air through the house.

Function

The primary role of a limit control switch is to determine when to turn off the furnace blower fan. Turning off the fan too soon means the heater element is active but no warm air is being circulated. Turning it off too late means blowing cool air through house since no heat is being generated by the heater element. The same applies in air-conditioning systems. By regulating the blower fan, the control switch maintains the temperature that's been set on the thermostat and keeps the heating element from overheating by blowing the accumulated heat through the duct work. A supply air plenum houses the heat exchanger and allows heated air to accumulate. After a while, this heat supply can warp or even crack the heat exchanger unless the blower works to circulate air through the system.

Effects

The inner workings of a limit control switch are designed to switch on the blower fan in accordance with the temperature setting and sense when the air plenum has reached a certain temperature. Inside this switch resides a bi-metallic spring. One end of this spring protrudes into the air-plenum compartment, while the other end is attached to a gear device that turns the fan limit-control dial. When the temperature in the air plenum reaches a certain point, the spring expands. As the spring expands, it begins to turn the fan limit-control dial. The turning of this dial regulates the blower fan. Electrical contacts reside on the back of the fan limit-control dial. These contacts respond according to the spring's movement and so turn the heating element on and off. Consequently, these electrical contacts are directly affected by changes made to a room's thermostat settings.

References

• Guide to Furnace Fan Limit Switch Control

Resources

• Limit Switch Manufacturers & Limit Switch Suppliers