What Is a Heater's Integral Limit Control?

Heater Controls

• Furnace heater controls allow for easy operation of the system. Thermostat devices include temperature sensors, limit control mechanisms and signal output elements that maintain the desired temperature setting within the home. As different heater systems incorporate various components and heating processes, the furnace heater control unit should be compatible with how a particular system operates. Thermostat operations may consist of a basic control design or include the additional features found in integral limit controls. In effect, an integral limit control allows for more precise temperature regulation.

Types of Limit Controls

• The most basic thermostat limit control appears as an “On/Off” lever or switch. In effect, the “On” position turns the furnace component on, and the “Off” position turns the furnace off. A more sophisticated design includes a proportional limit control that adjusts furnace output on a gradual basis. The proportional controller slows or speeds up furnace output as room temperatures approach or exceed the desired temperature setting. An integral limit control -- also known as a PID control -- uses a proportional control, combined with two other control methods -- integral and derivative -- to fine-tune how a furnace responds to the thermostat’s signals and settings.

Integral Controls

• The integral control feature found on PID thermostat models regulates furnace operations by using error signals as a reference point. Error signals result whenever room temperatures differ from the desired temperature setting, also known as the setpoint. Integral control actions coordinate furnace operations based on a rate or time setting, meaning the length of time it takes to resume the desired temperature setting. Once room temperatures reach a pre-set “error” level, the integral controller signals the furnace to either boost or reduce output levels to ensure the setpoint temperature is reached within the pre-defined time frame.

Derivative Controls

• Within a PID thermostat design, proportional, integral and derivative control features all work together to optimize the heating system's energy efficiency levels. The derivative control feature further enhances accuracy levels in terms of how furnace operations respond to error signal response times. Derivative control actions work to anticipate rate or response times recorded by the integral controller. Derivative controls also identify differences in response times when room temperatures exceed pre-set levels versus when temperatures dip below pre-set levels. In effect, this control feature helps shorten signal response times based on the furnace system’s past performance.