What type of signal is typically associated with the control output in an HVAC system?

The 4-20 mA current signal is commonly used in HVAC systems for control outputs due to its reliability and efficiency over long distances. This type of signal is advantageous because it is less susceptible to electrical noise and voltage drops, which can distort the signal if a voltage output were used. The 4-20 mA current loop provides a clear range of measurement, where a 4 mA signal typically represents the minimum value of a variable (like a temperature or pressure) and 20 mA represents the maximum value.

This current signal also has the benefit of allowing the control system to detect any issues, such as a broken wire, since a current of less than 4 mA would indicate a fault in the system. Furthermore, the use of a current signal allows for easy integration with various types of sensors and controllers, making it a standard in industrial control applications, including HVAC systems.

While digital pulses can be used for certain types of on/off control actions, and analog voltage outputs can work for some applications, 4-20 mA is more widely accepted and preferred for continuous measurement and control due to its stability and robustness.