Specifications

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Some important factors when choosing the best relay to purchase for the HVAC
control system are:
 Affordability, whether it is cost efficient for your budget in designing an
HVAC system
 Contact configuration: Single Pole Single Throw, Single Pole Double
Throw, Double Pole Double Throw, Three Pole Single Throw, Four Pole
Single Throw, or Four Pole Double Throw
 Coil voltage, how much voltage do you want the coils of the relay to
operate at from the available supply source in order for it to switch the
Heating, Cooling, and Air Circulation as a result of the measured
temperature and humidity.
 Type of mounting preferred: Usually surface mounting requires a soldering
iron, socket mounted for plugging into a circuit board for easy
replacement.
 Contact protection: This involves the relay‟s contacts that, due to arching
wear over time. Contact protection is the number one concern and priority
in relays. Relays that use relatively small contacts for the voltage and
current in an inductive circuit will result in arching and excess wear over
time.
 Low Power Consumption: Low voltage relay coils are less dangerous to
persons working with them, easier to install and do not come under the
stricter codes that apply to wiring over 30 volts.
 Max voltage sink by microcontroller: Depending on your microcontroller of
choice, there is a maximum voltage that the I/O pins can produce.
 The voltage produced by the microcontroller determines how the buffer
circuit must be set up so that the relay receives the correct voltage and
current.
In typical HVAC control systems there are two types of relays involved. These
relays are the control relays and the power relays. The control relays are
switched on by the main control unit when a decision to turn their specific
component on has been made. The output of each control relay is 24V AC, and
is known as the control voltage. This control voltage is the input to the next relay
which is the power relay. There is typically a power relay located outside at the
compressor and another located at the inside air handler. When the power relay
is switched to the on position, 220/240V AC is connected to the compressor or
air handler and it runs until the main control unit determines it is time to stop and
the relay is switched back to the off position. Figure 10 below is shown to better
illustrate this system.