What are the different types of HVAC systems?
This page details common HVAC systems and how to identify the system type.
How to identify an HVAC system type
While most customers will know which HVAC system type their home or business uses, there are a few ways to verify the system type.
Use the current thermostat wiring as a guide
For the most common HVAC types, certain wires are required in order to operate effectively. Reference the Expected wires sections for each system type below to compare to the current thermostat.
Verify the information located on the outdoor unit
The outdoor unit of the HVAC system often contains a label with helpful information about the system type. Sometimes the system type is directly labeled within the model information.
If the label is missing or otherwise illegible, use the outdoor unit's model number to search for the device's manual online. The system type will often appear within the device's installation manual.
Contact an HVAC professional
If the wiring or the information from the outdoor unit does not provide the needed confirmation, an HVAC technician should be able to analyze the system and verify.
Heat Pump
Expected wires
| Wire | Description |
|---|---|
| R or RH/RC | Relay to close electrical circuit to control board |
| Y | Operates condenser of Heat Pump systems |
| W, W2, Aux, E | Auxiliary and emergency heating stages |
| O or B | One required to operate the reversing valve, could have both |
| G | Operates fan |
| C | A/C power |
Caution: These are not guaranteed to be correct for every install. There are no standards for HVAC wiring. These are good suggestions, but if unsure, check the HVAC system to confirm how wires are connected.
How it works
Cool Mode (Reversing valve: O Energized, B De-Energized)
- Warm gas refrigerant is pumped to the outdoor unit.
- It goes through the compressor and condenser, turning it into a liquid and cooling using the outside air.
- Now a cool liquid, the refrigerant flows back indoors.
- It passes through an evaporating valve, turning it into a gas and supercooling the refrigerant.
- Air is blown over the tubes containing the refrigerant, warming the gas refrigerant and cooling the air.
Heat Mode (Reversing valve: B Energized, O De-Energized)
- Cool liquid refrigerant is pumped to the outdoor unit.
- It passes through an evaporating valve and condenser, turning it into a gas and warming the refrigerant with the outside air.
- Warm gas is sent to the indoor unit.
- It goes through a compressor, turning the refrigerant into a liquid and superheating it.
- Air is blown over the tubing, heating the air and cooling the refrigerant.
Normal (Conventional)
Expected wires
| Wire | Description |
|---|---|
| R or RH/RC | Relay to close electrical circuit to control board |
| W | Operates heating element |
| Y | Operates condenser |
| G | Operates fan |
| C | A/C power |
Caution: These are not guaranteed to be correct for every install. There are no standards for HVAC wiring. These are good suggestions, but if unsure, check the HVAC system to confirm how wires are connected.
How it works
Cool Mode (Same as a Heat Pump)
- Warm gas refrigerant is pumped to the outdoor unit.
- It goes through the compressor and condenser, turning it into a liquid and cooling using the outside air.
- Now a cool liquid, the refrigerant flows back indoors.
- It passes through an evaporating valve, turning it into a gas and supercooling the refrigerant.
- Air is blown over the tubes containing the refrigerant, warming the gas refrigerant and cooling the air.
Heat Mode with a furnace
- Fuel is burned.
- This is generally some kind of fossil fuel (i.e., natural gas, oil, etc).
- The exhaust is passed over ducts carrying air (forced air, steps 3-4) or water (hydronic [radiators], steps 5-6).
- The hot air is blown through the house.
- Vents circulate the air into a room.
- Cool Air is pulled back to the furnace.
- Returns circulate air out of the room.
- Water Boils in the furnace.
- Turning to steam, it propels itself through the home.
- Heat is exchanged at the radiators.
- As the steam cools, it falls back.
Heat Mode with a baseboard (electric)
- Electric current is passed through resistor wiring.
- Resistors heat up, heating the ambient air.
Multi-stage
Specialty wires
| Wire | Description |
|---|---|
| Y2 | Additional stages of a heat pump/condenser, could also be additional units. |
| W2, Aux, E, Z | Additional stages of a heat, could also be additional units. On some thermostats, the E terminal's function is to turn off a heat pump and only use auxiliary stages when the thermostat is put into emergency mode. The Alarm.com Smart Thermostat does this automatically, so a jumper between W and E can be ignored. |
How it works
Heating/Cooling Differential
- The differential is set to X degrees.
- If the difference between Ambient Temp and Target Temp reaches X degrees, the next stage will be triggered.
- For each stage, it will add another X degrees (i.e., if it's 3 degrees, stage 2 will start at a 3-degree difference, stage 3 will be 6, etc.).
Staging Delay
- Staging Delay is set to Y minutes
- If the Ambient Temp does not reach the Target Temp within Y minutes, the next stage will be triggered.
- For each stage, it will add another Y minutes (i.e., if it's 3 minutes, stage 2 will start after 4, stage 3 will be 4 min later, etc.).
Master/accessory Thermostat Relationship (O/B Zoning)
Specialty wires
| Wire | Description |
|---|---|
| RH, RC |
At the master thermostat, these will wire to HVAC Controller. At the accessory, they are O and B wires from the master. |
| Z |
At the master thermostat, will have the B wire when configured for O/B Zoning. This is not present at the accessory. |
| O/B |
At the master thermostat, will have the O wire when configured for O. This is not present at the accessory. |
How it works
Master thermostat
Will be configured and wired similarly to a Heat Pump setup.
- Heat Mode: Z Terminal energized
- Cool Mode: O/B Terminal energized
Accessory thermostat
Will be wired and configured the same as any other thermostat.
- Relay wires will come from the master thermostat
- Sometimes these will have W, R, and Y wires, but no G wire to run the fan. This is fine, as any necessary fan activation will be controlled through the master thermostat. The install wizard will say you need a G wire, so just tell the wizard that there is a G wire