Thermostat Wiring Diagrams – Heat pumps are wired for HVAC control far differently than air conditioning systems so make sure you know the difference and correctly identify the type of HVAC system you have installed.
Thermostat Wiring Diagrams
With the top diagram showing an air conditioning system and the second diagram showing a heat pump system and finally the third diagram showing the average type of split system with an air conditioner or gas or oil furnace used for heating. The heating system could also be a boiler, however, is it necessary to ensure with split systems and the type of heating systems described that you do not have dual transformers in the system or to at least be aware that the system has dual transformers. Whenever in doubt call a professional.
Thermostat Wiring Diagrams - HVAC Control - Heat Pump Systems
Heat pumps are different than air conditioners because heat pumps use the process of refrigeration to heat and cool. While an air conditioner uses the process of refrigeration to only cool the central air conditioner will usually be paired with a gas furnace, an electric furnace, or some other method of heating. So while the air conditioner is used extensively in warm or hot weather the heat pump is used year round. The heat pump system has different controls than an air conditioner, it has its own heat pump sequence of operation so a heat pump works differently than an air conditioner so will require a different thermostat and different wiring than an air conditioner. Sometimes, depending on the thermostat and the manufacturer, the thermostat can be a dual heat pump thermostat or it can be used for an air conditioner and gas furnace system depending on how the thermostat is wired. Always make sure you know what type of system you have before doing any wiring.
Honeywell Thermostat Wiring Diagrams
This is a basic Honeywell thermostat wiring diagram for a single stage heat pump. If you have a two-stage heat pump then you will also utilize terminal Y2 for the second stage. This is thermostat wiring diagram is specifically for a system with two transformers. Your system likely only has one transformer as most typical residential systems only use a single transformer for control. If you do have two transformers then your old thermostat had a red wire on the RC terminal and a red wire on the RH terminal. Single transformer systems typically use a jumper for thermostats that have two R terminals (RC and R or RC and RH). Another diagram of thermostat wiring for two transformers. The thermostat wiring diagram below utilizes the Y2 terminal meaning the system has two stages.
Heating Only Thermostat Wiring Diagrams
If you only have a furnace such as a gas furnace, oil furnace, electric furnace, or a boiler then you will use the following for simple thermostat wiring:
- Terminal R or terminal RH for the red wire. This originates from the transformer. If the thermostat has an RC terminal it is not needed for this application. It does not matter if you leave the jumper in the thermostat or take the jumper out if a factory jumper is installed between RH and RC provided there is no cooling for this particular system.
- Terminal W for the heating relay. for hot water boilers this will likely go to your aquastat on the boiler, however, it could also control a zone valve if you have a zoning system. After the zone valve opens it closes a switch and tells the boiler to maintain a temperature setpoint for the water.
- Terminal C if the thermostat is digital and has no batteries. The common originates from the transformer and is required to power up the thermostat.
Cool only thermostat wiring diagram will mirror the above instructions except you use the Y terminal instead of the W terminal.
White Rodgers Thermostat Wiring Diagram
White-Rodgers shows a slightly different type of wiring diagram that mirrors a ladder logic diagram. It is clear and understandable. At least to an expert. This diagram shows what each terminal designation that goes to the HVAC system. The illustration below is for a system with a single transformer.
Thermostat Wiring Diagrams
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