How to Wire a Heat Pump for Control - There may be a time when you will want to change your old heat pump thermostat with a new heat pump thermostat. We have other articles here that will help you with thermostat wiring. These other articles help you with thermostat wiring colors and thermostat wiring diagrams. In this article, we will provide specific information for wiring a heat pump for control.
Wiring a heat pump is different than wiring an air conditioner with a gas furnace or boiler. There are extra controls in heat pumps that require extra attention and understanding to get the wiring correct. Additionally, changing an older mechanical thermostat to a newer digital thermostat will require some extra wiring that also may possibly require you to pull new wire to the thermostat from the air handler.
Variations for Heat Pump Thermostat Wiring
There are many variations to wiring a heat pump for control because there are many types of heat pumps. Heat pumps provide heating and cooling for a home or business. Heat pumps use the process of refrigeration to move heat in one direction or another depending on the season and the setting of the thermostat. Air source heat pumps have backup methods of heating for two reasons.
- when the air source heat pump system kicks into defrost it needs a backup heat source.
- if the temperature falls below a certain point the heat pump needs a supplemental heat source to keep up with the thermostat setting.
The most basic method of backup or supplemental heating for heat pumps is electric heat strips. The next most common backup heating source is a gas furnace. Both of these different methods can require a separate way of wiring your heat pump thermostat.
Heat Pump Thermostat Wiring Designations / Terminals and Colors (Thermostat Wiring Colors are Industry Standard (typical))
R Wire Designation and Terminal - The R wire is is the 24 volt hot originating from the 24-volt control transformer. This is the power to energize the control relays and contactors in your system. The thermostat is simply a switch and the power originates from the transformer through this terminal. For those with a power stealing thermostat (without batteries) the is also where the thermostat gets its power. The color of the R wire is typically red.
C Wire Designation and Terminal - the old C wire? This is the common wire and it has caused many people indigestion and sleepless nights. The reason for this is most modern thermostats require or need a C wire to function. The C wire is a common wire and it originates from the control transformer also as with the R wire mentioned above. A control transformer for an HVAC system has 4 wires. One side is for line voltage into the transformer and the other side is the 24-volt control voltage coming out the other side of the transformer (see illustration above right).
To have a complete circuit you need a source, a path, and a load. In this case, the transformer is the source, the R wire and the C wire is the path, and the thermostat is the load. That is if you have a modern digital thermostat that doesn’t require batteries. The thermostat needs power to function.
When is the C Wire not needed?
There are some digital thermostats that use batteries for power. In that case, you will not need to worry about a C-wire or as it is sometimes labeled C1 wire. This information should be provided to you with the thermostat instructions.
The C Wire Problem
The reason for all the consternation mentioned above is that many older thermostats did not require a C wire. They were simple mechanical thermostats with a mercury switching mechanism inside. BTW, mercury has been banned by the federal gubmint )) as a product that cannot be used in HVAC products. If you are changing an older mercury thermostat make sure you dispose of it properly. Mercury is very toxic to the environment. Anyhow, these thermostats never required a C wire so a newer modern thermostat would require an extra wire (the C wire) to work properly. The color for the C Wire will typically be dark blue.
Consequently, this means if you don’t have the right amount of wires at the thermostat you would need to run new thermostat wire to the new thermostat. That is specifically so you will have the correct number of conductors to make the new thermostat work properly. A heat pump thermostat requires more conductors than an air conditioner thermostat. Even providing for the C wire. This applies to Honeywell, Emerson, Ecobee, Nest, White Rodgers and all other modern thermostats that are digital. They need the C wire to complete the circuit and have power.
Other R Wire Terminal Designations and Wires
Many thermostats have RC and RH instead of a single R terminal. What does this mean? These thermostats are designed to handle separate systems that each have their own control transformer. You can’t have two transformers powering a single thermostat. If you only have a single transformer then you will likely have a jumper wire (or brass strip) between the RH and RC terminals. If you do not have the jumper you will separate red wires. The control transformer in the heating system will have a red wire terminated at the RH. The control transformer for the cooling system will have a red wire terminated at the RC terminal.
I had an apprentice ask me one time why is only one common wire or c wire used in this situation. Great question especially from an apprentice. It shows they are paying attention. It is because all neutrals are the same (in most cases). If it doesn’t cost too much I always advise the customer to simply rewire for a single transformer. Most transformers in either system can handle the VA rating required to power all controls in both systems and the re-wiring is simple (at least for me it is).
Other Heat Pump Control Terminals and Wiring Designations
Y Wire Terminal Designation - This color is usually a yellow wire and goes to the Y terminal. In a heat pump, this is for heating and cooling. It energizes the compressor contactor in the condenser.
O Wire Terminal Designation - This color is typically orange and goes to the O terminal (in Rheem and Ruud it goes to the B terminal). This control for the reversing valve in the condensing unit. The reversing valve switches the condenser from heating to cooling and vice versa depending on the thermostat setting.
W Wire Terminal Designation - This color is typically white and goes to the W terminal. In a heat pump, this is for the backup heat source. Whether it is electric heat strips or a gas furnace (dual fuel) the W terminal controls this. This wire will also run to the air handler/furnace and the condenser. The defrost control board in the condenser will automatically defrost the system. When it goes into defrost mode the backup heat is energized. In most cases, if you are simply changing your thermostat, you do not have to worry about any wiring at the condenser.
Auxiliary Thermostat Wiring for Heat Pump Thermostats
In some case, you will have a system that requires staging. Most residential systems that are staged are two-stage systems. In this case, you will need a thermostat that has a W2 terminal and a Y2 terminal. Colors for these terminals vary but in most cases, W2 will be black and Y2 will be light blue. In some cases, depending on your system, W2 can be used simply for emergency heat which is your backup heat source. It is sometimes referred to as auxiliary heating and it is used in case something goes wrong with the condensing unit.
In some cases, the terminal for the thermostat will be labeled Aux/E/W2. If your old thermostat uses two separate wires and they were terminated at Aux and E separately, then use a wire and an extra wire to bundle these wires together. Then terminate the extra wire to the Aux/E/W2*. *Always follow the manufacturers wiring instructions concerning this. Nearly every manufacturer is different and will have different instructions for wiring to get it right.
Conclusion | How to Wire a Heat Pump for Control - Heating and Cooling
Lastly, always make sure to follow the instructions for the thermostat manufacturer for wiring and setup. Something newly installed thermostats require a setup specifically for a heat pump. The instruction guide should help you navigate through the settings to make the appropiate changes to the thermostat so it will function properly as a heat pump thermostat.
There other possible wiring configurations for your heat pump thermostat wiring but these are the basics. Hopefully, these are explained in a way that will help you understand the wiring. If you have a humidifier for home humidification and an electronic air cleaner for cleaner air there will be some extra wires for those devices. Follow the basic thermostat wiring instructions for those wires. If you get confused, call in a professional. Believe me, it will be cheaper, in the long run, to get a professional HVAC technician do this for you rather than burn something up for miswiring.
How to Wire a Heat Pump for Control - Thermostat Wiring Guide