Heating and Cooling Control Wiring

Note: This is an informational/instructional page. The author does not accept any responsibility for the misuse or damage to equipment if information is misunderstood.

Like many do-it-yourselfers, I have had to troubleshoot, repair, and install residential HVAC systems. Since I don't do it often enough, certain things are forgotten so I created this page as a reference to aid and assist in repairing or installing control systems for heating and cooling systems.

This page primarily discusses the electrical controls of the system but in order to clarify certain things, I will discuss briefly the theory of refrigeration techniques.

Typical systems here in the United States follow common guidelines when it comes to wiring controls in air conditioning systems. Industrial and commercial systems are more elaborate but still follow basic principles and wiring.

A typical residential HVAC system has three primary parts. The thermostat, the indoor fan/heating unit, and the outdoor unit.

All control voltages to and from the system are low voltage, 24 volts AC.

For the purpose of this page, I will refer to the unit on the outside of a dwelling as the "outdoor unit" ODU, and the unit in the house as the "indoor unit" IDU. The Indoor unit can be located in the attic, a closet in the living space on the main floor or the basement. The actual location is determined by where and how all the supply vents and returns are accessed.

Heat only

Heating systems were made up of a furnace and a thermostat. The thermostat controlled the indoor temperature and the furnace generated the heat. Heat was in the form of oil, natural gas, or electric. Safeguards were built onto the furnace to prevent out of control heating resulting in an explosion or fire. I won't get involved too deeply in the actual theory of operation but touch on how they work.


As you can see, when the inside temperature drops to a set level, a contact is made and the furnace starts. When the temperature reaches another point, the contact opens and the furnace shuts down. In a typical oil or gas furnace, there is the heating unit itself and a fan blower. When a request is made to heat, the oil or gas unit fires. When an internal temperature is reached, the fan turns on to circulate the heat. When the request for heat is met, the heating unit shuts off and the fan continues to blow until most of the heat has been removed from the furnace unit. This "delay on", on the fan keeps cold air from blowing until there is sufficient heat to circulate. The "delay off" evacuates the heat from the unit to prevent excessive heat buildup within the unit and also moves the heat into the rooms thus not wasting trapped heat. There is also an "overheat" or overtemp control switch to prevent temperatures rising to an excessive level and damage the unit or cause a fire. An overheat condition can be caused by fan motor or belt failure, the air filter clogged and air isn't able to move out of the unit, or if vents are clogged, shut off or blocked in any way and heat isn't removed from the unit. Sometimes in normal operation if there is too much heat being generated, the overtemp will cycle throughout the heating cycle.

Cooling (Air Conditioning) only

The term "air conditioner" usually refers to cooling only even though air conditioning refers to conditioning the air in a given space.

An air conditioner is made up of a thermostat and a cooling unit. The cooling unit is usually made up of two components. The indoor unit (indoor fan unit) and the outdoor unit (condenser unit). Window type air conditioners incorporate all three into one relatively small enclosure.


When the temperature rises to a set point, the outdoor unit (compressor and fan) turn on together and cool air emits from the indoor unit. When the temperature drops to a set point, both units turn off together. This works a little different than a heating unit where both units go on and off together. If you let the fan keep running when the compressor turns off, you will smell a musty odor coming out of the air supplies. This is the evaporator cooling off.

Theory: This is an over-simplified description how cooling works. There are two sets of coils and fins. One set is in the outdoor unit (the compressor with a fan), usually on the outside of the house. The other is in the indoor unit (inside of the house) and has a fan blowing air through it. There is a refrigerant gas usually Freon® F-22 in the closed circuit of the two coils. The compressor compresses the gas into a liquid. When this happens the liquid is hot. This heat is sent through the outside coil and the fan removes the heat from the liquid. The liquid is also at a high pressure. This liquid is sent to the indoor coil by way of a valve called an expansion valve. When the Freon® passes through this valve, it enters the indoor coil at a much lower pressure. As the liquid turns into a gas, evaporation takes place and the gas becomes very cool. This cold gas is what the indoor fan releases through the cooling coil and fins. This is why the indoor unit is called an evaporator. The gas is returned to the outdoor compressor where the cycle starts over again.

Heat pump system

This refers to a common system to heat and cool a dwelling. Note: Heat pump systems only work in moderate climates where temperatures don't often exceed 100° F or drop below 20° F. If temperatures drop below 20° F, heat pumps cannot provide optimum heating for a dwelling without some assistance by conventional electrical resistance heating. The term "heat pump" means to remove heat from the outside air and transferring it into a dwelling. Obviously, with temperatures below 20° F and efficiencies of current compressor technology, heat pumps are poor in efficiency and not very economical.

A heat pump system is a combination heating and cooling system using all the same components. What makes the system work is the fact that it reverses the process of conventional air conditioned cooling simply by making the outdoor coil and indoor coil reverse its mode of operation. This is done with a special valve in the outdoor unit called a "change-over-valve" COV or "switch-over-valve" SOV. It may also be called a reverser valve. Different manufacturers may call it a different way. The valve reverses the process of heating and cooling as the compressor can only pump one direction. The typical expansion valve has a bypass check valve to allow gas to freely return to the compressor without having to go through the restricted orifice of the expander valve.

Since the evaporation takes place on the outdoor unit, if there is any moisture in the air, frost or ice will form on the coil and fins eventually clogging them and blocking the air flow. Because of this, there is a timer and or sensor mounted on the outdoor fins that detects the amount of frost or ice that forms. Some units just use a timer circuit and at a predetermined time settable by the installer, the unit will reverse itself and go into the cooling mode to heat up the outside coil and defrost the coil. At the same time, heating elements in the indoor unit will come on and supplement the heating process in the house so the homeowner won't feel uncomfortable during this cycle. The cycle usually lasts about 5 minutes or so depending on the ice buildup.

There are many types of thermostats but only one correct way to hook them up for any given system. Most thermostats are designed to work in just about all types of systems as long as they are wired correctly. Hooking them up wrong will mean a system not functioning properly or blowing the fuse in the indoor unit and possible burning out the power transformer, or damaging the thermostat.

Digital thermostats derive their power from the 24 volt transformer in the IDU and most have two AA batteries to provide settings and preferences in the thermostat if there is a power failure. One feature in the digital thermostats is the timer that prevents damage to the compressor if there is a power failure and pressure hasn't bled off enough for the compressor to start.

Older conventional mercury thermostats are being replaced with newer digital thermostats and some are programmable for each day. Newer manual digital thermostats are still "programmable" in the field depending on the type of system you have.

Depending on your circumstances, installing a new system or replacing an old thermostat, you must check and verify that the color code matches your system otherwise it will not work.

Most residential heat pump systems are known as 2H/1C. this means that there are two heating systems (Freon and resistance) and one cooling system.

There is an industry standard color code for the wires going to and from the thermostat. The following documentation is most typically the industry standard.

Designation Color Description                         
R Red 24 volt AC from IDU
G Green Fan Relay
Y Yellow ODU compressor relay
B or C Blue Common - gnd. return
O/B Orange SOV/COV relay valve
W or AUX or Em White Sec/Electric heat relay

Click here for a color printout of the above code

Other wires may be used for commercial applications. Pink, Black, Brown, Violet. These may be used for motorized damper controls or for bringing in outside air.

Glossary of Terms

Conditioned Air - refers to air space that is controlled (heat or cold) by an HVAC appliance.

Programmable Digital - thermostats that have an electronic digital display and can be uniquely programmed for a seven day period. Conventional non-programmable digital thermometers are still field programmable for types of systems and upper and lower limits.

HVAC - Heat - Ventilation - Air Conditioning.

Em Heat - Emergency or auxiliary heat. Usually electrical resistance heat.

Emergency Heat - Heat provided in the event the Freon based system fails.

Auxiliary Heat - Usually denotes heat other than the FreonŽ based system.

IDU - Indoor unit (air handler)- nicked named "fan-coil"

ODU - Outdoor unit (compressor/fan assembly).

Condenser - Primarily refers to the coil in the outdoor unit when in the Cool mode.

EER or SEER - Energy Efficiency Ratio or Seasonal Energy Efficiency Ratio. The higher the number the more efficient the compressor is to be able to draw heat from the air. Newer Energy Star rated units have an SEER of 13 or better. Anything less than an EER of 10 should be replaced for better heating/cooling efficiency.

Evaporator - Usually refers to the coil in the indoor unit when in Cool mode.

Expansion Valve - Used to convert high pressure liquid to vapor.

Anticipator - Heat compensator located in a mercury type heat thermostat.

Mercury Thermostat - Older style bi-metallic thermostats with a mercury tilt switch.

Resistance Heat - Electric heating coil located in the IDU.

Heat Pump - Typical cooling/heating system.

Furnace only - A heating system only using fuel such as gas, oil, or propane.

Air Conditioned only - Unit that cools only.

SOV or COV - Switch-over valve or Change-over valve. Used to change from heating to cooling.

Mars(TM) relay - A slave fan relay and sometimes a transformer assembly added. Usually used in oil furnaces to turn a fan on.

Compressor - Pump unit to convert Freon® from a vapor to a liquid.

Fan - Refers to a squirrel cage or belt driven fan to circulate air in a living space.

Contactor - A slave relay usually a 24 volt AC coil to energize a 240 volt circuit.

Split System - Two completely separate HVAC units usually used to split a first and second floor system.

1H/1C or 2H/2C, etc. - Designates how may stages of heating and cooling.

Condensate remover - A float activated pump to raise waste condensate to a higher level to eliminate it.

Supply/Return - Supply is the conditioned air output. Return take the conditioned air back to the IDU.

Copyright© 2008 Rick C.