<P> Most programmable thermostats will control these systems . </P> <P> Depending on what is being controlled, a forced - air air conditioning thermostat generally has an external switch for heat / off / cool, and another on / auto to turn the blower fan on constantly or only when heating and cooling are running . Four wires come to the centrally - located thermostat from the main heating / cooling unit (usually located in a closet, basement, or occasionally in the attic): One wire, usually red, supplies 24 volts AC power to the thermostat, while the other three supply control signals from the thermostat, usually white for heat, yellow for cooling, and green to turn on the blower fan . The power is supplied by a transformer, and when the thermostat makes contact between the 24 volt power and one or two of the other wires, a relay back at the heating / cooling unit activates the corresponding heat / fan / cool function of the unit (s). </P> <P> A thermostat, when set to "cool", will only turn on when the ambient temperature of the surrounding room is above the set temperature . Thus, if the controlled space has a temperature normally above the desired setting when the heating / cooling system is off, it would be wise to keep the thermostat set to "cool", despite what the temperature is outside . On the other hand, if the temperature of the controlled area falls below the desired degree, then it is advisable to turn the thermostat to "heat". </P> <P> The heat pump is a refrigeration based appliance which reverses refrigerant flow between the indoor and outdoor coils . This is done by energizing a reversing valve (also known as a "4 - way" or "change - over" valve). During cooling, the indoor coil is an evaporator removing heat from the indoor air and transferring it to the outdoor coil where it is rejected to the outdoor air . During heating, the outdoor coil becomes the evaporator and heat is removed from the outdoor air and transferred to the indoor air through the indoor coil . The reversing valve, controlled by the thermostat, causes the change - over from heat to cool . Residential heat pump thermostats generally have an "O" terminal to energize the reversing valve in cooling . Some residential and many commercial heat pump thermostats use a "B" terminal to energize the reversing valve in heating . The heating capacity of a heat pump decreases as outdoor temperatures fall . At some outdoor temperature (called the balance point) the ability of the refrigeration system to transfer heat into the building falls below the heating needs of the building . A typical heat pump is fitted with electric heating elements to supplement the refrigeration heat when the outdoor temperature is below this balance point . Operation of the supplemental heat is controlled by a second stage heating contact in the heat pump thermostat . During heating, the outdoor coil is operating at a temperature below the outdoor temperature and condensation on the coil may take place . This condensation may then freeze onto the coil, reducing its heat transfer capacity . Heat pumps therefore have a provision for occasional defrost of the outdoor coil . This is done by reversing the cycle to the cooling mode, shutting off the outdoor fan, and energizing the electric heating elements . The electric heat in defrost mode is needed to keep the system from blowing cold air inside the building . The elements are then used in the "reheat" function . Although the thermostat may indicate the system is in defrost and electric heat is activated, the defrost function is not controlled by the thermostat . Since the heat pump has electric heat elements for supplemental and reheats, the heat pump thermostat provides for use of the electric heat elements should the refrigeration system fail . This function is normally activated by an "E" terminal on the thermostat . When in emergency heat, the thermostat makes no attempt to operate the compressor or outdoor fan . </P>

What should be the resistance of a new thermostat