While many people think of heat pumps as only heating the air or water in houses, the heat pump is actually part of a larger system that controls both the heating and cooling of the air. Rather than split heating and cooling operating into two separate devices, most modern air conditioning units combine all the functions into one system, which can handle either job as needed.

Heat Pump Operation

The heat pump works to heat or cool the air by the use of coolant gases, very much like a refrigerator. The gas is compressed to concentrate its heat, cooled to turn it into a liquid state, heated to turn it back into a gas, then compressed again in a constant cycle. This trading of heat is what allows the pump system to warm the air in the house, or dissipate warm air outside the house. The system uses specialized coils to radiate heat into the air or collect the heat from air that is already warm. These systems, usually referred to as HVAC (heating, ventilation, air conditioning) can work both ways thanks to the heat pump, which carries out the compression and decompression of the gas using a motor, compressor and heat exchanging coils. The heat pump is usually controlled by an electronic control module, or directly by a thermostat. This is where you come in--the heat pump responds directly to the settings on these electronic devices.

Switching Between Heating and Cooling

In order to switch between heating and cooling on your heat pump system, you will need to set your thermostat on the correct settings. There are several different kinds of thermostats, and each works in a slightly different way. You may have a thermostat that only has "On" and "Auto" settings. In this case, the thermostat itself decides whether the system will be heating or cooling the air, and you do not really have a choice. Setting the thermostat to on keeps the air flowing constantly, while the auto mode lets it start and stop in typical cycles. Your thermostat may also have a dial switch that allows you to change the settings from "heat" to "off" and then to "cool". This allows you to control what the heat pump does, but with limitations. For instance, if you skip from heating immediately to cooling on the dial, the thermostat will not instantly respond--it needs a cooling down or warming up period to adjust, and switching between the two rapidly can damage the system. The off setting should be used to let the heat pump settle before reversing its function. Other thermostats, especially the digital versions, may simply have a "mode" button that you can press to switch between heating and cooling, or varying degrees of either. These digital systems typically take care of the transition period themselves so you do not need to worry as much about strain on the system.

Source:

Inspectapedia: Guide to Air Conditioning & Heating Blower Fan AUTO ON & COOL OFF HEAT Switches on Thermostats

Custom Cooling Solutions: Heat Pump Info

Energy Control: Operating Your Heat Pump

theres probably a switch for that somewheres