Before you analyze heat pumps to allow for troubleshooting when problems occur, it is best that one understands how a heat pump works.
A heat pump is a device that transfers thermal energy from a heat source to a heat sink. Heat pumps can move thermal energy in a direction which is opposite to the direction of spontaneous heat flow. A heat pump uses energy to accomplish the desired transfer of thermal energy from heat source to heat sink.
Compressor-driven air conditioners and freezers are examples of heat pumps. However, the term "heat pump" is more general and applies to devices which are used for space heating, or space cooling.
When a heat pump is used for heating, it uses the same basic refrigeration-type cycle employed by an air conditioner or a refrigerator, but releasing heat into the conditioned-space rather than into the surrounding environment. In this use, heat pumps generally draw heat from the cooler external air or from the ground.
Heat pumps are used to provide heating because less high-grade (i.e., low-entropy) energy is required for their operation, than appears in the released heat.
Most of the energy for heating comes from the external environment, and only a fraction comes from electricity (or some other high-grade energy source).
In electrically powered heat pumps, the heat transferred can be three or four times larger than the electrical power consumed, giving the system a Coefficient of Performance (COP) of 3 or 4, as opposed to a COP of 1 of a conventional electrical resistance heater, in which all heat is produced from input electrical energy.
Reversible heat pumps are designed to work in either thermal direction, in order to provide heating or cooling to the internal space. They operate by changing which coil is the condenser and which coil is the evaporator, rather than physically turn the device around.
Such a function is achieved by a "reversing valve." In heating and air conditioning applications, the term heat pump usually refers to easily reversible vapor-compression refrigeration devices that are optimized for high efficiency in both directions of thermal energy transfer.
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