A heat pump always moves thermal energy in the opposite direction from temperature, but a heat pump that maintains a thermally conditioned-space can be used to provide either heating or cooling, depending upon whether the environment is cooler or warmer than the conditioned-space. When pumps are used to provide heating, they are used because less input from a commercial-energy source is required than is required for newly-creating thermal energy by transforming heat-free sources of energy (for example, electricity) or low-entropy sources of energy (for example, a gas flame) directly into the required heating. This is because the heat pump utilizes some thermal energy from the environment for part of the delivered-heating, increasing the “efficiency” of the process. In cooler climates, it is common for heat pumps to be designed only to provide heating. Even when a heat pump is used for heating, it still uses the same basic refrigeration-type cycle to do the job (merely changing operation so that the warm end of the device is inside the conditioned space, heating it). Such heat pumps, which always provide heating of spaces, may be found in climates that never or rarely require cooling. For the class of “reversible-cycle heat pump” devices designed to work in either thermal direction, the device simply operates in a way that changes which coil is the condenser, and which coil is the evaporator, rather than physically turn the device around. Such a switch in function is normally achieved by a “reversing valve.” Reversible-cycle heat pumps are often seen in providing building-space heating in high latitude climates that are much warmer than comfortable in one season, but colder in another season. In heating, ventilation, and air conditioning applications, the term heat pump normally refers to a vapor-compression refrigeration device that includes a reversing valve and optimized heat exchangers so that the direction of thermal energy flow may be changed without loss of efficiency. Most commonly, when used in heating, heat pumps draw heat from the air or from the ground.