The number of wires required by a thermostat depends on the number of features the thermostat is intended to operate. The final, or in this case fifth, wire is necessary to complete the electric path, and provide power for the display and thermostat relays.
It is common for programmable thermostats, such as the Honeywell Prestige model, to require a five-wire set up to accommodate additional features provided by advanced models. These programmable thermostats include features that allow the temperature controls to automatically adjust. Instead of needing to manually adjust the temperature as the demands change throughout the day, these thermostats automatically adjust based on the user's preset schedule. Not only does this reduce the number of tasks for the user to perform, but it can also optimize the building's temperature.
The simpler models may not require a five-wire set up, allowing the user to set a specific temperature for the morning, afternoon and evening hours. However, the more sophisticated models allow the user to select specific temperatures for each hour of the day. This can be a useful feature for those who need to carefully monitor the building's temperature in order to conserve energy, and manage the corresponding heating and cooling bills.