A multistage thermostat is a special type of thermostat used when there is a need to have multiple settings on a heating and cooling system. Any heating or cooling system that has more than one speed must have a thermostat that is able to set to multiple heating and cooling levels and utilize the available features on the on the unit. Even on somewhat cheaper single thermostats there is usually a way to have multiple heating or cooling settings. The difference between a single and multistage thermostat is how and when features are used. As would be expected, a multistage thermostat has more options.
A multistage thermostat operates by switching between two different sources of heating or cooling to provide maximum efficiency and a stable temperature for an indoor area; it does this by switching between electric heating or cooling and gas heating or cooling. This switching is determined by the season and outdoor temperature. The heating or cooling system will use the main pump until it no longer can keep the temperature at a stable level and then switch to to the auxiliary or electric system. How effective this switching system is depends on the matching of the outdoor system for the environment it exists in to the auxiliary cooling and heating system. Most multistage systems have an internal clock that has the ability to set up a night program and switch back in the morning to a different program. If a change occurs during these programs, he system will also change over to auxiliary power, if necessary. This saves money and keeps the implementation of the multistage thermostat cost-effective.
Each multistage thermostat cycle has a small offset by a number of degrees. This prevents the changing of cycles too often or switching to a backup system prematurely. The exception to this is when the differential between the auxiliary heat and the regular heat is provided by a pump--there are a few extra degrees factored into the error calculation. This differential provides a stable separation if their is a composite error on the cycle output. This also allows the heat pump to be on whether or not the auxiliary heat source is on.