Beyond these two main differences, thermostats can differ by whether they are intended to control a furnace only, a furnaces and an air conditioner, a heat pump, or equipment that operates in multiple stages that kick-in as the need for heating or cooling increases.
Many thermostats have some type of adjustment that allows the device to suit the system: for example, they may have a small switch on the back or wires that are jumpered. This adjustment is necessary because some heating systems are much slower to heat up than others and cycle differently. And some types of heating require system fans and others don’t.
For example, most heat pumps have an auxiliary electric heating element that kicks on when the room’s temperature drops about 2 degrees lower than the thermostat’s setting (or “set point”). If the room temperature is allowed to drop to 60 degrees at night and is then turned up to 70 degrees in the morning, the auxiliary heat will come on until the room temperature reaches about 68 degrees. In most parts of the country, using this much electric heat is quite expensive.
A sophisticated electronic heat-pump thermostat automatically calculates when the heat must come on to bring a room’s temperature up to the set point by the time you’ve programmed. It tells the heat pump to bring the temperature up from 60 to 61 degrees, then from 61 to 62 degrees, and so forth. That way, the electric auxiliary heat is “fooled” into staying off.
If you have a system that provides multiple stages of heating and cooling, such as a dual-speed air conditioner, a furnace with two sets of burners, or a heat pump, you need a thermostat that is designed to handle this complication (some conventional thermostats have a jumper or switch that can be set to modify the thermostat for such equipment). Zoned heating systems that send warmth or cooling to various rooms of a home depending upon the needs of those rooms generally require special programmable electronic thermostats (you can really fine-tune your home’s areas for comfort with one of these).