Digital electronic thermostats

Newer digital thermostats have no moving parts to measure temperature,instead relying on thermistors or other semiconductor devices such as resistance thermometers (resistance temperature detectors).Usually one or more normal batteries must be installed to run it, and while some so called "power stealing" digital thermostats use normal 24 volt AC circuits as their power source, they won't run on thermopile powered "millivolt" circuits some furnace.Each has an LCD screen that displays the current temperature and current settings. Most also have a clock, time of day, and even day-of-week temperature settings for comfort and energy savings.Some premium models have touch screens, or are able to work with home automation or building automation systems.

Digital thermostats use relays or semiconductor devices such as triacs as switches to control HVAC installations.Units with relays will run the millivolt system, but will usually make a "click" when turned on or off.An HVAC system that can regulate its output can be combined with a thermostat that has a built-in PID controller for smoother operation.There are also modern thermostats with adaptive algorithms that further improve inertia-prone system behavior.For example, setting them so that it should be 21 °C (69.8 °F) at 7 a.m.ensures that it's 21 °C (69.8 °F) when a traditional thermostat kicks in.Algorithms determine when the system should be activated so that the desired temperature is reached at the desired time.Other thermostat PID controls for process/industrial controls where on/off control is not suitable can also ensure very stable temperature (e.g. by fine-tuning the PID constants of the set value (SV) to reduce overshoot or maintain the temperature within a range by deploying hysteretic control.).The digital thermostats used in most average homes in North America and Europe are programmable thermostats, which can typically save 30% of energy if you leave the default programs; adjustments to these default settings may increase or decrease energy savings.The programmable thermostat article provides basic information on the operation, selection and installation of this type of thermostat.

Thermostats and HVAC operation

Ignition sequences in modern conventional systems

Gas

1.Turn on the induced draft fan/blower (if the furnace is relatively new) to create a column of air that flows up the chimney

2.Hot igniter or start spark ignition system

3.Open the gas valve to ignite the main burner

4.Wait (if the furnace is relatively new) until the heat exchanger is at the proper operating temperature before starting the main blower or circulation pump

Oil

Similar to gas, except instead of opening the valve, the oil pump is started to inject oil into the burner electrical.The blower or circulation pump will start and a large electromechanical relay or TRIAC will turn on the heating element coal, grain or pellets

Generally rare today (although grains such as corn, wheat, and barley, or pellets made of wood, bark, or cardboard are increasingly popular); similar to gas, except instead of opening the valve, the stove will start the screw to turn the coal/ Grain/Pellets are pushed into the combustion chamber.For a non-zoned (typical home, one thermostat for the whole house) system, when the R (or Rh) and W terminals of the thermostat are connected, the furnace will complete the start-up sequence and generate heat.With zoned systems (some residential, many commercial systems several thermostats controlling different "zones" in the building), the thermostat will cause a small electric motor to open a valve or damper and start a furnace or boiler that isn't already running.Most programmable thermostats will control these systems.

Combination heating/cooling regulation

Depending on what's being controlled, forced air AC thermostats typically have an external switch for heat/off/cool, and another on/auto to turn on the blower continuously or only when heating and cooling are running.Four wires run from the main heating/cooling unit (usually in a closet, basement, or occasionally attic) to a centrally located thermostat: one wire, usually red, provides 24 VAC power to the thermostat, while the other wire from Three power control signals for the thermostat, usually white for heating, yellow for cooling, and green for turning on the blower. Power is provided by a transformer and when the thermostat establishes contact between the 24 volt supply and one or two other wires, the heating/cooling unit's relay activates the corresponding heating/fan/cooling function (in seconds).When set to Cool, the thermostat will only turn on when the ambient temperature of the surrounding room is higher than the set temperature.Therefore, if the temperature of the controlled space is generally higher than the desired setting when the heating/cooling system is turned off, it is wise to set the thermostat to "cool" regardless of the outside temperature.On the other hand, if the temperature in the controlled area is lower than desired, it is advisable to turn the thermostat to "heat".