A thermostat is a device that automatically turns a hvac system on and off. It does this by sensing the temperature around it and operating an electrical switch, as necessary, to maintain your desired temperature.
These units can either control line voltage (120 or 240 volt) or low voltage (usually 24 volts). Normally the low voltage units are used on forced air heating and cooling systems as well as some hydronic systems. The line voltage units are typically used on electric heating systems as well as combi boilers.
There are two main types mechanical or digital. The difference in them is what force closes the electrical switch.
A mechanical unit can contain a vial of mercury. This mercury moves and closes or opens the switch. These are being phased out for a couple of reasons. First, the mercury is bad for the environment. Secondly, if these are not perfectly level they will not operate properly. It is common for the temperature to vary as much as 5 degrees from the setpoint. Other mechanical units use a piece a metal that expands and contracts with changes in temperature. This expansion or contraction causes the electrical contacts to open and close. These units are commonly called "snap action" thermostats.
A digital unit opens or closes the switch using a small electrical signal. These units are a lot more accurate which usually results in saving money on utility bills. The temperature normally will not vary more than 1 degree. They also can have added features on them. They can be battery powered or powered by the furnace low voltage transfomer. Some can be used with both power sources. The units can be either single stage or multistage units. Which one you choose depends on the type of hvac systems you have. Most central air conditioners with a seer of 15 or more are two stage units. Some gas furnaces have two stages as well. Heat pumps usually require at least two stage heating.
Some digital units are programmable. This allows you to adjust the home's temperature to save energy when you are not there. Typically, they have four temperature setpoints and four times associated with them. Normally, they are referred to as wake, leave, return, and sleep. The most advanced units do much more. They sense the humidity in the home and correct as needed.
Some units allow the furnace fan to operate periodically. This can be used to ensure adequate fresh air is supplied to the home. This can improve overall indoor air quality significantly which can also improve the health of you and your family.
When combined with two stage furnaces and air conditioners, these units can save energy by running in the lower speed. Then, when necessary, they can automatically change the speed to high to meet the home's heating or cooling needs.
The latest innovation incorporates wireless technology. These units eliminate the need for control wires from the thermostat to the heating unit. They can be a very good option in older homes where the wiring is insufficient for modern hvac systems.
The most advanced units are incorporated into a home automation system. These are referred to as communicating thermostats and allow you to control them over the internet. Some of them even monitor the home and send email alerts if something is wrong.
One of the most important things about your thermostat is it's location. Unless it is wireless, the unit should be mounted on an interior wall. It should be about five feet up from the wall and out of direct sunlight. It should be in a room that is central and frequently used. Most times it is placed in a central hallway if one is available.
The unit should not be located near any appliance that gives off heat such as lamps etc. The unit should be where it is in the normal airflow patterns but not where it will be affected by opening and closing doors. It should not be located behind a door.
The most common mistake people make, when trying to replace their thermostat, is buying the wrong type of unit. With a little help, you can replace your old thermostat and avoid that mistake. If you have a mechanical or other non-programmable unit, you can save 10-15% on your utility bill.
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