Types of controls
There are two main types mechanical or digital. The difference in them is what force closes the
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.