The manufacturing engineers determine
what type and design of thermostat should be
installed in a particular system. Knowing and
u n d e r s t a n d i n g t h e a d v a n t a g e s a nd
disadvantages of different types of thermostats
will help you identify the type of thermostat
being used in a system and enable you to
troubleshoot an inoperative system efficiently.
Electrical room thermostats are in three
categories: line voltage, low voltage, and
millivoltage. Line-voltage thermostats are
u s u a l l y 1 1 5 v o l t s . W h e n l i n e - v o l t a ge
thermostats are installed, there is no need for
lowering voltage with a transformer.
However, line-voltage thermostats are
dangerous for the users and the cost is higher.
Normally line-voltage thermostats are located
only in industrial commercial applications.
Low-voltage thermostats (24 volts) are not
dangerous to the user. They are also more
cost efficient than line-voltage models. The
disadvantage of low-voltage thermostats is the
extra requirements of wiring and additional
components; they are less rugged than line-
The millivoltage thermostat operates at
750, 500, or 250 millivolts. This thermostat
uses its own power source for operation and
is not affected by power interruptions. The
system requires only a small amount of wiring
compared to other systems. However, this
system is limited for use only in heating
applications. The temperature control is less
precise than other systems, wire length and
size are critical, and the system requires a
separate device to power a 24-volt control, or
you must use a millivoltage control.
O n e c o m p o n e n t t h a t e n h a n c e s t he
operation of a thermostat is an anticipator.
Anticipators are of two types-heating and
cooling. The heating anticipator produces
false heat in a thermostat to prevent extreme
temperature changes within a space. The
false heat created by resistance increases the
thermostat rate of response. Basically, the
thermostat receives false heat which shuts
d o w n t h e h e a t i n g s o u r c e b e f o r e t he
thermostat reaches the desired temperature.
This action reduces overshooting and is
economically efficient. The heating source
shuts off and the blower continues to run
using the heat transferred from the surface of
the furnace and ductwork. When adjusting a
anticipator resistance to match the current
rating of the primary control.
The cooling anticipator adds false heat to
the thermostat bimetal element the same way
as a heating anticipator. Unlike the heating
anticipator, cooling anticipators are not
adjustable; they are sized by the manufacturer
of the thermostat. The cooling anticipator is
placed in parallel with the cooling contacts.
By studying figure 14-20(D), you can see that
the cooling anticipator is energized when the
unit is in the OFF cycle (thermostat contacts
open). The small amount of heat produced
by the resistance heat closes the TC before
the actual temperature in the space reaches
the thermostat cut-in setting. This action
allows the unit to start removing heat before
the temperature in the space climbs above the
desired temperature. When the cooling