Thermostat Basics: Types and Applications

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THERMOSTATS

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Types and Applications

PRODUCT DEFINITION A THERMOSTAT is a key component of any HVAC

system. It maintains the space temperature by cycling the

heating or cooling equipment on and off to maintain a

desired temperature which is referred to as the thermostat

setpoint.


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THERMOSTATS

PRODUCT DESCRIPTION Thermostats sense the change in space temperature and

turn the heating equipment on when the temperature drops

below the setpoint and off when the temperature reaches

setpoint. They also turn the cooling equipment on when

the temperature rises above the setpoint and off when the

temperature reaches setpoint.

The majority of thermostats operate on low voltage

(24VAC) control circuits. The source of the 24 VAC is a

transformer located in the heating/cooling equipment.


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THERMOSTATS

PRODUCT DESCRIPTION Line voltage 120VAC or 277VAC thermostats are also

available.

Modern thermostats are almost exclusively digital.

A digital thermostat uses a simple device called a

thermistor to measure temperature.

A thermistor is a resistor which changes its resistance value

as the temperature changes.

The microprocessor in a digital thermostat can measure the

resistance change and converts the resistance value to an

actual temperature reading.

© Jackson Systems, LLC 2011

Intellectual rights apply

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THERMOSTATS

PRODUCT DESCRIPTION Thermostats are categorized by their function and the type

of equipment being controlled.

Thermostats can have two (2) functions:

Programmable

Non-Programmable


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THERMOSTATS

PRODUCT DESCRIPTION Thermostats can control different types of HVAC

equipment.

Single stage (1 heat / 1 cool)

Multi-stage (2 heat / 2 cool)

Heat pump ( 2 heat / 1 cool)

Heat pump ( 3 heat / 2 cool)

Dual fuel ( 2 heat / 1 cool)

Dual fuel (3 heat / 2 cool)

A “universal thermostat” means that it can be configured

to work with any of the above systems.


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THERMOSTATS

PRODUCT DESCRIPTION One of the most useful features of a digital thermostat is

programmable settings. In the winter you can program the

thermostat to automatically turn up the heat for an hour in

the morning while you get ready for work, turn down the

heat while away from home, turn up the heat in the

evening and then turn down the heat while you sleep. The

same is true for cooling.

Programmable thermostats save energy by automatically

adjusting the temperature based on a user program.


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THERMOSTATS

PRODUCT DESCRIPTION Residential thermostats typically have 4 schedules per day

Wake or Morning

Leave or Day

Return or Evening

Sleep or Night

Commercial thermostats typically have 2 schedules per

day

Occupied or Day

Unoccupied or Night


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THERMOSTATS

PRODUCT DESCRIPTION Thermostats can be hardwired or wireless

A hardwired thermostat requires 18 gauge thermostat wire

running from the thermostat to the HVAC equipment.

A wireless thermostat does not wire directly to the

equipment but rather wirelessly communicates via radio

waves to an equipment interface module (EIM) mounted

near the HVAC equipment. The EIM is then hardwired to

the equipment. A wireless thermostat can be used when it

is difficult or impossible to run wires from the equipment to

the thermostat. Wireless thermostats are almost always

powered by batteries.


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THERMOSTATS

PRODUCT DESCRIPTION Powering thermostats

The three most common types of thermostats are 24VAC

hardwired, battery powered, and dual powered.

24VAC hardwired stats require constant 24 Volts AC from

the equipment transformer.

Battery powered stats do not require 24VAC to power the

thermostat.

Dual powered stats can be powered by the equipment

transformer or by batteries.


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THERMOSTATS

ADVANCED INSTALLER SETTINGS Most, if not all, digital thermostats today contain “Advanced

Installer Settings”.

Advanced installer settings are typically accessed only by the

contractor during installation.

Advanced installer settings allow for equipment configuration

and selected user preferences.

Depending on the thermostat manufacturer and model, there

can be dozens of advanced installer setting options. Advanced

installer settings are either set by configurable dip switches or

programmed through the key pad or touchscreen.


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THERMOSTATS

ADVANCED INSTALLER SETTINGS SYSTEM TYPE

Matches the thermostat to the equipment such as single

stage, multi- stage, or heat pump system.

FAN CONTROL

Allows the equipment to energize the fan or the thermostat

to energize the fan during a call for heat.

SCHEDULE FORMAT

Thermostat can be set up as programmable or non-

programmable.


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THERMOSTATS

ADVANCED INSTALLER SETTINGS CHANGOVER VALVE

Selects O or B reversing valve for heat pumps.

AUXILIARY HEAT

Selects electric or fossil fuel auxiliary heat for heat pumps.

BACKLIGHT

Selects backlight always on or on after a key is pressed.

MANUAL/AUTO-CHANGOVER

Allows the stat to have both heating and cooling available

(auto changeover) or just heat or cool available (manual).


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THERMOSTATS

ADVANCED INSTALLER SETTINGS TEMPERATURE DISPLAY

Allows selection of Fahrenheit or Celsius to be displayed.

REMOTE SENSOR

Allows selection of no remote sensor, outdoor sensor, or

indoor sensor.

Some stats allow both an indoor and outdoor sensor.

LOW BALANCE POINT

Allows the heat pump to be locked out on a call for heat

below a certain outside temperature (i.e. 30°F) and only

allow the auxiliary heat to be used.


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THERMOSTATS

ADVANCED INSTALLER SETTINGS HIGH BALANCE POINT

Allows the auxiliary heat to be locked out above a certain

outside temperature (i.e. 60°F) and only allows the heat

pump to be used.


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THERMOSTATS

ADVANCED INSTALLER SETTINGS ADAPTIVE RECOVERY

When adaptive recovery is OFF, the thermostat will bring

on the HVAC equipment at the exact time of a scheduled

event.

When adaptive recovery is ON, the thermostat will bring

on the HVAC equipment earlier than the schedule event

time. The thermostat anticipates how long it will take to

reach the next scheduled setpoint based on the temperature

in the space and the rate of recovery of the equipment.

PROGRAM PERIODS

Allows 4 or 2 programs per day.


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THERMOSTATS

ADVANCED INSTALLER SETTINGS SHORT CYCLE PROTECTION

Allows a time delay (typically 5 minutes) between cooling

calls for conventional equipment and both heating and

cooling calls for heat pumps.


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THERMOSTATS

ADVANCED INSTALLER SETTINGS HEATING LIMIT

Allows selection of a maximum heating setpoint

adjustment.

COOLING LIMIT

Allows selection of a minimum cooling setpoint

adjustment.

KEYPAD LOCKOUT

Allows the thermostat buttons or touch screen icons to

unlocked or locked to limit user access to certain functions.

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Intellectual rights apply

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THERMOSTATS

ADVANCED INSTALLER SETTINGS INTERNAL SENSOR CALIBRATION

Allows calibration of the thermostat temperature sensor.

FAN PURGE

Allows selection (0-5 minutes) of extended fan run time

after a heating or cooling call ends.

STAGE DIFFERENTIAL

Allows selection of the temperature difference above or

below the setpoint before the equipment will be allowed to

run.

Example: 70 degrees heating setpoint with a selected 2

degree differential will allow the heating to come on at 68

degrees.


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THERMOSTATS

INSTALLATION Typical terminal designations for a thermostat controlling

conventional equipment.

R-Wires to the hot side 24VAC equipment transformer

C-Wires to the common side 24VAC equipment

transformer

W1-Wires to the first stage heating relay

W2-Wires to the second stage heating relay

Y1-Wires to the first stage cooling relay

Y2-Wires to the second stage cooling relay

G-Wires to the fan relay


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THERMOSTATS

INSTALLATION Typical terminal designations for a thermostat controlling

heat pump equipment.

R-Wires to the hot side 24VAC equipment transformer

C-Wires to the common side 24VAC equipment

transformer

W1-Wires to the auxiliary heat

O/B-Wires to the heat pump reversing valve

Y1-Wires to the first stage compressor

Y2-Wires to the second stage compressor

G-Wires to the fan relay


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THERMOSTATS

INSTALLATION An important part of thermostat operation is location.

The location should represent the ambient space

temperature.

The stat should be mounted on an interior wall.

The stat should be about five (5) feet up from the floor.

The stat should be out of direct sunlight.

The stat should be mounted in a room that is centrally

located and frequently used.

The thermostat should not be installed in an area where

drafts are present, near the floor, behind doors or on an

exterior wall.


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THERMOSTATS

SAMPLE TEST Thermostats should be configured to match the . . .

A. Color of the house

B. HVAC equipment

C. Location where it will be installed

D. All of the above

For a conventional thermostat, Y1 is for . . .

A. First stage cool

B. Second stage cool

C. First stage heat

D. Second stage heat


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THERMOSTATS

SAMPLE TEST For a conventional thermostat, W2 is for . . .

A. First stage cool

B. Second stage cool

C. First stage heat

D. Second stage heat

For a heat pump thermostat, Y1 is for . . .

A. First stage compressor

B. Second stage compressor

C. Fan

D. Auxiliary heat


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THERMOSTATS

SAMPLE TEST Which of the following is an advanced installer option?

A. Adaptive recovery

B. Low balance point

C. Backlight display

D. All of the above


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THERMOSTATS

SAMPLE TEST Residential programmable thermostats typically have how

many schedules per day?

A. 1

B. 2

C. 3

D. 4

Commercial programmable thermostats typically have

how many schedules per day?

A. 1

B. 2

C. 3

D. 4


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THERMOSTATS

THERMOSTAT BASICS

Types and Applications

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THERMOSTATS


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Thermostat Basics: Types and Applications