Thermostats
I would recommend that as you proceed
through this power point presentation that
you refer to your textbook, Unit 14, for some
great visuals and additional information.
There used to be two major players when it came
to the ID (identification code) used on thermostats.
Those two players were:
ITT (International
Telephone and Telegraph) and Honeywell.
You
may still see some ‘old’ thermostats with the ITT
ID code on them, but Honeywell’s ID code is what
you will see today.
There may also be some manufacturer specific ID
code as well. This presentation will briefly mention
the ITT code, but will focus on Honeywell’s ID
code.
Honeywell ID code ITT ID code
Meaning
R
V
Voltage/Power
W
H
Heating
Y
C
Cooling
G
G or F
G or Fan
The terminals shown above are basic and there
are more than these four.
Additional terminal
letters and their ID meaning can found via the
internet and/or manufacturer’s literature.
The ‘R’ or ‘V’ ID is the terminal on the thermostat
that is connected to the ‘P’ power side of the low
voltage transformer.
unofficially ‘red.’
The color of the wire is
The ‘W’ or ‘H’ ID is the terminal on the thermostat
that is connected to the heating equipment. The
color of the wire is unofficially ‘white.’
The ‘Y’ or ‘C’ ID is the terminal on the thermostat
that is connected to the cooling equipment. The
color of the wire is unofficially ‘yellow.’
The ‘G’ or ‘F’ ID is the terminal on the thermostat
that is connected to the fan relay which controls
the fan speed for heating and cooling. The color of
the wire is unofficially ‘green.’
Moving onto the application of thermostats.
HEATING ONLY thermostat
This would be used where air conditioning would
not be required. The heating only thermostat has
a base and a mounting plate for installation.
HEATING ONLY
Equipment
Thermostat
R
W
RED Wire
WHITE Wire
R
W
This circuit would require only two wires to function.
The ‘older’ electromechanical thermostats had an
‘anticipator.’ There is a good discussion of this in
your textbook.
What is important for you to know about the
electromechanical thermostat heating anticipator?
1.
is wired in series
2.
if it should fail open the heating equipment will not
work
3.
it is adjustable and needs to be adjusted based on the
actual load current in the heating circuit. The amp.
(current) value for the circuit is performed with an
ammeter and a 10 loop coil. The 10 loop is wired in
series with the W (heating) circuit.
The electronic thermostats do not use an
anticipator
like
the
older
electromechanical
thermostats. BUT they do need to be adjusted.
This is not done with the 10 loop method, but one
must
check
with
the
manufacturer
thermostat and follow their directions.
of
the
COOLING ONLY thermostat
This would be used where heating would not be
required. The cooling only thermostat has a base
and a subbase for installation.
The ‘older’ electromechanical thermostats had an
‘anticipator.’ There is a good discussion of this in
your textbook.
What is important for you to know about the
electromechanical thermostat cooling anticipator?
1.
is wired in parallel with the thermostat contacts
2.
if it should fail open the cooling equipment will
continue to work
3.
it is NOT adjustable
4.
occupant would probably never know it is
defective.
The electronic thermostats do not use an
anticipator
thermostats.
like
the
older
electromechanical
BUT some of them may require
adjustment. Check with the manufacturer of the
thermostat and follow their directions.
COOLING ONLY
Equipment
Thermostat
R
Y
RED Wire
BLUE Wire
R
Y
GREEN Wire
G
G
This circuit would require three wires to function. The
unofficial color code for the wires could be what is shown,
or different. Sometimes the RED wire is a BLACK wire.
COOLING ONLY thermostat
The cooling only thermostat would have a subbase with the
following two switches on it. A system switch to set the
thermostat for CLG – OFF and a fan switch with the
functions: AUTO – ON.
COOLING ONLY thermostat
The system switch has two positions designated to turn the
air conditioner on (CLG.) or off (OFF).
COOLING ONLY thermostat
The fan switch has two positions designated to turn the
fan on (ON) or auto (AUTO). Please refer to your textbook
for some visuals on this switch. The fan ON position will turn
the fan on continuously for the speed required for cooling.
The AUTO position will cycle the fan when the air conditioner
is operating.
COOLING ONLY thermostat
In both situations, this fan switch found on the thermostat will
only operate the fan on the speed for air conditioning. So do
not confuse this fan switch with the fan switch on a furnace
that operates the fan on the speed for heating.
HEATING and COOLING
thermostat
The older electromechanical heating and cooling thermostats
required a base and a subbase. The subbase is where the
system and fan switch. The electronic thermostats are
usually a one piece assembly and do not have a subbase.
There are electronic touch screen programmable thermostats
that do not have a subbase either. All functions of the
thermostat are programmed at the time of installation.
HEATING and COOLING
Equipment
Thermostat
R
W
Y
G
RED Wire
WHITE Wire
YELLOW Wire
GREEN Wire
R
W
Y
G
This circuit would require four wires to function. The
unofficial color code for the wires could be what is shown.
Here is an example of an electronic thermostat with
its cover removed to expose the terminals.
System Switch
Fan Switch
Terminal Identification
The next thermostat circuit is going to show an
‘isolated’ heating and cooling circuit. This means
that there are two low voltage sources.
One
source would be for the heating system and the
other source would be for the cooling system. The
sources MUST not be put together, so they are
‘isolated’ at the thermostat.
Heating
Equipment
Thermostat
W
RH
WHITE Wire
RED Wire
W
R
AC Equipment
RC
Y
G
BLACK Wire
YELLOW Wire
GREEN Wire
R
Y
G
This circuit would require five wires to function. The
unofficial color code for the wires could be what is shown.
Did you notice that on the previous slide the
thermostat had 2 ‘R’ terminals? One labeled R or
RH for heating and the other one labeled RC for
cooling. The thermostat may have both terminals
with a ‘jumper’ connecting the two. If the ‘isolation’
is not required, the ‘jumper’ would be left alone. If
the system requires ‘isolation,’ the jumper would
have to be removed.
Terminal Identification
R and RC with jumper
installed from factory.
Examples of isolation:
1. Gas or Oil fired boiler with air conditioning.
2. Heating and cooling system with heating
system using DC volts and air conditioner
using AC volts.
There are heating systems with stages of heating
and this is represented on the thermostat and
equipment as:
W1 - first stage
W2 - second stage
There are cooling systems with stages of cooling
and this is represented on the thermostat and
equipment as:
Y1 - first stage
Y2 - second stage
A review of transformers:
1. Step down – output voltage lower than input
2. Step up – output voltage greater than input
3. Rating is: VA and Primary and Secondary E
4. Primary is connected to the line.
5. Secondary is connected to the load.
This concludes this review of thermostats.