Room Thermostats; Control Wiring - International comfort products N9MP1 Service Manual

Service Manual
Room thermostats are available from several different
manufactures in a wide variety of styles. They range from
the very simple and inexpensive Bi−metallic type to the
complex and costly electronic set−back type. They are sim-
ply a switch (or series of switches) designed to turn equip-
ment (or components) "ON" or "OFF" at the desired condi-
tions.
An improperly operating, or poorly located room thermostat
can be the source of perceived equipment problems. A
careful check of the thermostat and wiring must be made
then to insure that it is not the source of problems.
Figure 10
Thermostat Location
THERMOSTAT
SHIELD
5 ft.
LOCATION
The thermostat should not be mounted where it may be af-
fected by drafts, discharge air from registers (hot or cold),
or heat radiated from the sun or appliances. Never install in
alcoves, bathrooms or bedrooms.
The thermostat should be located about 5 ft. above the floor
in an area of average temperature, with good air circulation.
Normally, an area in close proximity to the return air grille
is the best choice.
Mercury bulb type thermostats MUST be level to control
temperature accurately to the desired set−point. Electronic
digital type thermostats SHOULD be level for aesthetics.
HEAT ANTICIPATORS
Heat anticipators are small resistance heaters built into
most electric−mechanical thermostats. Their purpose is to
prevent wide swings in room temperature during furnace
operation.
In order to accomplish this, the heat output from the antici-
pator must be the same regardless of the current flowing
through it. Consequently, most thermostats have an adjust-
Control wiring is an important part of the total equipment
installation, since it provides the vital communications link
between the thermostat, and the equipment. Control wiring
that is either too long, undersized, or improperly connected
14

ROOM THERMOSTATS

DRAFTS
SUN
LIGHT

CONTROL WIRING

Single Stage Multi Position Furnace
ment to compensate for varying current draw in the thermo-
stat circuit.
The proper setting of heat anticipators then is important to
insure proper temperature control and customer satisfac-
tion.
Figure 11
Measuring Current Draw
W
R
Subbase
The best method to obtain the required setting for the heat
anticipator, is to measure the actual current draw in the con-
trol circuit ("W") using a low range (0−2.0 Amps) Ammeter.
(See Figure 11) After measuring the current draw, simply
set the heat anticipator to match that value.
If a low range ammeter is not available, a "Clamp−on" type
meter may be used as follows:
1. Wrap EXACTLY ten (10) turns of wire around the jaws
of a clamp−on type ammeter.
2. Connect one end of the wire to the "W" terminal of the
thermostat sub−base, and the other to the "R" termi-
nal.
3. Turn power on, and wait approximately 1 minute, then
read meter.
4. Divide meter reading by 10 to obtain correct anticipa-
tor setting.
Electronic thermostats do not use a resistance type antici-
pator. These thermostats use a microprocessor (computer)
that determines a cycle rate based on a program loaded into
it at the factory.
These cycle rates are normally field adjustable for different
types to equipment. The method of adjustment, however,
varies from one thermostat manufacturer to another. Check
with the thermostat manufacturer to find out the proper way
of adjusting the cycle rate.
(be it simply loose, or on the wrong terminal) can in fact be
the source of many equipment problems.
Amps
Ammeter
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