T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES
60-2485—822
CAUTION
Equipment Damage Hazard.
Due to calibration techni
ues used for T874
Thermostats with outdoor reset, the C815A Thermistor
must be wired into the s
stem at all times. Failure to
do so will result in serious de
radation of performance.
Service and Replacement of C815A Outdoor
Thermistor
To check and verif
thermistor operation, perform the
followin
steps:
1.
Disconnect wire from T terminal on subbase.
2.
Measure resistance with ohmmeter across the T sub-
base wire and X subbase terminal.
3.
Take outdoor temperature measurement at thermistor
location and find correct thermistor resistance on the
chart in Fi
. 29.
4.
Compare resistance on the chart with measured resis-
tance. Replace C815A if resistance varies more than 15
percent. Contact installin
dealer for packa
ed replace-
ment outdoor thermistor.
Fig. 29. Thermistor resistance chart.
Features
Two of the features of the T874/Q674 include LED indicators
and restricted setpoint.
LED Indicators
The li
ht-emittin
diodes
LED
indicators on the subbase
li
ht on command when somethin
specific happens in the
s
stem. See Fi
. 30.
Up to four different LEDs are available. The thermostat has a
clear lens window for viewin
each LED. On TRADELINE
models, a small insert is used so the LED function desired can
be selected.
This must be done during installation.
A blank insert is factor
-installed in some T874 models. To
remove it, push both temperature settin
levers to the far ends
of the thermostat. Use index fin
ernail to
entl
pull out the
scaleplate a fraction of an inch. Turn thermostat upside-down,
and the blank insert falls out.
A strip of four inserts is included with TRADELINE T874. Drop
a strip into the recessed area behind the scaleplate so
selected LEDs show. Make sure insert is completel
seated in
recessed area. Let scaleplate pop back; then set levers to
desired position.
• FILTER LED li
hts when the filter is clo
ed and needs
replacement.
• CHECK LED li
hts when somethin
needs to be checked
or done to maintain efficient operation of s
stem. See
heatin
s
stem instructions for CHECK LED meanin
.
• EM. HT. LED li
hts when the emer
enc
heat is operatin
.
• SUPL. HT. LED li
hts when the supplemental heat is
operatin
.
• LOCKOUT LED li
hts when the s
stem is shut down and
needs maintenance.
• AUXILIARY HEAT LED li
hts when the auxiliar
heat is
operatin
.
• SERVICE or MALFUNCTION LED can have several
meanin
s. Consult heatin
s
stem instructions.
LEDs cannot be replaced or added in the field.
Fig. 30. T874/Q674 LED location.
Restricted Setpoint (DoD)
The Department of Defense
DoD
models are e
uipped with
a restricted setpoint feature for fuel efficienc
.
Fixed stops are factor
-set so setpoint levers cannot be set
above 72°F
22°C
on heatin
, or below 78°F
26°C
on coolin
.
Applications
The T874/Q674 can be applied to standard residential
s
stems for automatic or manual chan
eover, to commercial
rooftop applications, or to heat pump applications.
Changeover on Standard Residential
Systems
In a standard residential heatin
-coolin
circuit, chan
eover
between heatin
and coolin
can be done either automaticall
or manuall
.
Automatic chan
eover is done b
an AUTO position on the
subbase s
stem switch. See Fi
. 31. When the switch is in the
AUTO position, the thermostat automaticall
chan
es between
heat and cool modes, dependin
on the indoor temperature.
THERMISTOR RESISTANCE (ohms)
C815A THERMISTOR RESISTANCE
R = 400 ohms ± 10% AT 77°F (25°C)
4600
4400
4200
4000
3800
3600
3400
3200
3000
2800
2600
2400
2200
2000
1800
1600
1400
1200
1000
800
600
400
200
0
-20 0 20 40 60 80 100 120 140
TEMPERATURE OF THERMISTOR (°F)
M1590A
80
50 60 70 80
COOL
EM.
HEAT
FILTER SERV.
AUX
HEAT
M5830
LEDS
