T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES
60-2485—812
anticipator at 140 percent of the actual primar
y
con-
trol current draw to reduce the c
y
clin
g
rate. See Fi
g
.
11.
Most heat pump s
y
stems should c
y
cle 2-1/2 to 3 times per hour.
4. Han
g
the upper ed
g
e of the thermostat cover on top of
the thermostat base and swin
g
the cover downward
until it en
g
a
g
es with the cover clip.
Fig. 11. Adjustable heat anticipator scales.
Temperature Setting
Move the heatin
g
and coolin
g
levers to the desired comfort
positions. See Fi
g
. 12. On some models with two sta
g
es of
heatin
g
or coolin
g
, the same lever controls both sta
g
es. The
minimum differential between heatin
g
and coolin
g
setpoints is
4°F
(
2°C
)
(
5°F [3°C]
)
on T874W.
If model has optional screws to lock temperature control
levers, loosen these screws before makin
g
temperature
ad
j
ustment; ti
g
hten the screws when levers are set at desired
position.
Fig. 12. Internal view of T874W (three stages of
heating, two stages of cooling).
Subbase Setting
The subbase switchin
g
positions control the s
y
stem operation
as described below.
SYSTEM SWITCH
(
see subbase for positions
)
:
OFF—both the heatin
g
and coolin
g
s
y
stems are off. If the
fan switch is at the AUTO position, the coolin
g
fan is
also off.
HEAT—heatin
g
s
y
stem is automaticall
y
controlled b
y
the
thermostat. Coolin
g
s
y
stem is off.
AUTO—thermostat automaticall
y
chan
g
es between heat-
in
g
and coolin
g
s
y
stem operation, dependin
g
on the
indoor temperature.
COOL—coolin
g
s
y
stem is automaticall
y
controlled b
y
the
thermostat. Heatin
g
s
y
stem is off.
EM.HT.—emer
g
enc
y
heat rela
y
is automaticall
y
controlled
b
y
the thermostat. Coolin
g
s
y
stem is off. Compressor is
de-ener
g
ized.
SUPL.HT.—supplemental heat rela
y
is ener
g
ized. Coolin
g
s
y
stem is off. Compressor is de-ener
g
ized.
WOOD—heatin
g
s
y
stem is operatin
g
with onl
y
the wood-
burnin
g
sta
g
e.
OIL—heatin
g
s
y
stem is operatin
g
with onl
y
the oil-burnin
g
sta
g
e.
WOOD/OIL—wood and oil sta
g
es operate se
q
uentiall
y
;
first the WOOD sta
g
e operates, then the OIL sta
g
e
operates if the WOOD sta
g
e cannot handle the load.
EVAP—controls coolin
g
s
y
stem b
y
water evaporation; see
e
q
uipment instructions for further information.
OVERRIDE—ni
g
ht setback is disabled.
ON—heatin
g
s
y
stem is controlled b
y
the thermostat. EM.
HT. or SUPL. HT. rela
y
is not ener
g
ized.
FAN SWITCH positions control fan operation as follows:
ON or CONT.—fan operates continuousl
y
.
AUTO—fan operates as controlled b
y
the thermostat in
heat pump s
y
stems or conventional coolin
g
mode; fan
operates as controlled b
y
the plenum switch in conven-
tional heatin
g
mode.
LO—fan operates constantl
y
at low speed.
MED—fan operates constantl
y
at medium speed.
HI—fan operates constantl
y
at hi
g
h speed.
To move the subbase switches to the desired control
positions, use thumb and index fin
g
er to slide the lever. The
lever must stop over desired function indicator position for
proper circuit operation.
Sprin
g
return momentar
y
position switchin
g
feature is
available on selected subbase models. On these models, the
fan switch is positioned to the ri
g
ht of the s
y
stem switch. B
y
movin
g
the fan switch to the far ri
g
ht and releasin
g
it, the ON
position circuit makes. The lever sprin
g
s back on release. This
position is not marked on the subbase.
Setting the Adjustable Differential
The ad
j
ustable intersta
g
e differential feature, on a selected
T874D model onl
y
, can be identified b
y
the scale and tension
screw near the heatin
g
and coolin
g
mercur
y
switches. See
Fi
g
. 13. On this model, the number of de
g
rees between the
makin
g
of the first and second sta
g
e mercur
y
bulbs is
ad
j
ustable. This feature is especiall
y
useful if the first sta
g
e
controls the comfort temperature, and the second sta
g
e
controls the ener
gy
savin
g
s temperature. Timers, such as the
S6005, for insertion between the first and second sta
g
e
control points must be ordered separatel
y
.
1.2
.4
.6
.3
.2 .15
.12
.10
.8
1.2
.4
.6
.3
.2 .15
.12
.8
.5
MOVE INDICATOR TO
MATCH CURRENT RATING
OF PRIMARY CONTROL
STAGE ONE
ANTICIPATOR
HEATING
CONTROL
STAGE TWO
ANTICIPATOR
HEATING CONTROL
M5069
STAGE 1
HEATING
STAGE 3
HEATING
STAGE 2
HEATING
HEATING
LEVER COOLING
LEVER STAGE 1
COOLING
STAGE 2
COOLING
CAPTIVE
MOUNTING
SCREWS (2) M7625