T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES
13 60-2485—8
Each mark on the scale represents 1°F
0.6°C
The
differential is factor
set at 2°F
1°C
the differential can be set
as hi
h as 12°F
7°C
To set the ad
ustable intersta
e
differential, loosen the tension screw. See Fi
. 13. Slide the
ad
ustable scale to ali
n with the number of de
rees desired
between sta
es. Use the lower ed
e of the tension screw
bracket as a
uide for ali
nment. In heatin
, slide the lever
wider
apart for a
larger
differential, or
closer
to
ether for a
smaller
differential. In coolin
, slide the lever
closer
to
ether
for a
larger
differential, or
wider
apart for a
smaller
differential.
While supportin
the scale with hand, ti
hten the tension screw.
IMPORTANT
Support the scale with hand while tightening tension
screw. See Fig. 13. Failure to do so can result in
twisting and damaging bimetal coil.
Fig. 13. Set adjustable interstage differential.
Equipment Damage Hazard.
When the thermostat is used to control a two-sta
e
heatin
or coolin
s
stem, the second sta
e mercur
bulb must never make before the first sta
e bulb, or
severe e
uipment dama
e could result. To prevent
this problem, provide at least 2°F
1°C
differential
between sta
e-one and sta
e-two make points.
Example: in heatin
, if sta
e-one makes at 70°F
21°C
sta
e-two should make at 68°F
20°C
or lower.
Verify the Adjustment
Heating
Start with the heatin
setpoint lever all the wa
to the left.
Slowl
move the lever to the ri
ht,
ust until the first sta
e bulb
makes
mercur
rolls to the ri
ht side of the bulb
. Note the
settin
on the temperature scale. Slowl
move the lever to the
ri
ht until the second sta
e bulb makes. Note the settin
on
the temperature scale. The difference between the two
temperatures is the
interstage differential,
which should match
the number set on the scale with the tension screw.
Cooling
Start with the coolin
setpoint lever all the wa
to the ri
ht.
Slowl
move the lever to the left,
ust until the first sta
e bulb
makes
mercur
rolls to the left side of the bulb
. Note the
settin
on the temperature scale. Slowl
move the lever to the
left until the second sta
e bulb makes. Note the settin
on the
temperature scale. The difference between the two
temperatures is the
interstage differential,
which should match
the number set on the scale with the tension screw.
CHECKOUT
Heating
Move the s
stem switch on the Q674 Subbase to HEAT or
AUTO. Move the heat lever on the T874 about 10°F
6°C
above room temperature. See Fi
. 12. Heatin
s
stem should
start and the fan should run after a short dela
. Move the heat
lever about 10°F
6°C
below room temperature. The heatin
e
uipment should shut off, and the fan should run for a short
time, then shut off.
In heat pump applications, sometimes time dela
s are
involved before the compressor and auxiliar
heat are
activated. This is due to a minimum-off timer, which prevents
the compressor from restartin
for five minutes from when the
thermostat last turned off the compressor, or from when the
s
stem first received power.
Cooling
Equipment Damage Hazard.
Do not operate coolin
if outdoor temperature is below
50°F
10°C
. Refer to manufacturer recommendations.
TENSION
SCREW
ALIGN LOWER
EDGE WITH
SCALE
NOTCH
10°F
SCALE
1
SLIDE LEVER
WIDER APART
FOR LARGER
DIFFERENTIAL
HEATING
TENSION
SCREW
ALIGN LOWER
EDGE WITH
SCALE
NOTCH
4°F
SCALE
1
SLIDE LEVER
CLOSER TOGETHER
FOR LARGER
DIFFERENTIAL
COOLING
1 EACH MARK ON THE SCALE REPRESENTS 1°F (0.6°C).
M937
1.2
.8
.6
.3
.2
1
.5
.4
50 60
70 80
HEAT
TENSION
SCREW
HEATING SET-
POINT LEVER
FIRST
STAGE
SWITCH
SECOND STAGE
(ADJUSTABLE)
SWITCH
SCALE
