Honeywell T8424D User manual

® U.S. Registered Trademark
Copyright © 2001 Honeywell • • All Rights Reserved
INSTALLATION INSTRUCTIONS
69- 1386- 1
T8424D Electronic
Multistage Thermostat
The T8424 Thermostat provides electronic control of
single-zone, multistage heating and cooling systems.
First and second stage cooling cycle rates are fixed at 3
cph. Heating cycle rate is selectable. Temperature
indication can be set for °F or °C.
The T8424 Thermostat is powered from the system
transformer. Batteries are not required because setpoints
are held permanently by non-volatile memory.
The T8424 includes a thermostat, wallplate (for wiring
and mounting thermostat) and owner’s guide.
MERCURY NOTICE
If this control is replacing a control that contains
mercury in a sealed tube, do not place your old
control in the trash.
Contact your local waste management authority
for instructions regarding recycling and the
proper disposal of an old control containing
mercury in a sealed tube.
If you have questions, call Honeywell Inc. at
1-800-468-1502.
INSTALLATION
When Installing this Product…
1. Read these instructions carefully. Failure to follow
them could damage the product or cause a hazard-
ous condition.
2. Check the ratings given in the instructions and on
the product to make sure the product is suitable for
your application.
3. Installer must be a trained, experienced service
technician.
4. After installation is complete, check out product
operation as provided in these instructions.
CAUTION
Electrical Shock or Equipment Damage
Hazard.
Can shock individuals or short equipment
circuitry.
Disconnect power supply before installation.
Location
Install the thermostat about 5 ft (1.5m) above the floor in
an area with good air circulation at average temperature.
See Fig. 1. Do not install the thermostat where it can be
affected by:
— drafts or dead spots behind doors and in corners.
— hot or cold air from ducts.
— radiant heat from the sun or appliances.
— concealed pipes and chimneys.
— unheated (uncooled) areas such as an outside wall
behind the thermostat.
This thermostat is a precision instrument and was
carefully adjusted at the factory. Handle it carefully.
Mounting Wallplate to Wall
IMPORTANT
Level only for appearance. The thermostat func-
tions normally even when not level.
Mount wallplate, T8424 and the screws provided (see
Fig. 2) as follows:
1. Place the wallplate at the desired location on the
wall.
2. Pull the thermostat wire through the entrance hole
on the decorator cover plate, then through the wall-
plate entrance hole.
3. Select the two mounting holes that best fit the
application.
4. Fasten the wallplate to the wall using the anchors
and screws provided.
5. After wiring the wallplate, plug the hole to prevent
drafts from affecting the thermostat; see Wiring
section.

T8424D ELECTRONIC MULTISTAGE THERMOSTAT
69-1386-1 2
Fig. 1. Typical location of thermostat.
Fig. 2. Mounting wallplate to wall.
Wiring
IMPORTANT
Use an 18-gauge maximum wire for wiring the
T8424 Thermostat.
All wiring must comply with local electrical codes and
ordinances. Disconnect the power supply to prevent
electrical shock or equipment damage.
NOTE: To ensure proper mounting of thermostat,
restrict all wiring to the shaded area. See Fig. 3.
The shape of the terminals permits insertion of straight or
wraparound wiring connections; either method is
acceptable. See Fig. 4.
The T8424 Thermostat is powered through the system
transformer and is adaptable to most 24 Vac multistage
heating-cooling systems. Refer to Fig. 5 for typical wiring
hookups.
Fig. 3. Restrict wiring to shaded area.
5 FEET
[1.5 METERS]
YES
NO
NO
NO
M11338
M12202A
WALL WALL
ANCHORS (2)
WALLPLATE
WHEN USING WALL ANCHORS, DRILL 3/16 INCH
HOLES FOR DRYWALL, 7/32 INCH HOLES FOR
PLASTER OR WOOD.
MOUNTING
SCREWS (2)
1
1
KEEP WIRING IN
SHADED AREA
MOUNTING
SCREW HOLE
MOUNTING
SCREW HOLE
WIRING ENTRANCE
HOLE
M20211
G
C
R
Y
W1 Y2
W2
B
O

T8424D ELECTRONIC MULTISTAGE THERMOSTAT
3 69-1386-1
Fig. 4. Wiring connections.
Fig. 5. Typical hookup of T8424 in two-stage heat and
two-stage cool system with manual changeover.
Mounting Thermostat to Wallplate
1. Engage the tabs at the top of the thermostat and
wallplate.
2. Swing down the thermostat and press the lower
edge of the thermostat onto the wallplate to latch.
See Fig. 6.
OPERATION
Setting FAN and SYSTEM Switches
Fan and system settings are controlled manually by
using the switches located at the bottom of the
thermostat case. See Fig. 7.
Fig. 6. Mounting thermostat wallplate.
FAN Switch
Fan switch settings are:
On: The fan runs continuously. Use for improved air
circulation and air quality.
Auto: Normal setting for most homes. In cooling, the
fan starts and stops with the cooling equipment. In
heating, the fan is controlled directly by the heating
equipment and may start a few minutes after the
heating equipment turns on (on most systems).
When using an electric heat thermostat, the fan
starts and stops with the heating equipment.
Slide the FAN switch in the bottom left corner of the
thermostat to select the desired fan setting.
SYSTEM Switch
System switch settings control thermostat operation as
follows:
Cool: The thermostat controls the cooling system.
Off: Both heating and cooling are off.
Heat: The thermostat controls the heating system.
Slide the SYSTEM switch in the bottom right corner of
the thermostat to select the desired system setting.
TERMINAL
SCREW
M20212
G
R
C
Y
W1 B
O
FOR STRAIGHT
INSERTION STRIP
5/16 IN. (8 MM)
FOR WRAPAROUND
STRIP 7/16 IN. (11 MM)
Y2
W2
L1
(HOT)
L2
1
1
W2
Y
G
R
C
1ST STAGE
COOLING
2ND STAGE
COOLING
FAN
RELAY
STAGE-TWO
HEATING
Y2
24V
POWER SUPPLY. PROVIDE DISCONNECT MEANS AND
OVERLOAD PROTECTION AS REQUIRED.
M20213
T8424
W1 STAGE-ONE
HEATING
M14677
SYSTEM
Cool Off Heat
Auto On
FAN
DASHED LINES INDICATE TABS
ON BACK OF THERMOSTAT
SYSTEM
Cool Off Heat
Auto On
FAN
ENGAGE TABS AT TOP OF THERMOSTAT
WITH SLOTS ON MOUNTING PLATE.
PRESS LOWER EDGE OF
CASE TO LATCH.
A
B

T8424D ELECTRONIC MULTISTAGE THERMOSTAT
69-1386-1 4
Fig. 7. Digital display and system
switches (FAN and SYSTEM).
Set Temperature Setpoint
NOTE: Temperature setpoint range is 40° to 99°F (5° to
37°C).
The temperature setpoint and the room temperature are
shown separately on the digital display. The ▼ indicator
points to Set when the temperature setpoint is displayed
and to Room when the room temperature is displayed
.
To set temperature setpoint:
1. Select Heat or Cool by sliding the SYSTEM switch
in the lower right corner of the thermostat to the
desired mode. See Fig. 7.
2. To display the temperature setpoint on the digital
display, press either the ▲or ▼key once. The
temperature setpoint is displayed for approxi-
mately five seconds as the indicator points to Set
and flashes.
3. To increase the temperature setpoint press the ▲
key. Each press changes the setpoint one degree;
press and hold to change the setpoint several
degrees.
4. To decrease the temperature setpoint press the ▼
key. Each press changes the setpoint one degree;
press and hold to change the setpoint several
degrees.
Setting °F/°C Indication and Heat Cycle Rate
NOTE: To save changes to the °F/°C indication and the
heat cycle rate, all seven steps must be com-
pleted.
To set the °F/°C indication and heat cycle rate:
1. If the temperature is displayed in °F, set the tem-
perature setpoint to 52°F. If the room temperature
is displayed in °C, set the temperature setpoint to
11°C.
2. Press the ▲▼ keys simultaneously for more than
one second to light all segments on display and to
enter installer setup mode. When the keys are
released, a two-digit software revision code is dis-
played.
OPTIONAL SYSTEM CHECKOUT
When in steps 2 and 3 only, the
▼
key can be used to
turn heat or cool outputs on and off. Change the
SYSTEM switch setting to test heat or cool outputs. No
action takes place If the system switch is in the Off
position.
Examples: System setting at HEAT: If heat is on, the ▼
key turns it off; if heat is off, ▼turns it on.
System setting at COOL: If cool is on, ▼key
turns it off; if cool is off, ▼turns it on.
NOTE: In Optional System Checkout for multistage
models, pressing the ▼key turns on the first
stage output. (The second press of the ▼key
turns on the second stage output.)
NOTE: In installer setup mode only, each press of the
▲key momentarily displays 01. Each press of
the ▼key momentarily displays 02. When the
keys are released, these two-digit codes are no
longer displayed.
SYSTEM
Cool Off Heat
Auto On
FAN
M14678
TEMPERATURE DISPLAY
INCREASE SETTING
DECREASE SETTING
FAN SWITCH
SYSTEM SWITCH
Set Room
INDICATOR
ROOM TEMPERATURE
SETPOINT TEMPERATURE
M14679
M14680
M14681
M14682
M14683
M14686
M14684
M14685

T8424D ELECTRONIC MULTISTAGE THERMOSTAT
5 69-1386-1
3. Press the ▲key. Factory information is displayed.
A typical example is shown, but information dis-
played varies by model. This information is for fac-
tory use only.
4. Press the ▲key again to display °C or °F.
5. Press the ▼ key to change the °C or °F indication.
NOTE: Steps 6-9 apply only to T8424 Thermostats.
6. Press ▲to display number of cooling stages.
7. Press ▼ to change number of cooling stages.
8. Press ▲to display number of heat stages.
9. Press ▼ to change number of heat stages.
10. Press the ▲key to display the heat cycle rate of 1,
3, 4, 5, 6, 9, or 12. (If the T8424 is set for two heat
stages, both stages operate at the selected cycle
rate.) If the desired cycle is displayed, press the ▲
key to exit the installer setup mode. To change the
heat cycle rate, press the ▼key to scroll between
1, 3, 4, 5, 6, 9, or 12. Stop scrolling when the
desired rate is displayed. See Table 1 for the cycle
rate options and the corresponding system equip-
ment.
11. Press the ▲key to display algorithm configuration
default.
12. Press the ▲key again to change algorithm to C1
or C3.
C1 = Standard algorithm.
C3 = Aggressive algorithm (can cause
overshooting).
13. Press the ▲key again. Current configuration (CC)
is displayed. A typical example is shown, but CC
varies by model. (This information is for factory use
only.)
Table 1. Heating Cycle Rates for Stage-Two Heating.
aHigh efficiency furnace.
bRefer to equipment manufacturer’s instructions.
14. Press the ▲key to save all changes, exit installer
setup mode and return to normal operation.
NOTE: After exiting installer setup mode, change the
setpoint to the desired room temperature.
CHECKOUT
Heating
1. Slide the SYSTEM switch to Heat and the FAN
switch to Auto.
2. Press and hold the ▲key to raise the temperature
setting several degrees above the room tempera-
ture. After approximately ten seconds, the heating
equipment should start. In conventional systems,
the system turns on the fan through the use of a
time delay relay or through a limit control. When
using an electric heat thermostat, the fan starts
immediately.
3. Press the ▼key to lower the temperature setting
below the room temperature. Heating equipment
should stop.
M14687
M14688
M14689
M20208
M20207
M20209
M20210
M18500
System Cycles Per Hour
Steam, gravity 1
Hydronic heat, condensing gas
furnacea
3
Special applicationsb4,5
Gas or oil forced air (default) 6
Electric heat 9
Special applicationsb12
M18419
M18420

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Cooling
CAUTION
Low Temperature Hazard.
Operating at too low of an outdoor
temperature may cause compressor damage.
Do not operate cooling if outdoor temperature is
below 50°F (10°C). Refer to manufacturer’s
recommendations.
• For compressor protection, during power
interruption and on initial startup, the T8424
Thermostat goes into a 5 minute delay. During
this delay the cooling icon, a snowflake ❄,
flashes.
1. Slide the SYSTEM switch to Cool and the FAN
switch to Auto.
2. Press the ▼ key to lower the temperature setting
several degrees below the room temperature. After
approximately five minutes, the cooling equipment
should start. The fan starts and stops with the cool-
ing equipment.
3. Press the ▲key to raise the temperature setting
above the room temperature. Cooling system
should shut down.
NOTE: To bypass the 5 minute delay, see the Optional
System Checkout section.
Fan
1. Slide the SYSTEM switch to Off and the FAN
switch to On. The fan should run continuously.
2. Slide the FAN switch to Auto. In heating, the fan is
controlled directly by the heating equipment and
may start a few minutes after the heating equip-
ment turns on (on most systems). When using an
electric heat thermostat, the fan starts and stops
with the heating equipment. In cooling, the fan
starts and stops with the cooling equipment.
Make certain all equipment responds properly to the
thermostat.