Honeywell T841A User manual

Honeywell
T841A
Heating-Cooling Thermostats
INSTALLATION INSTRUCTIONS
APPLICATION
The T841A Thermostats provide 24 Vac control of two-
stage heating and one-stage cooling in heat p mp
systems sing man al changeover.
The T841A provides EM.HT.-HEAT-OFF-COOL system
switching and AUTO-ON fan switching.
Test holes are provided on the front of the thermostat to
accommodate test meter leads witho t removing the
thermostat from the wall. Remove the cover of the
thermostat to expose test holes that are labeled to
correspond with the terminals on the back of the thermostat.
Table 1. Thermostat Feat res.
Thermostat Terminal Designations LEDs A to Fan in EM. HT. Fig re N mbers
T841A W3, W2, Y, G, R, E, X, O, B, L AUX. HT., EM. HT., CHECK Yes 2
W3, W2, Y, G, R, E, X, O, B, L AUX. HT., EM. HT. Yes 3, 4
W, Y, R, T, F, G, X2, O, B EM. HT., AUX. HT. Yes 5
W3, W2, Y, G, R, E, X, O, B —Yes 6
W3, W2, Y, G, R, E, X, O, B EM. HT., AUX. HT. Yes 7
W2, W1, Y1, G, R, E, X, O, B EM. HT., AUX. HT. Yes 8
W3, W2, Y, G, R, E, X, O, B EM. HT., AUX. HT. Yes 9
W2, W1, Y, G, R, K, Z, X EM. HT., SERVICE No 10
W2, H, Y, G, R, E, X, A, O, B EM. HT., COMP. FAULT No 11
W, Y, G, R, X, O, B EM. HT., AUX. HT. Yes 12
W2, W1, Y1, G, R, E, X, O, B, L EM. HT., AUX. HT. Yes 13
W2, W1, Y, G, R, E, C/X, O, B, EM. HT., AUX. HT. Yes 14
W, Y, G, R, T, F, X2, O, B, U EM. HT., AUX. HT. Yes 15
W2, Y, G, R, E, B, X —Yes 16
OPERATION
The stages of heat make seq entially as the temperat re
drops. Make refers to the merc ry switch initiating a call for
heat or cool.
There is abo t 1 °F (0.6°C) between stages so the second
stage (a xiliary heat) makes only when the first stage
cannot handle the load. This 1°F is the interstage
differential.
The light emitting diode (LED) indicators on certain
thermostat models light when something specific happens
within the system. The AUX. HT. LED lights when a xiliary
heat (second stage heat) is operating d e to extremely
cold weather that req ires additional heat to s pplement
the heat p mp. When the red EM. HT. LED lights, the
emergency heat is operating ( s ally electric strip
heaters), beca se the homeowner has physically switched
to the EM. HT. position. When the red CHECK LED lights,
it indicates that something needs to be done to maintain
efficient system operation. See heating/cooling
man fact rer's instr ctions for specific meaning. When the
yellow COMP. FAULT LED lights, a compressor problem
req ires attention. When the yellow SERVICE LED lights,
see heating/cooling man fact rer's instr ctions for specific
meaning.
C o p y rig h t © 1 99 8 H o neyw ell Inc. All R ights R e se r v e d 69-1218

T841A HEATING-COOLING THERMOSTATS
F R O N T O F D E V IC E B A C K O F D E V IC E
-SYSTEM SWITCH FAN SWITCH
THERMOMETER
SET POINT
SCALE
- LED (UP
TO THREE) MOUNTING
HOLES (4) L WIRING TERMINAL 1
(UP TO 12)
COVER
Fig. 1. External view of T841A.
® RECYCLING NOTICE
This control contains merc ry in a sealed t be. Do
ntplace control in the trash at the end of its sef l
life.
If this control is replacing a control that contains
merc ry in a sealed t be, do ntplace yo r old
control in the trash.
Contact yo r local waste management a thority for
instr ctions regarding recycling and the proper
disposal of this control, or any control containing
merc ry in a sealed t be.
INSTALLATION
When Installing this Prod ct...
1. Read these instr ctions caref lly. Fail re to follow
them co ld damage the prod ct or ca se a
hazardo s condition.
2. Check the ratings given in the instr ctions and on
the prod ct to make s re the prod ct is s itable for
yo r application.
3. Installer m st be a trained, experienced service
technician.
4. After installation is complete, check o t prod ct
operation as provided in these instr ctions.
A CAUTION
1. Disconnect power s pply before beginning
installation to prevent electrical shock or
eq ipment damage.
2. Do not short across coil terminals on relay.
This can b rn o t the thermostat heat
anticipator.
Location
Locate the thermostat abo t 5 ft (1.5m) above the floor in
an area with good circ lation at average temperat re.
Do not mo nt thermostat where it may be affected by:
— drafts, or dead spots behind doors and in corners.
— hot or cold air from d cts.
— radiant heat from s n or appliances.
— concealed pipes and chimneys.
— nheated ( ncooled) areas behind the thermostat, s ch
as an o tside wall.
This thermostat is a precision instr ment and was caref lly
adj sted at the factory. Handle it carefully.
Mo nting and Wiring
The T841 can be mo nted directly on a wall or a horizontal
o tlet box. Choose the method that best fits yo r
installation.
1. In replacement applications, check the existing
thermostat wiring for cracked or frayed ins lation.
Replace any wiring in poor condition. All wiring m st
comply with local codes and ordinances.
2. Grasp the thermostat cover at the top and bottom
with one hand. P ll o tward on the bottom edge of
the cover ntil it snaps free of the thermostat base.
Caref lly remove and save the packing material
s rro nding the merc ry switches.
3. R n wiring (if necessary) to the location. If the wiring
is plastered into the wall, make a hole next to the
cable and loosen the wires so they can be p shed
back into the wall later. Thread wires thro gh the
hole in the packing material saved in step 2.
Connect the wires to the terminals on the back of the
thermostat. See Fig. 2 thro gh 17.
4. Where applicable, set the second stage adj stable
heat anticipator to match the c rrent draw of the
primary heating control (see Heat Anticipator Setting
section).
5. P sh the excess wire back thro gh the hole and
pl g any opening with packing material to prevent
drafts that may affect thermostat performance.
69-1218 2

T841A HEATING-COOLING THERMOSTATS
6. Loosely sec re the therm ostat to the w all or o tlet
box w ith screw s thro gh the tw o m o nting holes in
the m iddle of the device. The sheetm etal screws
incl ded w ith the therm ostat are designed for se in
a plaster wall and anchors are not needed.
7. Level the therm ostat exactly sing a spirit level or
pl m b line. Tighten the two m o nting screw s at the
m iddle of the device.
8. Install two screw s in the top m o nting holes and
tighten.
IMPORTANT
An incorrectly leveled su ase will cause the
temperature control to deviate from setpoint. It is
not a cali ration pro lem.
9. R eplace the therm ostat cover.
ODT 1
L1
(HOT)
L2
CHANGEOVER
RELAY (COOL)
POWER SUPPLY. PROVIDE DISCONNECT MEANS AND OVERLOAD PROTECTION AS REQUIRED.
DO NOT JUMPER E TO W2 TERMINALS.
X TERMINAL MUST BE CONNECTED FOR PROPER OPERATION.
Fig. 2. Internal schematic and typical wiring diagram for T841A with CHECK, EM. HT., and AUX. HT. LEDs.
Provides both heat and cool changeover relay control. Common (X) wire m st be connected for proper operation.
369-1218

T841A HEATING-COOLING THERMOSTATS
L1 L2
(HOT)
Fig. 3. Internal schematic and typical wiring diagram for T841A with EM. HT. and AUX. HT. LEDs. Provides cool
changeover relay control only. Cool changeover (O) wire m st be connected for proper operation. Use L terminal
for compressor malf nction indication.
69-1218 4

T841A HEATING-COOLING THERMOSTATS
L1 L2
(HOT)
Fig. 4. Internal schematic and typical hook p for T841A with fan in Emergency Heat mode, 2-stage heat, 1-stage
cool, man al changeover.
L1 L2
(HOT)
Fi . 5. Internal schematic and typical wirin dia ram for T841A with EM. HT. and AUX. HT. LEDs. Provides
outdoor reset when used with C815 Thermistor. Provides cool chan eover relay control.
569-1218

T841A HEATING-COOLING THERMOSTATS
ODT 1
Fig. 6. Internal schematic and typical wiring diagram for T841A with EM. HT. LED. Provides both heat and cool
changeover relay control. Common (X) wire m st be connected for proper operation.
DISCONNECT MEANS AND
OVERLOAD PROTECTION
AS REQUIRED.
DO NOT JUMPER E TO W2 TERMINAL.
L1 L2
(HOT)
^W 3)-
a
<R
•(W2)-
AUX. HT.
LED (GREEN)
E
a y
RTD 1 ODT 1
J- H hH-T-HH l~-|-|
EHR 1 RTD 2
ODT 2
m -TEEh
EHR 2 RTD
y r m a t
J-°-V H ------------
A
EM. HT.
LED (RED)
< X h
Y
< c > - -
EM.HEAT
RELAY
-A V H — "
+ ° - V ° +
CHANGEOVER
VALVE (HEAT)
COMPRESSOR
CONTACTOR
+>VH—
CHANGEOVER
VALVE (COOL)
T<>v>|
Fi . 7. Internal schematic and typical wirin dia ram for T841A with AUX. HT. and EM. HT. LEDs. Provides both
heat and cool chan eover relay control. Common (X) wire must be connected for proper operation.
RD
RID 1
CHP
M8505
69-1218 6

T841A HEATING-COOLING THERMOSTATS
HEAT RELAY 1
Fig. 8. Internal schematic and typical wiring diagram for T841A with AUX. HT. and EM. HT. LEDs. Provides both
heat and cool changeover relay control.
CHANGEOVER
VALVE (COOL)
Fi . 9. Internal schematic and typical wirin dia ram with AUX. HT. and EM. HT. LEDs. Provides both heat and
cool chan eover relay control.
769-1218

T841A HEATING-COOLING THERMOSTATS
FAN RELAY
Fig. 11. Internal schematic and typical wiring diagram for T841A with EM. HT. and COMP. FAULT LEDs.
Common (X) wire m st be connected for proper operation.
69-1218 8

T841A HEATING-COOLING THERMOSTATS
FAN REIAY
Fig. 12. Internal schematic and typical wiring diagram for T841A with EM. HT. and AUX. HT. LEDs.
L1
(HOT) L2
/ K REMOVE W1-Y1 JUMPER WHEN HEAT RELAY 1 IS USED.
WHEN L TERMINAL IS CONNECTED TO SYSTEM MONITOR, EM. HEAT. LED ALSO INDICATES COMPRESSOR MALFUNCTION.
/ \ X TERMINAL MUST BE CONNECTED FOR PROPER OPERATION. M1103C
Fi . 13. Internal schematic and typical wirin dia ram for T841A with EM. HT. and AUX. HT. LEDs. Provides both
heat and cool chan eover relay control.
969-1218

? I > ^ > >
T841A HEATING-COOLING THERMOSTATS
L1 L2
Fig. 14. T841A with two a xiliary heat stages and AUX.HT. and EM.HT. LEDs. Provides a to fan in emergency heat.
Fig. 15. T841A with EM. HT. and AUX. HEAT LEDs. Provides o tdoor reset when sed with C815 Thermistor.
69-1218 10

T841A HEATING-COOLING THERMOSTATS
THERMOSTAT
___ A _____
SUBBASE
__ A ___ A / -
SYSTEM COMPONENTS
______ A ________
Fig. 16. Internal schematic and typical wiring diagram for T841A1720. Provides heat changeover relay control.
SYSTEM
SETTINGS
Heat Anticipator Setting
On m odel shown in Fig. 2, 3, 6, 7, 8, 11, 14, 16 and 17 the
second stage heat anticipator is adj stable. On these
m odels, m ove the adj stable indicator to m atch the c rrent
draw of the second stage heating prim ary control, or the
anticipator setting of the old therm ostat. See Fig. 18.
Temperat re Setting
M ove the se tpoint lever to the desired control point on the
tem perat re scale.
System Switch Setting
The system switching positions control the system
operation as follows.
EM. HT. (T841A only)— em ergency heat relay is
energized. Cooling system is off. Com pressor is de 
energized. Fan r ns w ith call for heat in A UTO position.
EM. HT. LED is on.
HEAT— heating system is a to m atically controlled by the
therm ostat. Cooling system is off.
O FF— both the heating and cooling system s are off. If the
fan is at A U TO position, the cooling fan is also off.
CO O L— cooling system is a tom atically controlled by the
therm ostat. Heating system is off.
Fig. 18. Second stage adj stable heat anticipator
indicator.
Fan Switch Setting
A U TO — fan operates a tom atically as heating or cooling
eq ip m ent cycles on.
ON— fan operates contin o sly.
11 69-1218

T841A HEATING-COOLING THERMOSTATS
CHECKOUT
Heating
Move the system switch on the thermostat to HEAT and
the fan switch to AUTO. Move the setpoint lever abo t
10°F (6°C) above room temperat re. Heating sho ld start
and the fan sho ld r n. Move the setpoint lever abo t 10°F
(6°C) below room temperat re. Heating and fan sho ld
sh t off.
NOTE: In heat p mp applications, compressor activation
is time delayed to prevent short cycling. The
minim m-off timer controls the delays and
prevents the compressor from starting for p to
five min tes, either from when the thermostat last
t rned off the compressor, or from when the
system first received power.
Cooling
A CAUTION
If o tdoor temperat re is below 50°F (10°C), do
not operate cooling. Refer to man fact rer's
recommendations.
Move the system switch on the thermostat to COOL and
the fan switch to AUTO. Move the set point lever abo t
10°F (6°C) below room temperat re. Cooling sho ld start,
followed by the fan (see NOTE above). Move the set point
lever abo t 10°F (6°C) above room temperat re. Cooling
and fan sho ld sh t off.
Fan
Move the system switch to OFF, and the fan switch to ON.
The fan sho ld r n contin o sly. Move the fan switch to
AUTO. In this position, fan operation is controlled by the
heating or cooling system control circ it.
O tdoor Reset Thermistor (Honeywell
C815A1005)
For system shown in Fig. 5, the thermistor m st be sed; if
not, thermostat performance deviates radically from proper
operation.
The proper thermostat operation m st be verified to ass re
the correct operation of the thermostat. Check thermistor
operation as follows:
1. Disconnect the Twire on the thermostat.
2. Use an ohmmeter to meas re resistance between
the Twire and the ^thermostat terminal.
3. Take o tdoor temperat re at thermistor location and
find the correct thermistor resistance on the chart in
Fig. 19.
4. If the resistance meas red in step 2 and the
calc lated resistance in step 3 vary by more than 15
percent, the thermistor req ires replacement.
Contact Original Eq ipment Man fact rer or
installation dealer for o tdoor thermistor
replacement.
-20 0 20 40 60 80 100 120 140
TEMPERATURE OF THERMISTOR (°F) M1590A
Fig. 19. Thermistor resistance chart.
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