Tracing Method; Key To Hookup Symbols - Honeywell T874 Product Data

thermostats. The schematics can be field-modified as
required (EX: if T874C is being used, eliminate second
stage of heat).
For heat pump circuits:
Cool changeover valve—operates on cooling. The revers-
ing valve or relay is activated either by moving the sys-
tem switch to COOL (manual changeover) or by a
mercury switch that makes on a temperature rise (auto
changeover).
Heat changeover valve—operates on heating. The revers-
ing valve or relay is activated either by moving the sys-
tem switch to HEAT (manual changeover) or by a
mercury switch that makes on a temperature fall (auto
changeover).
System monitor relay—optional equipment on some heat
pumps includes an R4222P1065 or equivalent. This
system monitor relay detects a malfunction in the com-
pressor and indicates the malfunction by activating the
EMERGENCY HEAT LED on the Q674 Switching Sub-
base. The system monitor relay is usually wired into the
L terminal on the Q674.
Each mercury switch is identified by function:
H1—Stage 1 Heating.
H2—Stage 2 Heating.
H3—Stage 3 Heating.
C1—Stage 1 Cooling.
C2—Stage 2 Cooling.
C3—Stage 3 Cooling.
C/O—Changeover (heat pumps).
Each anticipator is identified and each switch affected is
named (EX: H1 anticipator, C1 anticipator).
All T874 Multistage Thermostats use mercury switches. Each
schematic indicates switch operation by being drawn in the
open position with an arrow indicating operation with a
temperature RISE or FALL.
One circuit has been selected that is typical of various models
used with heat pumps. This circuit has been traced to illustrate
the functions performed by these control systems.

Tracing Method:

1. Always begin at the system transformer or R terminal.
You may want to draw the switch contacts in each
switch position to aid in tracing. Colored pencils are
helpful when only one copy of the circuit is available.
See Fig. 16 for a description of the hookup symbols and
Fig. 17 through 25 for typical hookups.
2. Completely trace only one circuit at a time (for example:
heat or cool).
3. Connections are indicated by small dots at the point of
intersection. If there is no dot, there is no connection.
4. The left portion of the circuit (the thermostat) contains
the mercury switches. The heat switches make on a
temperature fall, and the cool switches make on a tem-
T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES
perature rise. Fixed anticipation is represented by a zig-
zag line and adjustable anticipation is a zigzag with an
arrow. The resistance of the fixed anticipator is so large
it limits current so that a system relay cannot be pulled
in from a circuit path going through the fixed anticipator.
The relay can be pulled in through an adjustable antici-
pator because its resistance is generally 0 to 5 ohms.
5. The center portion (the subbase) contains the switches.
The fan switch is above the system switch. The small
circles on the switch represent the maximum possible
contacts available on the Q674 Subbase. The larger cir-
cles represent the switch positions available on this par-
ticular Q674, with the solid circle representing where it
is actually switched on the diagram.
NOTE: Solid circles are not interconnected electrically.
At the right, the relays and contactors are shown, attached to
the proper terminals. The terminals are represented by large
circles with terminal designations in capital letters. See Table
3 for the meaning of each lettered terminal.
Sometimes power for a fixed anticipator is brought through an
off system relay like the changeover relay shown in Fig. 20.
This current is kept low by the high resistance of the fixed
anticipator so that relay does not pull in.

KEY TO HOOKUP SYMBOLS

TRANSFORMER
(24 VAC SECONDARY)
RELAY OR CONTACTOR COIL
MERCURY SWITCH
FIXED ANTICIPATOR
HIGH RESISTANCE
(TYPICALLY 5 KILOHMS)
ADJUSTABLE ANTICIPATOR
LOW RESISTANCE
(TYPICALLY 0 TO 5 OHMS)
Fig. 16. Key to hookup symbols.
15
RELAY/CONTACTOR CONTACTS
B
TERMINAL
LED
ODT
OUTDOOR THERMOSTAT
EHR
EMERGENCY HEAT RELAY
RTD
TIME DELAY RELAY
RD
DEFROST RELAY
CHP
PRESSURE SWITCH
LACO
LOW AMBIENT CUTOFF
M5848
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