Understanding Circuits - Honeywell Heat/1Cool - T841A1563 Premier 2 Stage Thermostat User Manual

HEAT PUMP THERMOSTAT CROSS REFERENCE/SELECTION GUIDE

Understanding Circuits

To understand wiring diagrams, it is important to know what all the symbols mean and how to trace the path of the
circuits from the transformer. Circuit descriptions and terminology are defined as follows:
Auto changeover—refers to the presence of an AUTO position in the system switching (EX: Q674F with OFF-COOL
AUTO-HEAT-EM. HEAT switching). The thermostat automatically changes between heat and cool modes as
indoor temperature changes.
Manual changeover—requires a system switch movement to change mode (EX: Q674L with EM. HT.-HEAT-OFF-
COOL switching).
Cool changeover valve—operates on cooling. The reversing valve or relay is activated either by moving the system
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 reversing valve or relay is activated either by moving the system
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. This system monitor relay detects a malfunction in
the compressor and indicates the malfunction by activating the EMERGENCY HEAT LED on the switching
subbase. The system monitor relay may be wired to the L terminal on some thermostats
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.
KEY TO HOOKUP SYMBOLS
RELAY/CONTACTOR CONTACTS
TRANSFORMER
(24 VAC SECONDARY)
RELAY OR CONTACTOR COIL
ODT
OUTDOOR THERMOSTAT
MERCURY SWITCH
EHR
EMERGENCY HEAT RELAY
FIXED ANTICIPATOR
RTD
HIGH RESISTANCE
TIME DELAY RELAY
(TYPICALLY 5 KILOHMS)
RD
DEFROST RELAY
CHP
PRESSURE SWITCH
ADJUSTABLE ANTICIPATOR
LOW RESISTANCE
LACO
(TYPICALLY 0 TO 5 OHMS)
LOW AMBIENT CUTOFF
Fig. 14. Key to hookup symbols.
B
TERMINAL
LED
M5848
70-6627 • 13