Filter
Top Products
07/05 505,069M
*2P0705* *P505069M*
Page 1
E2005 Lennox Industries Inc.
Dallas, Texas, USA
RETAIN THESE INSTRUCTIONS
FOR FUTURE REFERENCE
51M42 Merit
®
Series Thermostat
The Lennox Merit
®
Series 7−day programmable electronic
thermostat 51M42 provides excellent temperature control
and a large, easy-to-read display. It includes a
programmable filter change reminder, an equipment
maintenance reminder, and a system check indicator to
notify the user when the equipment requires service.
Thermostat 51M42 is suitable for heat pump, three-stage
heat/two-stage cool applications using a gas or electric
auxiliary heat source. In addition to displaying outdoor
temperature, the outdoor temperature sensor provides
auxiliary heat lockout, balance point operation, and dual−
fuel compatibility. Also, the thermostat provides auto-
changeover capability.
OPERATION
MANUAL
51M42 Merit
®
Series
7−Day Programmable Thermostat
CONTROLS
505,069M
07/05
Table of Contents
51M42 Series Thermostat 1. . . . . . . . . . . . . . . . . . . . . .
General 1. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Introduction 1. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Initial Thermostat Power-up 1. . . . . . . . . . . . . . . . . . . . .
Buttons, Backlight, Timers & Settings 2. . . . . . . . . . . .
DAY/TIME − Set Day and Time 2. . . . . . . . . . . . . . . . . .
HEAT − Heat Mode 3. . . . . . . . . . . . . . . . . . . . . . . . . . . .
COOL − Cool Mode 4. . . . . . . . . . . . . . . . . . . . . . . . . . . .
AUTO − Using the Autochangeover Mode 4. . . . . . . . .
HOLD − Temperature Hold Mode 4. . . . . . . . . . . . . . . .
PROG − Thermostat Programming 5. . . . . . . . . . . . . . .
COPY − Copy a Program 5. . . . . . . . . . . . . . . . . . . . . . .
FAN − Controlling Fan Operation 6. . . . . . . . . . . . . . . . .
Displaying Outside Temperature 6. . . . . . . . . . . . . . . . .
SETTINGS − Filter and Maintenance Reminders 6. . .
SETTINGS − Balance Point 7. . . . . . . . . . . . . . . . . . . . .
Service Indicator 7. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Thermostat RESET 7. . . . . . . . . . . . . . . . . . . . . . . . . . . .
Removing/Installing Thermostat 7. . . . . . . . . . . . . . . . .
Default Thermostat Settings 7. . . . . . . . . . . . . . . . . . . .
Technical Specifications 9. . . . . . . . . . . . . . . . . . . . . . . .
Thermostat Output Table 8. . . . . . . . . . . . . . . . . . . . . . .
General
These instructions are intended as a general guide and do
not supersede local codes in any way. Consult authorities
having jurisdiction before installation.
Check equipment for shipping damage. If you find any
damage, immediately contact the last carrier.
Introduction
This document describes the operation of Lennox thermostat
51M42. Refer to the installation manual for instructions re-
garding installation and wiring of the thermostat.
Initial Thermostat Power-up
When power is initially applied to the thermostat, the dis-
play will appear as shown in figure 1.
HEAT SETPOINT
COOL
OFF
AUTO
EMG.
HEAT
I8:88
i88
Figure 1
AM
PM
OUTROOM
MO TU WE TH FR SA SU
COOL SETPOINT
Initial Power−Up Display
FILTER
MAINTENANCE
Hi
Lo
FAN ON
AUTO
°
°
°
SERVICE
Copy tofrom
PROG
A B C D
HOLD
All display segments are momentarily activated. This oc-
curs as a normal part of thermostat initialization.
Litho U.S.A.