51M37 Merit® Series Thermostat - Operation Manual - 5/2 Day Programmable Thermostat

Oct 23, 2026

Uploaded by Jiven

51M37 Merit® Series 5/2 day programmable electronic thermostat operation instructions focus on temperature control and user interaction. The manual explains initialization and home-screen behavior, default Fahrenheit settings, and how to confirm and adjust day/time within time limits. It details button functions and backlight operation, including flashing input fields, setpoint display timing, and screen return logic. Heating modes cover normal heat with auxiliary backup activation and emergency heat that disables the heat pump until a heat demand occurs.

LanguageEnglish

CategoryHealthcare

FormatPDF

Pages54

Reading Time2 h 16 min

AccessFree to download

51M37 Series Thermostat

General

Introduction

Initial Thermostat Power-up

Buttons, Backlight, Timers & Settings

DAY/TIME - Set Day and Time

HEAT - Heat Mode

COOL - Cool Mode

HOLD - Temperature Hold Mode

PROG - Thermostat Programming

FAN - Controlling Fan Operation

SETTINGS - Filter and Maintenance Reminders

SETTINGS - Balance Point

Service Indicator

Thermostat RESET

Removing/Installing Thermostat

Default Thermostat Settings

Technical Specifications

Thermostat Output Table

􀀁2005 Lennox Industries Inc. Dallas, Texas, USA RETAIN THESE INSTRUCTIONS FOR FUTURE REFERENCE 51M37 Merit® Series Thermostat The Lennox Merit® Series 5/2 day programmable electronic thermostat 51M37 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. The 51M37 is suitable for heat pump, 3-stage heat/2-stage cool applications using a gas or electric auxiliary heat source. An optional outdoor temperature sensor provides auxiliary heat lockout, balance point operation, and dual−fuel compatibility. 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 51M37 . Refer to the installation manual for instructions regarding installation and wiring of the thermostat. Initial Thermostat Power-up When power is initially applied to the thermostat, the display will appear as shown in figure 1. OPERATION MANUAL 51M37 Merit® Series 5/2 Day Programmable Thermostat CONTROLS 505,050M 05/05 Supersedes 03/05 Table of Contents 51M37 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 HOLD − Temperature Hold Mode ................ 5 PROG − Thermostat Programming ............... 5 FAN − Controlling Fan Operation ................. 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 ........................ 8 Thermostat Output Table ....................... 10 | Initial Power−Up Display | | | | --- | --- | --- | | MO TU WE TH FR SA SU
AMPM I8:88
PROG A | B | C | D HOLD | |
| | ROOM
88° | SETPOINT
88 ° | | Figure 1 All display segments are momentarily activated. This occurs as a normal part of thermostat initialization. 05/05 􀀁􀀂􀀃􀀄􀀅􀀄􀀅􀀁 Page 1 505,050M 􀀁􀀃􀀅􀀄􀀅􀀄􀀅􀀄􀀆􀀁 Within a few seconds, the Home Screen appears (see figure 2) with default settings as shown. After about 1 minute of initialization time, the actual room temperature will be displayed. NOTE − Tables 2 and 3 on page 8 show all system and programming defaults. Home Screen | MO
AM
I2:00
HOLD | | | | --- | --- | --- | | | | FAN
AUTO | | ROOM
70 ° | SYSTEM
HEAT | | Figure 2 At this point, the thermostat will be fully functional; its default temperature setpoint (not shown) is 70°F. At this point, if the equipment has been fully powered and if a heat demand were present, the system would begin operating. NOTE − Temperature scale default is Fahrenheit units but may be reset to show Celsius, if desired. See page 8. Buttons, Backlight, Timers & Settings Buttons are located behind the small door on the right− hand side of the thermostat (see figure 3) . | IMPORTANT | | --- | | Do NOT begin pressing buttons until after you read the following section describing each button. | A pale blue display backlight illuminates for 30 seconds each time any button is pressed. When PROG or DAY/TIME is pressed, a field begins flashing, expecting another input. Start making changes within 15 seconds o

FAQs

What happens when power is first applied to the Lennox 51M37 thermostat?

The display segments activate as part of initialization. After a short time, the home screen appears with default settings, and about 1 minute later the actual room temperature is shown.

How do I set the day and time on the thermostat?

Press the DAY/TIME button and adjust hour, minutes, and day of week using the Up/Down arrows. Confirm by pressing DAY/TIME at each step; daylight savings adjustments can be stored by pressing ENTER.

What is the difference between normal heat mode and emergency heat mode?

In normal heat mode, both the heat pump and backup heat source can provide heat, with the backup activated when the heat pump cannot meet demand. In emergency heat mode, only the backup heat source provides heat and the heat pump is disabled, activating only upon a heat demand.

Oct 23, 2026

Uploaded by Jiven