Lennox Merit 10HPB Series

This document provides installation instructions for Lennox Merit® Series 10HPB heat pump outdoor units, ranging from 1-1/2 through 5 tons. It covers various aspects from initial setup to maintenance and troubleshooting.

Function Description:

The Lennox Merit® Series 10HPB outdoor units are heat pump systems designed for residential and light commercial applications. They are approved and warranted for installation with specially matched indoor coils, line sets, and refrigerant control systems as designated by Lennox. These units operate under a wide range of weather conditions, providing both cooling and heating functions. The system includes a defrost system to prevent ice buildup on the outdoor coil during heating operation in colder temperatures. An emergency heat function is also integrated, typically used during outdoor unit shutdown or when auxiliary electric heat is staged by outdoor thermostats.

Important Technical Specifications:

Model Range: 10HPB Series Units, 1-1/2 through 5 tons.
Refrigerant: HCFC-22. The unit is factory charged, and adjustments for varying line set lengths are provided (e.g., 2 ounces per 5 ft. for 5/16 in. liquid line, 3 ounces per 5 ft. for 3/8 in. liquid line).
Electrical: Units are approved for use only with copper conductors. Voltage supply must be within the range listed on the unit nameplate. 24V, Class II circuit connections are made in the low voltage junction box.
Defrost System: Includes a defrost thermostat and a defrost control board. The defrost thermostat senses 42°F (5.5°C) or cooler to initiate defrost timing and terminates defrost when the liquid line warms up to 70°F (21°C). The defrost control accumulates compressor run times at 30-, 60-, or 90-minute intervals (field-adjustable) before initiating defrost if the thermostat is closed. The maximum defrost period is 14 minutes.
Refrigerant Metering Device: 10HPB units are designed for use with check expansion valve systems only. Expansion valve kits (e.g., LB-85759F, LB-85759G) must be ordered separately. If installing a check expansion valve with an indoor coil that includes a fixed orifice, the orifice must be removed.
Pressure Switch Circuit: The defrost control incorporates a pressure switch circuit for an optional high-pressure switch, which can lock out the unit if activated.
Unit Dimensions:
- 10HPB18/24: 25 in. (635 mm) height, 24-1/4 in. (616 mm) width.
- 10HPB30: 29 in. (737 mm) height, 28-1/4 in. (718 mm) width.
- 10HPB36/42/48/60: 33 in. (838 mm) height, 32-1/4 in. (819 mm) width.
Line Set Connections:
- 10HPB18: 3/8 in. liquid, 5/8 in. vapor.
- 10HPB24/30: 3/8 in. liquid, 3/4 in. vapor.
- 10HPB36: 3/8 in. liquid, 3/4 in. vapor.
- 10HPB42/48: 3/8 in. liquid, 7/8 in. vapor.
- 10HPB60: 3/8 in. liquid, 1-1/8 in. vapor.
- Units are designed for line sets up to 50 feet (15 m).
Torque Requirements for Service Valves:
- Service valve cap: 8 ft.-lb. (11 NM)
- Sheet metal screws: 16 in.-lb. (2 NM)
- Machine screws #10: 28 in.-lb. (3 NM)
- Compressor bolts: 90 in.-lb. (10 NM)
- Gauge port seal cap: 8 ft.-lb. (11 NM)

Usage Features:

Installation Clearances: Minimum clearance of 24 inches (610 mm) between multiple units. Service clearance of 30 inches (762 mm) on one side adjacent to the control box, 36 inches (914 mm) on another side, 12 inches (304 mm) on one of the remaining two sides, and 6 inches (152 mm) on the final side. 48 inches (1219 mm) clearance required on top. Maximum soffit overhang is 36 inches (914 mm).
Slab Mounting: Top of slab should be high enough to prevent water collection, with a slope tolerance away from the building of 2 degrees or 2 inches per 5 feet (51 mm per 1524 mm).
Roof Mounting: Install a minimum of 6 inches (152 mm) above the roof surface. Locate above a load-bearing wall or area that can adequately support the unit. A wind barrier may be required if the coil cannot be mounted away from prevailing winter winds.
Refrigerant Piping: Proper isolation of refrigerant lines is crucial to prevent vibration and noise. Use either Lennox L15 series line sets or field-fabricated lines according to the Refrigerant Piping Guide (Corp. 9351-L9). Brazing connections require dry nitrogen purging to prevent oxidation and moisture.
Leak Testing: After line set connections, check for leaks using an electronic leak detector or halide. Pressurize the system with dry nitrogen (regulated to 1-2 psig) and a trace amount of HCFC-22 (max 2 ounces or 57g).
Evacuation: Evacuate the system to an absolute pressure of 23,000 microns (29.01 inches of mercury) to remove noncondensables and water vapor. A rapid rise in pressure indicates a leak. Final evacuation should reach below 500 microns (29.9 inches of mercury) within a 20-minute period.
Start-Up: If equipped with a crankcase heater, it should be energized 24 hours before start-up. Check fan rotation, wiring connections, and voltage supply. Open liquid and vapor line service valves after evacuation.
Charging: Unit is factory charged for a 15 ft. (4.6 m) line set. Adjust charge based on line set length.
- Weighing in the Charge (Outdoor Temp < 65°F / 18°C): Recover refrigerant, leak check, evacuate, then weigh in the nameplate charge.
- Subcooling Method (Outdoor Temp < 65°F / 18°C): Restrict airflow through the outdoor coil to achieve 200-250 psig (1379-1724 kPa) liquid pressure. Measure liquid line temperature and pressure, then calculate subcooling. Adjust charge to match specified subcooling values (e.g., 5°F for 10HPB18, 7°F for 10HPB24).
- Normal Operating Pressures / Approach Method (Outdoor Temp ≥ 65°F / 18°C): Compare stabilized pressures with provided tables. Adjust charge to match approach temperature values (liquid temp. minus ambient temp., e.g., 12°F for 10HPB18, 13°F for 10HPB24).
Emergency Heat: Activates when the room thermostat is placed in emergency heat mode, isolating the outdoor unit control circuit. It should be used if power has been off for over an hour and outdoor temperature is below 50°F (10°C) to allow the crankcase heater to prevent compressor slugging.

Maintenance Features:

General Maintenance: Performed by a qualified installer or service agency at the beginning of each cooling season.
Outdoor Unit:
- Clean and inspect outdoor coil (with power off).
- Outdoor unit fan motor is pre-lubricated and sealed (no further lubrication needed).
- Visually inspect all connecting lines, joints, and coils for oil leaks.
- Check all wiring for loose connections.
- Check for correct voltage and amp draw on outdoor fan motor.
- Inspect and clean drain holes in the coil compartment base.
- Gauge and check refrigerant charge if insufficient cooling is reported.
Indoor Coil:
- Clean coil if necessary.
- Check connecting lines, joints, and coil for oil leaks.
- Check condensate line and clean if necessary.
Indoor Unit:
- Clean or change filters.
- Adjust blower speed for cooling. Measure pressure drop over the coil to determine CFM.
- For belt drive blowers, check belt for wear and proper tension.
- Check all wiring for loose connections.
- Check for correct voltage and amp draw on blower motor.
Filter Drier: The unit is equipped with a biflow filter drier. Replace with one of like design if necessary.
Crankcase Heater: If equipped, ensure it is energized 24 hours before start-up to prevent compressor damage.
Safety: Contains warnings regarding fiberglass wool, electric shock, refrigerant inhalation, explosion hazards from using oxygen to pressurize the system, and equipment damage from deep vacuum operation. All installations and services must comply with national and local codes.