Lennox OF25 Series Oil Furnace Installation Instructions

General Information

These instructions provide a general guide for the installation and servicing of Lennox Elite® Series OF25 oil furnaces. They do not supersede local codes. Installation and service must be performed by qualified technicians. In Canada, refer to CSA B139 for recommended installation procedures. Always consult authorities having jurisdiction before installation.

⚠️ WARNING: Do not store or use gasoline or other flammable vapors and liquids in the vicinity of this or any other appliance.

⚠️ CAUTION: Never burn garbage or paper in the heating system. Never leave papers near or around the unit.

Shipping & Packing List

• 1 - Assembled oil furnace
• 1 - Barometric draft control
• 1 - Bypass plug (used on two oil line lift systems)
• 1 - Side filter adapter kit (includes filter)
• 1 - Return line adapter

Check components for shipping damage. If damage is found, immediately contact the last carrier.

OF25 Unit Dimensions

Diagrams show the following views and dimensions:

• Top View: Shows return air opening (16" x 18"), supply air opening (18" x 21"), flue connection (3/4"), and top flue outlet. Overall dimensions are approximately 52-1/8" wide.
• Side View: Shows overall height of 37", depth of 19-1/2". Includes flue connection (3/4"), electrical inlets (Right Side Only), oil piping inlet (Both Sides), and side flue outlet centering hole (field fabricate either side). Optional outdoor combustion air inlet centering hole (field fabricate right side only).
• Front View: Shows overall height of 37" and depth of 19-1/2".

OF25 Unit Parts Arrangement

Figure 1: Unit Components

• Heat exchanger
• Flue opening
• Clean-out port
• Limit switch
• Riello burner
• Indoor blower
• Blower motor
• Observation/sampling port
• Combustion air intake

Figure 2: OF25 Riello Oil Burner Parts Arrangement

• Lockout Indicator Lamp & Reset Button
• Primary Control
• Primary Control Sub-Base
• Pump Pressure Regulator
• Capacitor
• Motor
• Turbulator Adjustment Screw
• Air Tube Cover
• Coil
• Combustion Head
• Air Adjustment Fixing Screw
• Capillary Tube
• Supply Fuel Line Port
• Return Fuel Line Port
• Pressure Tube & Gauge Port
• Vacuum Gauge Port

Locate & Level the Unit

Choose a central location for the furnace to ensure balanced heat distribution. The furnace should be placed to minimize flue connection length and elbows. Ensure adequate clearances as per Table 2 and for servicing. Level the unit using shims if necessary, ensuring weight is evenly distributed on all four corners to prevent cabinet strain.

⚠️ WARNING: If the unit is installed where oil damage may occur, a drain pan must be installed. The drain pan and its piping must be sized to prevent oil damage and route oil back to the tank.

Unit Set-Up

Input Selection

1. Determine the desired furnace firing rate using Table 1 to select the proper nozzle and pump pressure. Replace the nozzle if necessary for higher input rates.
2. Remove the oil burner cover.
3. Locate the drawer assembly. Loosen the screw holding the control box and carefully remove it by pulling back and then up (See Figure 3).
4. Loosen the screw securing the air tube cover plate and remove it.
5. Loosen the screw securing the drawer assembly and slide it out of the combustion head (See Figure 3).
6. Remove the screw shown in Figure 4 and slide the nozzle adapter from the drawer assembly.
7. Insert the proper nozzle into the adapter and tighten securely, but do not overtighten.
8. Reverse steps to re-assemble the burner.

Figure 3: Drawer Assembly Removal Depicts the removal of the control box and drawer assembly from the combustion head, showing screws and component locations.

Figure 4: Nozzle Replacement Illustrates the removal of the nozzle adapter from the drawer assembly and the insertion of a new nozzle.

Table 1: Burner Set-Up

¹ Denotes factory default burner configuration.
² Denotes optional firing rate and burner configuration.
³ Burner is pre-set for single-line gravity-fed oil supply.

Electrode Gap Verification

Figure 5: Electrode Gap Shows critical dimensions for electrode gap verification: 5/32" (4mm) and 5/64" to 7/64" (2 to 2.5mm). Also shows a 13/64" (5mm) dimension.

Turbulator Setting

Figure 6: Turbulator Setting Illustrates adjusting the turbulator screw until the index marker aligns with the desired input setting (e.g., '2' on a scale of 4 to 0). Retighten the retaining nut.

Air Adjustment Plate Positioning

Figure 7: Air Adjustment Plate Position Shows the air adjustment plate with a setting indicator. Loosen screws, align the desired value from Table 1 with the indicator, and retighten screws. Fine-tuning may be needed at start-up.

Requirements

⚠️ WARNING: Product contains fiberglass wool. Disturbing insulation can expose you to fiberglass wool dust, which may cause lung cancer (known carcinogen in California) and respiratory, skin, and eye irritation. Consult Material Safety Data Sheets or supervisor for more information. Contact: Lennox Industries Inc., P.O. Box 799900, Dallas, TX 75379-9900.

⚠️ WARNING: Improper installation, adjustment, alteration, service, or maintenance can cause property damage, personal injury, or loss of life. Must be performed by a qualified installer or service agency.

Installation must conform to NFPA No. 31, National Electrical Code (ANSI/NFPA No.70), CSA Standard CAN/CSA-B139, Canadian Electrical Code Part 1 (CSA 22.1), and recommendations from NECA. Consult local authorities. Chimneys and connectors must meet NFPA No. 31 section 160 requirements.

Clearances: Unit is approved for combustible flooring. Refer to unit rating plate and Table 2 for clearances to combustible material. Service and accessibility clearances take precedence over fire protection clearances.

Table 2: Installation Clearances (inches/mm)

*Service access clearance of 24" in front is required for burner service.
**A passage for a large person between furnace and chimney is needed for flue collector service. 24" clearance at rear and one side for blower service and cleaning.
**Minimum flue pipe clearance may be reduced with special protection per local codes.

Temperature Rise

Unit must be adjusted for the temperature rise range listed on the unit rating plate. Ensure the furnace is electrically grounded per local codes or the National Electric Code (NEC)/Canadian Electric Code (CEC).

Field Wiring

Field wiring connections must meet or exceed Type T wire specifications and withstand a 63°F (17°C) temperature rise.

Cooling Applications

If used with cooling units, install in parallel or upstream of cooling units to avoid condensation. Use a damper to control airflow and prevent chilled air entry. Ensure units cannot operate unless damper is in the correct position.

Combustion and Ventilation Air (Confined and Unconfined Spaces)

Modern, tightly constructed homes require dedicated outdoor air for combustion. Exhaust fans, vents, chimneys, and fireplaces can remove air needed for combustion, potentially causing dangerous downdrafts and carbon monoxide issues. Users may inadvertently block fresh air intakes.

Combustion Air Requirements

⚠️ CAUTION: Insufficient combustion air can cause headaches, nausea, dizziness, asphyxiation, excess water in the heat exchanger (leading to rusting and premature failure), and property damage.

All oil-fired appliances need combustion air. Insufficient air leads to inefficient, unsafe operation, sooting, and potential downdrafts. Requirements depend on whether the space is confined or unconfined.

Combustion air must conform to NFPA No. 31 or CSA B139. In confined spaces (e.g., utility rooms), two combustion air openings are required, one within 12" of the ceiling and one within 12" of the floor (See Table 3).

❗ IMPORTANT: An opening to the outside for combustion air is strongly recommended, especially in new homes. Refer to Table 2 or the unit rating plate for specific dimensions.

Table 3: Combustion Air Opening Dimensions (2 required)

Unconfined Space

An area greater than 50 cubic feet per 1,000 Btu/hr of combined input rating of all appliances. Includes adjacent rooms not separated by a door. Outdoor air may be needed if infiltration is insufficient, especially in tight construction.

Confined Space

An area less than 50 cubic feet per 1,000 Btu/hr of combined input rating. Includes furnace closets or small equipment rooms. If supply ducts carry air outside the space, return air must be sealed and terminate outside the space. Leaks can cause dangerous negative pressure.

Air from an Adjacent Space

Figure 8: Equipment in Confined Space - All Air from Inside Depicts a confined space with two openings to an adjacent room for air intake. Each opening requires a minimum free area of 1 sq. inch per 1,000 Btu/hr, but not less than 100 sq. inches.

Air from Outside

Figure 9: Equipment in Confined Space - All Air from Outside Shows a confined space drawing air from a crawl space and exhausting to an attic. Requires two permanent openings (top and bottom) communicating with the outdoors. Each opening needs a minimum free area of 1 sq. inch per 4,000 Btu/hr (or 1 sq. inch per 2,000 Btu/hr if using horizontal ducts).

Figure 10: Equipment in Confined Space - All Air from Outside (All Air Through Ventilated Attic) Illustrates a setup where air is drawn from a ventilated attic.

Venting

⚠️ WARNING: Keep combustion air openings clear of obstructions to prevent improper burner operation and fire hazards.

⚠️ WARNING: The barometric damper must be in the same atmospheric pressure zone as the combustion air inlet to prevent improper burner operation and fire hazards.

⚠️ CAUTION: Do not store combustible materials near the furnace or supply air ducts, as they may ignite spontaneously.

⚠️ WARNING: This furnace is certified for type “L” vent. Type “B” vent must not be used with oil furnaces. Type “L” vent is similar to type “B” gas vent but has a higher temperature rating and a stainless steel inner liner.

Ducts used for combustion air must have a cross-sectional area matching the openings. Minimum rectangular duct dimension is 3" (76 mm). Consider the blocking effect of louvers/grilles (20-25% free area for wood, 60-75% for metal). Louvers must be fixed open or interlocked with equipment.

A return air duct system is recommended, sealed to prevent air circulation from the confined space. Minimum mixed return air temperature of 60°F to 65°F is required.

Venting Details

Inspect the chimney for repairs before installation. Ensure it is properly constructed and sized per NFPA standards. The chimney's smallest dimension must be at least equal to the furnace vent connector diameter. A draft of at least .04" w.c. (9.9 Pa) is required.

Local building codes may have stricter requirements. Vent connectors should be as short as possible and not smaller than the furnace outlet diameter. Use at least 24-gauge galvanized pipe. Single-wall vent pipe should not run outside or through unconditioned space.

Chimneys must extend 3 ft (0.9 m) above flat roofs within 10 ft (3 m) and 2 ft (0.6 m) above the highest roof peak within 10 ft (3 m). Vent must not pass through floors or ceilings. Clearances to single-wall vent pipe should be at least 6" (152 mm).

Figure 12: Wall Thimble Shows a wall thimble for passing vent pipe through a combustible wall.

Vent pipe should slope upward towards the chimney at least 1/4 inch (6 mm) per foot (0.3 m) and be supported independently. Extend vent pipe flush with the chimney liner and seal the joint.

Furnaces must connect to factory-built chimneys or masonry chimneys lined with acceptable material. If multiple appliances vent into a common vent, its area must be at least the largest vent area plus 50% of additional vent areas. The vent pipe shall not connect to a chimney serving a solid fuel appliance or mechanical draft system. Close all unused chimney openings.

Figure 13: Venting Configurations Illustrates various venting setups: Front/Rear Flue with Factory-Built Chimney and Masonry Chimney, including condensate drains.

Figure 14: Horizontal Venting Shows configurations for horizontal venting using approved mechanical vent systems (Tjernlund, Field Controls). Maximum permissible vent length is 100 equivalent feet, minimum 15 feet. Elbows add to equivalent length (90° = 10 ft, 45° = 5 ft).

Figure 15: Horizontal Venting Diagrams Depicts top and rear flue horizontal venting with barometric control placement.

Vent Connector (Smoke Pipe) and Draft Control

❗ IMPORTANT: If smoke pipe is installed at less than minimum clearance (Table 2), radiation shields (Figure 16) must be installed.

Use 24-gauge or heavier galvanized pipe. Connect to chimney with minimal elbows. Insert pipe into but not beyond the chimney flue wall. Ensure common flue area is adequate for multiple appliances. Install barometric draft control as instructed. Inspect flue pipe annually.

Figure 16: Radiation Shield Installation Details the installation of radiation shields, specifying minimum dimensions and when they are required (when distance 'A' is less than 9").

Removal of Unit from Common Venting System: If removing a furnace from a shared venting system, test the remaining appliances for proper venting. Seal unused openings. Inspect the system for blockages, leaks, or corrosion. Close building openings and operate other appliances to simulate worst-case venting conditions.

Supply & Return Air Plenums

Secure the return air plenum with sheet metal screws. Use fiberglass sealing strips for the supply air plenum. Supply and return air ducts must match the furnace opening dimensions and be at least 18" long.

Oil Supply Piping

Use continuous heavy wall copper tubing or steel pipe. Install piping under floors or near walls for protection. Avoid running pipes along joists or reverberating surfaces. Use flare fittings; all fittings must be accessible. Do not use compression fittings.

Manual bleeding of the fuel pump is required on start-up to prevent air locks. Cycle the heating on and off ten times after bleeding to eliminate air in the gun assembly.

⚠️ WARNING: Do not use pipe dope or Teflon tape on any direct oil connection to the fuel pump.

⚠️ WARNING: Operating vacuum should not exceed 11.44 inches of mercury.

Figure 17: Oil Pump Diagram Shows the Riello oil pump with ports labeled: Pump Cover, Pump Nozzle Port, O-Ring, Supply Line Port, Bypass Plug, Adapter with Two Female 1/4" NPT, Vacuum Port, Pressure Port, Bleeder Valve, Return Line Port, Return Line Adapter, and Supply Line Adapter.

Oil Filter

An oil filter is required, installed indoors between the tank shut-off valve and the burner, at least 3 feet from the burner to avoid pressure drop. Table 4 lists available filters.

Table 4: Oil Filters

Piping System

The OF25 unit is shipped for a single-line system. Ensure piping system restrictions and lift do not exceed pump capability. Two pipe connectors and NPT adapters are supplied. Pump port threads are British parallel thread; NPT threads can damage the pump body. Riello gauges connect directly; metric NPT adapters are needed for other gauges.

Single-Pipe Systems

⚠️ CAUTION: Do not install the bypass plug into the pump pressure port on single-line systems, as it will damage the pump shaft seal.

Size oil piping per Table 5 and Figure 18. Connect pipe to the factory-installed adapter on the pump supply port. Ensure the return port plug is tight.

Table 5: Single-Pipe Sizing

Figure 18: Oil Piping Single-Pipe System Diagram showing a single-pipe oil supply system with maximum height and tubing length considerations.

Two-Pipe Systems

Use the provided 2.5mm hex key to install the bypass plug in the pump return port. Install the NPT adapter on the pump return port. Size piping per Table 6 and Figure 19. Supply and return piping must be the same diameter and extend to the same depth in the fuel tank.

Table 6: Two-Pipe Sizing

Figure 19: Oil Piping Two-Pipe System Diagram showing a two-pipe oil supply system with maximum height and tubing length considerations.

❗ IMPORTANT: Both oil supply and return pipes must be submerged in oil in the supply tank.

Leak Check

After oil piping is completed, check all connections for oil leaks.

Electrical Wiring

All wiring must conform to the National Electric Code (NEC) or Canadian Electric Code (CEC) and local codes. Refer to Figure 20 for wiring diagrams.

1. Refer to the appliance rating plate for proper fuse size.
2. Install the room thermostat and connect wires to the unit. Avoid installing the thermostat on an outside wall or where it can be affected by radiant heat. Set the adjustable heat anticipator according to the wiring diagram sticker on the unit.
3. Install a separate fused disconnect switch near the unit for servicing.
4. Complete line voltage wiring from the disconnect switch to the make-up box.

An equipment ground screw is provided. Ground the unit using a suitable ground wire.

Figure 20: Typical OF25 Wiring Diagram This diagram illustrates the electrical connections for the OF25 furnace, including line voltage, low voltage, thermostat, fan/limit switch, transformer/relay, blower motor, and Riello oil burner connections. It also includes a chart for lowest permissible heating speeds based on nozzle size (GPH) for upflow, counterflow/horizontal, and basement installations, and a motor lead color chart.

Pump Purging

Ensure the oil tank is adequately filled with clean No. 2 furnace oil before starting. Contaminants in the oil supply can cause fuel pump malfunction.

Single-Pipe Systems

1. Loosen the pump bleeder valve until oil flows out. If no oil flows, re-tighten and add oil through the vacuum port (use 4mm hex key).
2. Disconnect the nozzle oil supply line at the pump nozzle port.
3. Attach a flexible plastic tube to the pump nozzle port and direct the other end into a bucket to catch oil.
4. Loosen the screw securing the air tube cover.
5. Hold the air tube cover in position and start the burner.
6. Allow the solenoid valve to engage (approx. 10 seconds after burner fires). Remove the air tube cover and shine a light source on the photo-cell; it must sense light.
7. Run the burner until the fuel pump is purged of air, then tighten the bleeder valve and shut down the burner.
8. Reinstall the air tube cover and nozzle line.
9. Start the burner normally.

⚠️ WARNING: Omitting steps 2 or 3 can lead to unburned oil accumulation in the combustion chamber, creating a hazardous condition.

Two-Pipe Systems

1. Turn off main power to the burner and remove the air tube cover.
2. Shine a light source on the photo-cell. The solenoid valve will not open, and the burner will remain in pre-purge mode as long as the light is present.
3. Restore power and fire the burner.
4. Run the burner until the fuel pump is purged of air.
5. Replace the air tube cover.
6. Restore power and start the burner normally.

Unit Start-Up & Adjustments

⚠️ CAUTION: Never burn garbage or paper in the heating system. Never leave papers near or around the unit.

⚠️ CAUTION: Blower door and all burner covers must be in place before start-up.

⚠️ WARNING: Do not start the burner if excess oil has accumulated in the chamber, if the furnace is full of vapor, or if the combustion chamber is very hot. This could result in property damage, personal injury, or death.

1. Set thermostat for heating demand and turn on electrical supply.
2. If the burner fails to start, push the reset button on the primary safety control and the burner motor reset button.
3. ⚠️ CAUTION: Do not push the primary control reset button more than once.
4. If the burner still fails to light, refer to the troubleshooting section.

Fuel Pump Pressure

Use an 11mm wrench to remove the bleeder port tubing adapter. Attach a gauge to the bleeder port. Turn on the unit and measure fuel pump pressure. Compare to Table 1 and adjust if necessary.

Vacuum Pressure

Use a 4mm hex key to remove the vacuum port cover. Attach a gauge to the bleeder port. Start the burner and measure operating vacuum. It should not exceed 11.44 inches w.c.

Temperature Rise

Place thermometers in warm air and return air plenums. Locate the warm air thermometer where it won't see the heat exchanger directly. Set the thermostat to its highest setting. After readings stabilize, subtract return air temp from supply air temp. The difference should match the temperature rise range on the unit rating plate. Adjust blower motor pulley to change blower speed if needed.

Fan and Limit Control

The fan and limit control is factory installed and wired.

Burner Adjustment

⚠️ WARNING: Service must be performed by a qualified installer or service agency.

Proper adjustment ensures quiet, clean fire, preventing sooting. Use a CO2 analyzer and smoke gun. Do not exceed #0 smoke for flexibility.

1. Punch a 5/16" diameter service hole in the flue outlet, at least two flue diameters above the breeching.
2. Place burner in operation. Adjust air setting by visual observation until a stable, predominantly yellow-white flame is achieved, contained within the combustion chamber. If the flame turns orange or rises, adjust air. Run for at least 10 minutes.
3. Take a draft reading. Adjust barometric draft control for an overfire draft of 0" to -.01" and a breach draft of -.02" to -.04".
4. Use a smoke tester. If smoke is evident, check nozzle, combustion setting, oil, or installation.
5. If producing more than #0 smoke, loosen the air adjustment plate screw and rotate it until the flame is clean. Retighten the screw. Adjust turbulator settings slightly (higher number) if needed.
6. Reset draft if combustion head or air settings were adjusted.
7. Use an emissions test instrument to measure CO2 in the stack (should be 11-12%).
8. Obtain flue gas temperature at the service hole.
9. Adjust air plate if necessary for the desired operating range.
10. When proper combustion and smoke readings are achieved, tighten the air adjustment plate screw.
11. Re-secure the burner cover and re-check draft, smoke, combustion, and flue gas temperature.

Service

⚠️ CAUTION: Do not tamper with unit controls. Call a qualified service technician.

Filter

Ensure access panels are in place. Check and replace filters monthly with like kind and size filters for proper operation.

Blower

Blower motor is prelubricated and sealed. No lubrication required. Remove blower assembly for cleaning if necessary. Disconnect power before checking for debris.

Flue Pipe

Inspect annually. Clean soot/ash. Inspect for holes or rust. Replace with the same size and type as required by code. Inspect and replace flue draft control device if defective.

Burner

Check nozzle and oil filter before each heating season. Close oil supply line shut-off valve when unit is shut down for extended periods.

Cleaning the Heat Exchanger

1. Remove vent pipe.
2. Loosen clean-out cap screw, rotate cap to notch, remove cap, and remove flue access elbow.
3. Use a spiral wire brush to sweep the outer drum of the heat exchanger. Vacuum loose debris.
4. Loosen observation port cap screw, rotate cap to notch, remove cap. Use a spiral wire brush to reach the rear of the heat exchanger and clean the crossover tube. Replace cap and screw.
5. Replace clean-out caps and flue access elbow. Ensure locking screws are secure.
6. Brush and vacuum the vent outlet area and replace the vent pipe.
7. Clean the burner, blower deck, and vestibule area.

NOTE: A heat exchanger brush ABRSH380 (35K09) is available from Lennox.

⚠️ CAUTION: Do not attempt to clean the combustion chamber; it can be easily damaged.

OF25 Start-Up & Performance Check List

OIL FURNACE START-UP CHECKLIST (Complete and keep for future reference)

• Customer Name: _________________________
• Address: ________________________________________________________________________________
• Furnace Model #: _________________________ Serial #: ___________________
• Input Rate: _____________________________ Nozzle Used: _______________
• New Construction: _______________________ Replacement: ______________
• Date of Installation: _____________________________

Start-Up Procedure

1. A. Close disconnect switch _______
2. B. Set thermostat to call for heat _______
3. C. Bleed air from lines and pump; run for 20 seconds after bubbles disappear _______
4. D. Install vacuum gauge; check pump vacuum _______
5. E. Install pressure gauge; adjust pressure to 140 psig. Always verify proper pump pressure per tables with unit instructions.
6. F. After 10 minutes of operation, obtain flue temperature reading: 1st ___________ 2nd ____________ 3rd _____________
7. G. Obtain smoke reading: 1st ____________ 2nd ______________ 3rd _____________
8. H. Measure CO2: 1st __________ 2nd ____________ 3rd __________
9. I. Check draft: overfire _______ Breech _______
10. J. Air shutter setting __________ Locked _______
11. K. Measure static pressure in duct system: Static pressure on supply side _______, Static pressure on return side ________, Static pressure drop _______
12. L. Temperature rise after steady state conditions: Supply side ____, Return side ____
13. M. Block off return air (limit control checkout); burner should shut down in 2 or 3 minutes _______

Owner Record

• Installed By: __________________________________
• Dealer: ____________________________________
• Address: ____________________________________ ____________________________________ ____________________________________
• Telephone #: __________________________________
• License #: ____________________________________