Service Guidelines For Direct Vented Oilheat Systems

There has been a significant increase in direct vent installations in recent years. These systems have benefited our industry in many ways, such as eliminating the need for a chimney in new home construction, reducing installation costs, providing fresh outdoor air for combustion, enabling sealed combustion, reducing standby draft losses and reducing system noise.

Certainly, direct vent installations represent a major advancement in technology and appliance design. The systems may require special intake air piping, exhaust piping, vent terminals, clearances, controls and code requirements. Unique and challenging service issues can arise. For instance, service technicians have requested information regarding how to handle the impact on combustion triggered by cold outside air, cold oil and exhaust gas recirculation into the combustion air intake that can occur under certain conditions. These factors can be problematic because they create conditions leading to random primary control lockouts.

When dealing with random lockouts, there are several important factors that need to be carefully examined before making changes to the heating system or burner specifications. We have provided these guidelines to assist you in professionally diagnosing and handling these issues.

NOTICE: This guide is provided as supplemental information and is not intended to supersede the appliance manufacturer’s published specifications. Always follow the appliance manufacturer’s instructions, strictly adhere to applicable codes, standards, ordinances and all authorities having jurisdiction.

STEP 1 – Check Burner Specifications

Refer to appliance manufacturer’s specifications, burner installation manual, R.W. Beckett Residential Burner OEM Spec Guide part number 6711, or visit our website at Verify the burner is set to the specifications for the specific appliance, as follows:

  • The exact nozzle for the appliance is installed.
  • The electrode settings are correct. (See Figure 1)
  • The correct pump pressure level is set.
  • The head/air adjustments are correct and adjusted for proper combustion, as outlined next.

NOTE: The OEM Spec Guide air setting is the starting point. This setting must be adjusted to obtain the correct CO2 target.

Figure 1. – Electrode Settings

Burner Model & Heads Electrode Tip Gap Tip Height Above Nozzle Centerline Tip Forward of Nozzle Face
NX 5/32″ 1/4″ 3/32″
Standard AFG – (F, L, V), AF- F, AFII – FBX 5/32″ 5/16″ 1/16″
Alternate AFG – L & V heads 5/32″ 1/4″ 1/8″
AFII – HLX/AF2 1/8 – 5/32″ 1/4″ 3/32″

STEP 2 – Check Burner Combustion Levels

Using combustion test instruments, measure the following combustion outputs and verify they are adjusted to the appliance manufacturer’s requirements:

  • Carbon Dioxide (CO2) or Oxygen (O2) flue gas level. (Typically 11.5% CO2/5.3% O2 at zero smoke.)
  • Over-fire and stack draft in Inches W.C.

NOTE: If the lockouts persist after completing the above tests, consider the following:

STEP 3 – Check For Cold Oil Conditions

WARNING: Improper Installation of the Oil Supply System Could Cause Oil Leaks or Fire

Before altering the system, make sure that any changes comply with the pump manufacturer’s instructions, the latest editions of NFPA 31, CAN/CSA-B139 and CSA B-140 in Canada and all authorities having jurisdiction.

Review the existing oil tank location (inside or outside) and the piping system (whether it is a one-pipe system or a two-pipe system) to see if cold oil is a potential issue. Two-pipe systems permit very little increase in oil temperature, since the oil circulates through the pipes at the pump gearset capacity, which is typically 17 to 20 gph. It may be possible to change from a two-pipe system to a one-pipe system, which only moves the nozzle flow rate. This greatly reduced fl ow volume allows the oil more time to gradually warm to the
indoor air temperature.

Evaluate the oil supply system by making the following checks:

  • What is the vertical lift and horizontal run length of the oil pipe? (It must be within the pump manufacturer’s specifications).
  • What type of filter is being used? (It must be rated to filter 10 microns).
  • Is the pump pressure stable? (It should not vary more than 2 to 3 psig)
  • Measure system vacuum at the pump, following the pump manufacturer’s instructions.
  • The maximum vacuum for a single stage pump, one-pipe system is 6” Hg and 12” Hg for a two-pipe system.
  • The maximum vacuum for a two stage pump, two-pipe system is 17” Hg.

Tech Note: Fuel oil will boil or de-gas at high vacuum levels, creating bubbles that enter the nozzle line causing a smoky or unstable fire and after-drip.

STEP 4 – Check For Cold Intake Air Conditions

WARNING: Smoke, Fire and Asphyxiation Hazard

The vent system must comply with the appliance manufacturer’s specifications, the latest editions of NFPA 31, CAN/CSA-B139 and CSA B-140 in Canada and all authorities having jurisdiction.

Examine the vent system arrangement to check if cold air is a potential contributor to the random lockouts.

  • Verify the intake and exhaust lengths meet specifications and standards. Shorter intake lengths will not allow the incoming cold combustion air enough time to gain temperature from exposure to warmer building air before reaching the burner.
  • If possible, rearrange the intake piping to create a “U-shape”, which will act as a pressure break.
  • If the system allows, lengthen the incoming air run to the burner intake, allowing the air more time to pick up temperature. The total length and number of elbows cannot exceed the appliance manufacturer’s specification.
  • With any change to the intake piping, recheck combustion levels and make adjustments, if necessary.

STEP 5 – Exhaust Recirculation Into Air Intake

Review the installation of the vent terminals. Make sure the terminals are installed to comply with all specified and required clearances. Are there any obstructions or architecture that may impact recirculation of gases at the terminal? Cross-contamination of the flue gases with the incoming air can starve a fire (reduce oxygen content) and cause the burner control to go into lockout. It can also create soot/carbon build up.

  • Test for cross-contamination using a combustion analyzer that measures CO2 (Carbon Dioxide), or O2 (Oxygen) and CO (Carbon Monoxide). Drill a 1/4” hole in the intake pipe near the burner and insert the probe.
  • If a vacuum breaker is installed, the sample should be taken 4” to 6” away from the vacuum breaker, between the vacuum breaker and the air inlet terminal.
  • If there is CO2 or CO present, or if the O2 level is below 20.9%, there is contamination present.

STEP 6 – Check Cad Cell Flame Sighting Effectiveness

Check the cad cell to verify a clean lens surface, correct bracket alignment and proper flame sensing.

  • Measure the cad cell ohms with flame. If the measurement is above 2000 ohms (using a R7184 Series control), replace the cad cell with a new one.
  • For NX burners: Check the cad cell sighting by making sure the cad cell is aligned with the sighting holes in the nozzle line assembly (both the hole in the throttle cup and the hole in the head).
  • For NX burners: If there is no reduction in the cad cell resistance, make sure there is stable combustion at the manufacturer’s specifications.
  • For NX burners: If the combustion is satisfactory, replace the nozzle line assembly with an alternate construction unit equipped with a mica window in the throttle cup. To determine the correct nozzle line assembly for your burner, please provide the NX burner serial number and manufacturer’s appliance model number.

STEP 7 – Additional Measures – If Lockouts Persist

If the system installation and cad cell sighting appears to be satisfactory, but lockouts persist, there is still a chance that cold air or a combination of cold air and cold oil might be causing flame instability or a nonlight-off condition. Sometimes, raising the pump pressure and using a slightly smaller nozzle will improve oil droplet atomization and stabilize the combustion, as outlined below.

  • If the burner spec requires a pump pressure of 140 to 150 psig, replace the nozzle with one that is rated 0.10 (one tenth) gph below the original nozzle specification and increase the pump pressure to achieve the desired input rate for the appliance. Check combustion with instruments and observe the flame to ensure it does not impinge on any surface in the combustion chamber or heat exchanger.
    Example: A 0.75 gph nozzle at a 140 psig pump pressure provides 0.89 gph. Changing to a 0.65 gph nozzle and increasing the pump pressure to 175 psig will provide a fl ow rate of 0.86 gph. (See Figure 2.)
    Tech Note: The maximum rated pressure for CleanCut and most other pumps is 200 psig.
  • The temperature of the oil can be increased with the use of an oil line de-aerator. There are several products available, but for reference, the Tiger Loop™ is a typical oil de-aerator commonly used in our industry. This system allows the oil in the reservoir to circulate to the pump and back during the run cycle. The pump gearset friction and warm ambient air causes the oil temperature to increase slightly.
  • Another way to raise the oil temperature is by installing a Nozzle Line Heater/Start Helper, Beckett Part Number 51621. This device can assist with burner light off by maintaining consistent oil viscosity at start-up. For further details on the operation of the Beckett Start Helper see the Tech Bulletin dated August 15,1993, Part Number RWB 664839.


We hope these guidelines provide valuable assistance, as you service, troubleshoot, effectively diagnose and resolve direct vent system issues that may arise in your service routine.

If you have any questions, please contact Beckett Technical Service at 1-800-645-2876 in the USA. (1-800-665-6972 in Canada) Our hours are 8 AM to 5 PM Eastern Time.

Also, we invite you to visit our website at: Use Login: prof and Password: info, to enter our technical area. There are many features and downloads provided for service technicians to help stay up-to-date with our products and sharpen your troubleshooting skills.

Figure 2 – Pump Pressure/Nozzle Flow Rate
Nozzle flow rate U. S. gallons per hour of No. 2 fuel oil when pump pressure (psig) is:
Nozzle size
(rated at 100 psig)
125 psi 140 psi
(factory std.)
150 psi 175 psi 200 psi
0.40 0.45 0.47 0.49 0.53 0.56
0.50 0.56 0.59 0.61 0.66 0.71
0.60 0.67 0.71 0.74 0.79 0.85
0.65 0.73 0.77 0.80 0.86 0.92
0.75 0.84 0.89 0.92 0.99 1.06
0.85 0.95 1.01 1.04 1.13 1.20
0.90 1.01 1.07 1.10 1.19 1.27
1.00 1.12 1.18 1.23 1.32 1.41
1.10 1.23 1.30 1.35 1.46 1.56
1.20 1.34 1.42 1.47 1.59 1.70
1.25 1.39 1.48 1.53 1.65 1.77
1.35 1.51 1.60 1.65 1.79 1.91
1.50 1.68 1.77 1.84 1.98 2.12
1.65 1.84 1.95 2.02 2.18 2.33
1.75 1.96 2.07 2.14 2.32 2.48
2.00 2.24 2.37 2.45 2.65 2.83
2.25 2.52 2.66 2.76 2.98
2.50 2.80 2.96