I’m going to open this article with the very best piece of oil boiler advice I can give: Always check for oil leaks upon arrival and departure! Having said that, while oil burner technology has certainly progressed since the introduction of the power oil burner, some of the basic premises, which are critical to clean and reliable operation, have not. 

The most rudimentary yet critical step is to ensure an ample supply of clean oil. While this appears to be a simple concept, it’s one our tech support teams deal with frequently.

Examining the oil delivery system 

For clarity, it’s easiest to tackle this topic in stages. When on a no-heat call, the problem may be related to one of several different systems. One of these is the oil delivery system. 

First, we want to check oil delivery to the nozzle assembly. To do so, remove the nut on the high pressure (nozzle) line at the nozzle assembly. Divert the line into a container by either loosening the nut at the pump and spinning the line or attaching a hose to either the nozzle line or nozzle line port on the pump. 

Reset the boiler and monitor the discharge from the oil line after any solenoids open (if present). If the oil discharge is free of air, the problem is likely elsewhere in the burner or heating unit. If the oil flow is sporadic, foamy, or non-existent, this is a good indication of a restriction in the oil system, failed or slipping pump coupling, bad pump, solenoid, primary control solenoid output, or possibly an out-of-oil condition. 

If the burner motor runs, but no oil is present at the nozzle line during ignition, check for an empty oil tank, bad solenoid, broken or slipping pump coupling, air bound or restricted oil line, bad primary, or possibly a bad pump.

Once it’s determined that the issue is related to the oil supply, we first need to analyze the type of system we have. Is the oil line feeding the pump in a vacuum or gravity application? If it’s a gravity system, check the oil level in the tank. Also confirm the gauge is accurate; the gauge can stick and indicate oil is present when the tank is actually empty. 

Move on to the filter

If the tank has adequate oil, move on to the filter.

Assuming the system is a gravity feed type, close the firomatic or standard service valve at the bottom of the tank or inlet of the filter. 

Remove the filter from the filter canister. Open the valve between the tank and filter head. Oil should flow freely from the filter head. The higher the oil level in the tank, the more flow should be present at the filter head. 

A full tank with barely a trickle of flow at the filter head indicates a restriction in the tank, tank valve or filter head inlet. It is preferable but not always possible to use a push/pull pump to draw the blockage out of the tank with suction. 

Regardless of method, take steps to resolve the issue if this is the case. The above assumes the tech is using a bypass key if an oil safety valve is present.

The next step is up to your personal preference, but this is a way to confirm all elements of the system are clear.

I recommend installing a new filter, but not purging the air at the filter. On a pump with a removable strainer, (Suntec, Danfoss, Riello, etc.) remove the pump cover. On a pump with no strainer, (Webster or similar) remove the oil line from the pump and drain into a container.

If that’s not possible, attach a hose to the oil line and allow the hose to drain into a container. A 3/8-inch male flare x ¼-inch NPT fitting (½-inch flare x ¼-inch male if ½-inch line) with a hose connected to the threaded end will make this quick, neat and easy.

If working on a gravity feed system, turn on the oil valve at the tank. Check for OSV valves. These devices are designed to prevent oil spillage in the event of a line leak or pump seal failure.

If an oil safety valve is present insert the bypass key. Allow the air bubble from the filter to pass through the line and vent through the pump cover/oil line. Remember, gravity is working its magic here. The oil level in the tank must be higher than the discharge point. This can be an issue on highboy-style hot air furnaces with low oil tank levels, as the burner is at a higher elevation than on most boilers.  

If the oil flow at the pump is reduced excessively from the flow at the filter head, the oil line is restricted and must be cleared or replaced.

Check with your service manager or insurance company on oil line protocol. Restrictions may apply to using CO₂ charges to clear lines. 

Always pay attention to what equipment is attached to the oil lines. If the oil line serves multiple units, always isolate the other units from the oil line when cleaning. Do not trust the fire valve at the pump for isolation. More than a few pumps have gone to an early grave because the seal was damaged during oil line service.

A sure-fire way to protect attached pumps is to remove the pump cover when flushing the lines or remove the oil line and plug it with a flare plug. If removing the pump cover, close the firomatic valve fully and tap on the stem to fully seat. Wrap a rag around the cover and place a container beneath just in case the fire valve does not fully close. If you don’t take preventative steps and the valve is leaking during flushing it can get quite messy. Ask me how I know! Avoid using CO₂ or any high-pressure method to clear oil lines that are buried.

Once the oil line is confirmed to be clear, reassemble the system, installing a new strainer and gasket. Purge the pump of any air using the bleed port. If the pump coupling is intact, solenoid (if present) is functional and energized during ignition, and the oil system is in order, the next step is replacing the pump if the issue persists.

Two pipe/vacuum systems

When dealing with a two pipe, or suction, system, most of the above items are still relevant, but we have a few more things to check. 

Air leaks can be problematic and can cause atomization issues, oil leaking from the nozzle during the off cycle, and contribute to early pump failure. In addition to the items mentioned with gravity systems, it’s important to check for air or restrictions. Most often a leak in a suction system will leak air in, as opposed to a gravity system which will leak oil out.

An “oil watcher” is a clear plastic tube with a vacuum gage attached. This device typically features 3/8-inch flare fittings; a male on one end and female on the other. For larger oil lines, 3/8-inch x ½-inch adaptors are normally included. This tool can be used for checking air leaks or restrictions in the oil line, and when combined with a valve, it may also be used to check the pump, including the shaft seal, for leaks. 

Regarding pump seals, a visual inspection of the area around the pump shaft may reveal a pump seal issue.

If the area around the pump seal is wet with oil, this may indicate a seal leak.

To be sure, fill the pump and oil line with oil. Confirm fittings are tight with no leaks. With the oil watcher in place, close the valve on the inlet on the upstream end of the oil watcher. Start the boiler. The pump should develop a certain level of vacuum and maintain the level after the burner shuts off. (Check pump manufacturer’s specs for expected vacuum level and allowable deterioration rate) If the vacuum slowly dissipates, you either have a fitting leak or bad pump seal.

With the burner running, check for excessive vacuum or air bubbles at the pump inlet.

No. 2 oil can begin outgassing at vacuum levels as low as 6-inch wc. Excessive vacuum can be caused by oil line/ tank/filter blockages, jammed check or foot valves on old inground tanks or undersized lines/excessive line length. Air can be introduced from leaking fittings or by outgassing.

Calculate your expected vacuum. The old rule of thumb was 1-inch for every 1-foot of vertical rise not offset by a corresponding drop at the boiler, and 1-inch for every 10-feet of horizontal run for 3/8-inch oil line, plus additional loss for filters, fittings, etc. OSV’s can be as high as 3-inch wc. Divide by half for ½-inch oil line.

After diagnosing the oil system and resolving any issues, the final step is to install a new nozzle based on manufacturer’s recommendations.

Be aware that a nozzle’s flow rating is only valid at 100 psi. Many burners run higher nozzle pressures today, some as high as 180 psi. Service the electrode assembly, gap electrodes correctly and check the pump discharge pressure on the nozzle line. Confirm the nozzle line pressure meets the manufacturer’s specifications and that the pump cut-off is holding after shut-down. 

Give careful attention to potential gelling in outside tanks in areas subject to extremely low temperature, and be sure to check all devices and fittings for oil leaks prior to leaving.

If the pump must be replaced, be sure the replacement unit meets or exceeds the original specs or site requirements.

Most (if not all) pumps ship with the bypass plug OUT, and as such will install directly into a single line system. If you have a two-line system and forget to install the bypass plug, this is known in our multicultural industry as “No Bueno.” If you have a single pipe system and install the bypass plug, this is known as “Super-Extra No Bueno” and will necessitate a return to the supply house for another pump, as the second you turn it on, the pump seal will blow out. Always set the pump discharge pressure to the manufacturer’s specs.

A minimum basic oil service tool kit should consist of an “oil watcher” with vacuum gauge, pump pressure and cut off tester, push/pull gun, and miscellaneous tubes, hoses and fittings.

The final step 

After repairs are complete, always check for leaks!

Remember the old adage, “Leaks take weeks.” They may also cause tens to hundreds of thousands of dollars in damage if site contamination occurs. Wipe every line and fitting dry as you complete a task and be sure it’s still dry when you leave. Finally, after performing these tests, if you’re still unable to isolate the issue, consider contacting your local rep or corresponding manufacturer’s tech support.

In closing, while this article is comprehensive, it is by no means exhaustive. All major residential oil burners are substantially similar in function. But in form, each has their idiosyncrasies. Fortunately, manufacturers offer excellent training both in person and online.

Please feel free to reach out to me at [email protected] with any questions or comments. And finally, always check for leaks before and after your work!! I wish you all the best in the coming year.

Tom Secondino, residential products technical/application team leader, U.S. Boiler Co., has been in the mechanical trades for more than 40 years.