Waste Oil Furnace Secrets

This article explains the important points in converting a standard Beckett AFG oil burner to burn waste motor oil, used ATF, fryer grease or waste vegetable oil in your experimental furnace, heater or boiler. 

There are three main points that must be addressed to burn waste oil:

  • Preheat: The higher viscosity of waste oil requires it to be heated prior to entering the spray nozzle.
  • Nozzle: Despite preheating, waste oil requires a special air atomizing nozzle to spray well.
  • Flame Length: A waste oil flame tends to be longer than the original flame – often too long for the original combustion chamber.


Room temperature waste oil is too high in viscosity to atomize well or even flow easily through the system, so it must be pre-heated. The proper temperature varies greatly depending on the type of oil and its condition. While 160F may be adequate for waste motor oil, a temperature closer to 350F may be required for waste vegetable oil.

The first idea that occurs to many people is that they should simply heat the tank that holds their waste oil fuel. However, you may not realize just how slowly the fuel travels through the supply lines. For instance, if your fuel line is ¼” inside diameter and the fuel is flowing at 1gal/hr, it moves only about 1″ per second through the line. Every inch of fuel line between the tank and the nozzle means one second that cold, unburned fuel will be spraying into the chamber before it ignites. Once it’s finally burning, every inch is a second for the fuel to cool on its way to the nozzle. Prompt ignition is essential to minimizing maintenance and cleaning requirements.

In other words, the fuel must be preheated to its final temperature at or very close to the nozzle. This is not to say you should never heat the tank. If your fuel includes hydrogenated oil or animal fat that tends to solidify ambient temperature, you will need to warm the tank and supply lines enough to allow for the fuel to flow.

The final preheat is usually done with electrical heating elements regulated by some type of thermostat or temperature controller. The preaheater can be as simple as an aluminum block drilled with passages for the air and fuel. Cylindrical cartridge type heaters can be inserted into properly sized drilled holes. Preheaters have also been constructed from concentric pipes and heated with clamp-on band heaters.  There are two popular ways to implement the preheat – internal and external.

When using an external preheater, it is convenient thread a standard siphon nozzle adapter fitting onto use the existing “J-tube”. The J-tube was originally used to deliver fuel to the pressure nozzle and now it will carry the atomization air. A hole is drilled in the bottom of the blast tube and tubing threaded into the siphon nozzle adapter for the new fuel supply. A preheater is then attached to this tube directly below the blast tube. Because this now prevents the J-tube from being easily removed from the blast tube, you must drill a hole in the top of the blast tube to allow screwdriver access to the electrode clamp screw.

A completely internal preheater that takes the place of the J-tube can be constructed from an aluminum block that is machined to accept the siphon nozzle on one end, with ports for air and fuel at the other end. This has the advantage being as close as possible to the nozzle and heating the nozzle itself. This may allow a slightly lower temperature to be used than would be possible with an external preheater. The standard nozzle thread is 9/16-24 (extra fine thread).


Even when heated, waste oil fuel is difficult to atomize using a conventional pressure nozzle. Furthermore, the pressure nozzle has extremely fine openings that are clogged easily by impurities and deposits formed in the waste oil fuel. A solution to this problem is to use an air atomizing “siphon” nozzle. This type of nozzle works very much like an airbrush or paint spray gun, using compressed air to atomize the fuel.

Compressed air is fed around the periphery of the nozzle and fuel goes in through the center.  If you are using the manufacturer’s supplied adapter to hold the nozzle, the situation appears reversed. The air goes in the threaded port on the end opposite the nozzle and the fuel is fed through the port in the side

These air atomizing nozzles were developed for portable “torpedo” type shop heaters which use kerosene for fuel. Kerosene is lower in viscosity than even normal home heating oil and the air can actually siphon it up from a fuel tank several inches below the nozzle outlet. This does not usually work well for waste oil. Instead, the fuel must be supplied to the nozzle at a slight pressure and a higher than rated air pressure is also often used. These pressures typically fall in the range of 10-15 psi. Because of these differences, the performance specs given by the manufacturer do not apply. For instance, a siphon nozzle rated for 0.5 gal/hr can be used at an actual firing rate of 1 gal/hr.

Note that the compressed air is usually preheated along with the fuel.

Flame Length

The final obstacle is that a waste oil flame tends to be longer in length than a conventional flame. This partly due to the fact that waste oil takes longer to atomize, mix with air and combust than conventional fuel. The characteristics of the air atomizing nozzle, which tends to produce a long, narrow spray, only add to this problem. For this reason, the flame may impinge on the back wall of the combustion chamber. This situation can cause serious damage to the chamber, sometimes rapidly.

Perhaps the best solution is to use a furnace or boiler that already has a combustion chamber at least two feet long. Another solution is to fabricate an extension for a shorter combustion chamber. It must be properly insulated inside and out. Finally, switching to a “turbine” style retention head designed for commercial waste oil burners may shorten the flame enough to make a marginal situation work.  This part may be fabricated from sheet metal or purchased from commercial waste oil burner manufacturers such as Kagi or Econoheat as a spare part.

More Details

The amateur community seems to have “standardized” on a Beckett brand, AFG model oil burner “gun”. This is very common, easy to find model and parts are readily available. It is highly recommended that you obtain a spare unit to convert and leave your existing burner intact.

Compressed Air

If you already own an air compressor, this is the simplest solution. The air supply should be routed to an adjustable regulator where the pressure is maintained somewhere in the range of 10-15 psi. The air flow is turned on and off by a solenoid valve wired in parallel with the burner motor and ignition.

For quieter, more efficient operation and to reduce wear and tear on a large compressor, a small “medical” compressor can be used instead. These are quite expensive new but often found on eBay at a more reasonable cost. A high quality unit is required, as it must be able to continuously and last for many hours. If you use a piston type that produces a pulsating stream of air you may need to put a small tank in-line to smooth out the flow.

Fuel Feed

There are many ways to feed fuel to the nozzle. As mentioned before, fuel can be siphoned from a small reservoir below the nozzle, but this is often not successful. If it can be made to work, it does have the advantage of allowing the fuel to drain down from the nozzle when the burner is off, preventing nozzle drip.

If siphoning is not successful, the reservoir can be elevated above the nozzle. In this case, a solenoid valve must be used to stop the fuel flow when the burner is off. With either of these gravity regulated systems, the fuel level in the reservoir must be maintained at a constant level in order to ensure a consistent pressure head. This can be achieved with a float valve arrangement or by pumping an excess of fuel into the reservoir and allowing it to overflow and run back to the main tank. Obviously if using the overflow method, the burner must be elevated above the top of the main tank.

Another method is use the standard pressure pump but regulate the fuel pressure down to the required 10-15 psi.

Finally, perhaps the best solution is to us a positive displacement pump to meter the fuel to the nozzle at exactly the desired rate regardless of pressure and viscosity variations. This might be done using the standard pump driven at a slower than normal speed by a gear motor.

Combustion Air

The modified burner typically requires much less blower air than before. A standard accessory called a “low air baffle” from Beckett can be installed in the burner housing.

Back to HHO?

Once modified with a preheater and air-atomizing nozzle, what if I need to run on standard home heating oil again? For a long term change, simply re-install your original burner (you did buy a second burner to modify, right?). For the short term, simply disable the preheater and feed it HHO.

Safety & Legalities:

First, It is very important to preserve the function of the oil “primary” safety unit. This device monitors for the presence of the burner flame and shuts off the burner if a flame is not detected for a certain period of time, typically 30 seconds. This prevents unburned fuel from being pumped into the combustion chamber indefinitely in case the flame fails to ignite or goes out. The primary control’s alarm contacts can be used to open a relay in order to remove power from the preheater when the safety shut off is activated.

While there exist some commercial UL approved waste motor oil burners, they still may not be used legally in a single-family residence. The author is not aware of any waste vegetable oil burners that are approved for any application. It is extremely unlikely that any amateur built experimental burner would meet building codes and other regulations. It should be assumed that the presence of such a burner would void insurance coverage in the case of a fire, even if the burner was not the cause.

One possible way to increase safety would be to install the experimental burner outside of the structure being heated, possibly in a small shed or outbuilding. It would be prudent to ensure that all wiring and plumbing is neat and professional in appearance and done in accordance with good practices.

Naturally, no part of this document may be construed as legal advice as the author is not qualified to make any determination on these matters.