Make The Right Draft Choice
On the golf course it was obvious. My tee shot was aimed for the fairway, but I hit a trick shot into the woods. Trick shot my foot. I should’ve just put the driver back in my bag.
Now human nature kicks in. Rather than find out what’s wrong with my swing, I start the quest for a club that “works.” Sound familiar? That might make sense if the only goal is immediate results. But what if the goal is to solve the problem? Then there has to be an understanding of the dynamics involved, in this case, the dynamics of a proper golf swing.
Ever get a complaint about soot from that appliance you’re servicing? In the basement it seems obvious, soot from the burner. But what’s to blame? Do you use my golf example and start changing burners? Some people do, and sooner or later one may work - for a while.
It’s kind of like that driver you bought to fix your slice. It worked so well at the range but, low and behold, that slice comes back.
Blame The Burner: Like my errant driver, some choose the obvious as the problem. Soot comes from the flame. The flame comes from the burner. Ergo, the burner is to blame. However, it’s never that cut and dry, so let’s take a moment and run through the mechanics of a combustion cycle. With all the facts, we’ll find out why a perfectly good burner may be producing soot.
Three components need to exist in order for combustion to take place: fuel, heat and air - all commonly referred to as the “triangle of combustion.” All three have to be in proper ratio or the results will suffer. Visual examples work best, so here is a simple example of combustion that I use with my students.
Ever use a kerosene lantern? A kerosene lantern is an example of a vaporizing burner, as opposed to an oil burner, which is an atomizing burner. Even though vaporization and atomizing are two separate designs, the triangle of combustion is always boss. You can’t fight physics.
Here’s what the components of the triangle are when using a kerosene lantern: The wick provides the fuel with kerosene “wicking” up the fabric. A match provides the initial heat, which starts the combustion cycle. The surrounding atmosphere provides air. After using a match to establish a flame, the heat perpetuates, as long as the other two parts are present.
We have a fire, but no control or regulation. To regulate the flame, the outer glass “chimney” is lowered over the flame and air for combustion rises through slots around the base. Air mixes with the other two components, and the now heated air rises out of the top of the glass chimney.
Typically, the flame produces smoke on start up, caused by exposing too much wick. Too much wick creates a fuel-rich, but air-lean mixture. Regulation is quick and easy. To clean up the flame you “trim the wick,” an expression that describes lowering the wick until the smoke stops.
Here’s how all this relates to an oil burner: With an oil burner, we’ll energize the motor, which turns the fan and the pump. Oil is drawn into the fuel unit (pump), pressurized and sent through the nozzle. The nozzle regulates the volume and atomizes the fuel through a pressure drop. The fuel is now ready to assume its part in the triangle.
Oil burners provide heat for combustion through a spark across the electrodes. The current for the spark comes from a step-up ignition transformer or igniter, with voltages that exceed 10,000 volts. The high-voltage arc is adjusted so it is strong enough to exist within a turbulent flow of air, hot enough to exceed fuel “flame point” and can carry forward into the atomized pattern of oil.
Air for combustion is drawn into the burner chassis by the burner fan, which is powered by the burner motor. Now the air is directed down the air tube and into the mix at the retention head. If the triangle of combustion is close to balanced, there will be fire.
Soot: Now let’s figure why a perfectly fine burner may still produce soot. Soot comes from a smoky fire. Why? The ratio of fuel to air is off.
When you were adjusting the lantern, what caused the smoky fire? Smoke was the result of lots of wick, in other words, plenty of fuel. The combustion cycle was fuel rich, but air lean.
Now look at your oil burner. How is the air adjusted? Are the air bands closed? When you close the air bands, what changes? Your pump stayed at a fixed pressure. The electrodes are sparking. If the bands are closed, your burner is fuel rich, but air lean. When an oil burner is adjusted like this, you will have two distinct combustion characteristics, high CO2 and smoke on your smoke paper. Where there’s smoke, there’s soot!
Heat for combustion will continue as long as fuel and air are available in a ratio that isn’t too far from that required for clean combustion. When all three components are there, we’ll have combustion, though not always the results we’d consider proper.
Now with the kerosene lantern we lowered the wick. As the wick is “trimmed,” the smoke started to clear up. What happened? The ratio changed from fuel rich, air lean to a balance of fuel and air by reducing the wick’s surface area. With the oil burner, we opened the air band, changing the ratio just like the lantern.
Once you’ve selected the nozzle, an oil burner has two additional methods that can be adjusted to maintain combustion; either change the pump pressure or adjust the air. The pump delivers the oil at a pressure that allows for atomization, typically, at or above 100 psi. By increasing the pump pressure, you will change the input rate of the nozzle.
All nozzles are rated by gallons per hour at 100 psi. Typically, an oil burner manufacturer will recommend a pressure and nozzle size for an application. If atomization is a problem, then a nozzle pressure chart can be consulted to select a different nozzle for a higher-pressure setting.
Please, reach out for help from an industry resource or the burner manufacturer before attempting this for the first time. This could create an unsafe condition if applied improperly.
Fuel And Air: That provided our fuel, now let’s deal with the air. The air bands are adjusted to provide air for combustion. The retention head also must be set properly, so please consult the manufacturer’s literature for that setting.
Where’s the air coming from? NFPA 31 requires a minimum of 50 cubic feet of air per 1,000 Btus if combustion air is being provided by the building space. Consult your local code official or NFPA 31 if you can’t meet this requirement. For now, let’s just remember the 50-foot rule and that one gallon of No. 2 fuel oil is rated at 139,400 Btus. Please note, the 50-foot rule may not be sufficient with modern building construction. This rule expects that air used for combustion will be replenished through infiltration.
The result that we’re looking for is a flame with clear smoke. However, there’s one more source of air for combustion - draft from the flue venting. Let’s consider our kerosene lantern again.
If you take a glass ashtray and cover half of the top of the lantern’s chimney, the smoke will come back. Why? You changed the ratio of combustion air to fuel.
Here’s what happened: As the fire burns, combustion gases rise from the combustion area. Gases exit through the hole in the top. The rate at which they exit depends on the temperature of the gases and the rate of air exchange into the combustion area.
Close off the top and the combustion gases can’t get out. If the spent gases stay in the combustion area, it will be full of combustion byproducts, which don’t contain enough oxygen to support combustion. The fuel stayed the same, and the fire went out. Fuel rich, but air lean.
It’s the same with an oil burner. The fuel pump is churning away at a set pressure, delivering a consistent amount of fuel through the nozzle. Your question is, what’s keeping the gases in the chamber? It’s the same force that removes the gases, commonly referred to as draft. Maybe it’s the chimney, maybe the heat exchanger. One way or another, when draft is affected, combustion will change.
Here’s another twist: Increase the draft, and you’ll increase the amount of combustion air. There goes the triangle again.
So this season I’ll get help before buying another driver. If you have trouble with combustion, go back to the basics. And if you need help, reach out to your supplier. They can get in touch with the proper resources or give the manufacturer’s help-line a call.