Basic draft, Part II


By George Lanthier

In Part I, we established a lot of rules for draft and ended up by talking about outside air and the fact that although I’m the world’s greatest believer that you must have plenty of draft for combustion, I’m not an advocate of outside air. It’s funny, because over the years in my ruminations about combustion air people think I love cold outside air, NOT!

By now you’re probably asking yourself, is there any sure way to know if you have enough air for combustion? Well, there is, and you need a draft gauge. Got ya. I told you that air was draft in Part I, right? So what did you think I was going to tell you, that you have to have a pogo stick?

To work around draft you must have a really good draft (air) gauge. My personal favorite has been the Dwyer Magnehelic for a long time. (See Figure 1.) It’s accurate, reliable, inexpensive and very tough. No, it can’t take a drop from a 20-story building, but it can get banged around more than some others.

With this gauge in a .50-inch w.c. range I can set up both residential and most commercial equipment, including the Euroboilers. For heavy commercial and industrial grade equipment, I switch to another Magnehelic with a 1-inch w.c. range. Naturally, if you have an electronic tester you can use that if it will read the ranges.

To verify enough air for combustion and inlet draft, you need to do a simple test. To prepare for the test you are going to have to get your draft gauge to read the air in the room that the burner is in. You want to do this test on the solid door closest to the burner. It’s important that if the door is louvered you move away from this door and get to a solid one. The test won’t work unless we are testing the burner and its entire smoke enclosure. In many cases that door will be the one at the top of the basement stairs.

The easiest way to test the smoke enclosure is to remove the doorknob lockset from the solid door. Most come off very easily. With the lockset removed you will have a big hole. Close the lockset hole off with a piece of cardboard and some tape and then punch a half-inch hole in the cardboard. I have a piece of sheet metal that has foam border on the back and I tape that down. You can make up your own test plate the same way.

Now with the gauge and yourself outside of the room get the burner running. The gauge is now reading the smoke enclosure (room) and you must run the burner for at least 15 minutes to create at least one air change. Do what you have to do to make this happen, but keep that burner running. By the way, with commercial and industrial jobs you may have to drill a steel fire door, so bring some fender washers and a half-inch by 1-inch bolt and nut with you.

Okay, so the burner’s been running a while and the gauge is actually reading a negative draft like you were checking a chimney. This is not good. The higher that gauge climbs, the less air you have, but it doesn’t really matter because any reading other than zero is telling you that the burner is trying to suck your gauge through the door and you need to bring inlet draft in from someplace.

In 38 years I’ve only seen two cases where after the gauge read a negative it went positive after pressurizing the room. That is an indication of a defective venting device and your chimney, power-venter or terminal must be checked right away for blockage. It’s also an indicator that the burner is trying to get out of the room, so be careful.

Okay, so we’ve beaten inlet draft to death. Now we’re going to look at combustion draft. This is another area that the draft gauge is essential for. Good combustion draft must be maintained to have a great fire and burn all of the fuel prepared by the burner. Keep in mind that all fuel burners are 100 percent efficient. They burn 100 percent of the fuel that comes out of that oil burner nozzle and burn nothing when shut off. What we want to know is how well the burner is converting the fuel to useful heat and btus and how much help the heat exchanger is adding to the process.

A leak in the heat exchanger is a bad thing when we are working on a furnace. Due to smells, fumes and soot we smoke-bomb them like crazy and go nuts trying to find the cause, but have you ever smoke-bombed a boiler? How about a water heater? Why not? If you have an air leak, you have a draft leak, and regardless of the heat exchanger it just isn’t a good thing.

Many years ago I was taught how to do this test and when we went nuts over efficiency in the 1970s it kind of got lost in the shuffle. That’s too bad, because many of us remember it, but only use it when all else fails. In my opinion, it should be done with every new unit and at every routine service. Do you know what test I’m talking about? It’s called the over-fire C02 test and, yes, those of you with your electronic toys can do it, too. Although we’re going to measure CO2 or O2, we are really measuring draft.

After setting up your burner for optimum performance, not efficiency, take one final reading of flue gas O2 or CO2 over the fire. I don’t recommend inserting the curly end of a Fyrite sampling tube into the chamber and if you do that to an electronic unit you could fry or damage the thermocouple on the end of the probe. In all cases use a short piece of hose that will connect to your sampling device and a piece of half-inch stainless tubing. After obtaining the sample, compare it to the breeching CO2 or O2. There should be no difference in the readings. The greater the difference, the bigger the air leak and the more efficiency you’re losing. Don’t be afraid to check new units, either. As I’ve already said, equipment is assembled by people and we all can have a bad day.

The last thing to do is talk about the vent. Whether it’s a chimney, power-venter, stub-vent or through-the-wall termination hood of a direct-vent system you have to get that draft back outside, remember? With a chimney everyone seems to think it’s a piece of cake. Chimneys are not what they use to be and draft has actually changed due to technology.

In my book COMBUSTION & Oil Burning Equipment we have a chart that shows just how much draft can change, but to give you the skinny on it, it basically shows that when you cut your draft temperature in half you cut your ability to make thermal draft in half and chimneys rely on thermal draft to work more than anything else. When oil burners and heat exchangers produced 700F at the breeching of the appliance life was good and -.02-inch w.c. over-the-fire and -04-inch w.c. in the breeching even in the summer was a done deal. But, if you cut that temperature to 350F then the -.02-inch becomes -.01-inch and the -.04-inch becomes -.02-inch and you have a draft problem.

See ya.

George Lanthier is the owner of Firedragon Enterprises, a teaching, publishing and consulting firm. He is a proctor and trainer for the industry’s certification programs and is the author of nine books on oilheating and HVAC subjects. He can be reached at 132 Lowell Street, Arlington, MA, 02474-2756. His phone is (781) 646-2584, fax at (781_ 641-7099 and his e-mail is

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