Basic draft, Part 1
By George Lanthier
Over the years, I’ve written a lot of articles on draft and combustion, so many in fact they became the basis for one of my books, COMBUSTION & Oil Burning Equipment. Although oilheat service technicians have to know a lot about controls to systems to metal working and even some carpentry to get the job done, we sometimes forget that the most important job we are supposed to be doing is keeping the home fires of oilheat burning. Oh, yeah, and you’re supposed to keep those fires burning at true zero smoke, too.
After teaching this for a while, it still amazes me just how confused we can get over some basic principles, so I’m going to start at the beginning, square one. Draft science has not changed since day one, but boy, oh boy, the technology sure has. The area that we must know the most about and really understand completely is draft. Draft, in my opinion, falls into three areas of concern: inlet draft, combustion draft and venting draft. You may notice I didn’t use the term air; well, I shouldn’t have to. When someone says draft you should immediately think of air, because that’s what it is.
Consider these points. Normally at the end of a routine preventive maintenance service and upon completing a new installation we do a combustion test. Let’s look at that test. I’m going to use the old wet-kit for testing since it is still state-of-the-art, the most commonly used and everyone knows how to use them, or do they?
The last test we normally do should be the first one started, the stack temperature test. By using two test holes you can not only keep up with these high-efficiency units, but also keep an eye on what’s going on. Temperature is an indication of excess air and how well the heat exchanger is extracting the heat, and that’s what it’s all about.
Before recording the temperature we normally take the percentage of CO2. I’m doing these in reverse order, but you’ll see why in a moment. CO2 is like taking the blood pressure of the patient. We’ve already taken the temperature, only makes sense to take the BP, right?
A smoke test comes before that and although that has no relation to human analogy, to oil-fired equipment I feel that it is the second most important test done. By knowing the smoke, we can adjust our burner for peak operation and see just how well it’s running.
However, the first test we do is draft. It should always be the first test and draft should be adjusted as close to the OEM recommendations as possible. There are a lot of very good reasons for doing this test first and the most important is that I want you to think of draft as the pulse of the system. If your heart isn’t pumping, you’re dead, plain and simple. Air is the blood of combustion and draft is its pulse. Draft is alive, it fluctuates, varies and acts as an indicator of how the air is moving through our entire combustion system and from that point we move into the diagnosis.
The American Society of Heating, Refrigerating and Air-conditioning Engineers defines draft as ‘a column of air in an enclosed space” and a good dictionary will give you the same definition. So, why do so many of us think that draft only begins, well, wherever?
The first thing to do is picture yourself as draft or air and that you are outside the structure. You’re job is to get in, mix with the oil, do a few other little jobs and get back outside again. Have you got it? Let’s go exploring.
I first coined the term ‘inlet draft” while teaching basic students in 1975. I used it to try to show that there is only one current of air (draft) from start to finish in the combustion process. It must be there and it must be ‘balanced.” Look at the simple drawing in Figure 1. Draft is on a seesaw; if it has enough coming in it will balance, if it doesn’t, well, do you get it?
Anyway, let’s get going and look at how this is supposed to work. First, you must travel into the burner as inlet draft (inlet or combustion air). When you think about it, this has a lot of names and that’s really weird for something you can’t even see. This is even more amazing when you look at some of the things we do to get it where we need it. Remember, you started outside, so you must get in. That means through tiny cracks and crevices around doors, windows and the foundation sills. If you’re lucky there’s enough spaces for air to go through. If not? Remember the enclosed space?
Okay, so you’ve made it into the burner room. Now, you have to get into the burner, move across the fan and travel down the air tube. Remember, all this time you’re still inlet draft, but parts of you are spun off through the combustion components of the burner and now even your own mother would be impressed because you’ve acquired names like primary air, secondary air and tertiary. Instantly at the combustion head you become combustion draft as you mix with the fuel and become part of the flame process and start to find your way out because you’re getting bigger from the heat and you’ve put on a lot of weight by mixing with the fuel oil.
Finally, all you have to do is leave the unit and find the exit as outlet draft. In a boiler, you’re going to have to move through a maze of pins and plates and all kinds of things that are trying to slow you down. But you can’t be stopped now because that fan is big enough to move you and you fly through this clean heat exchanger. By the way, if it’s not clean, you start to backup from where you came. As combustion air you’ve become a very big part of the job since you must now move the bulk you’ve acquired due to the fuel, remove the gasses left over from the combustion process and leave the building, literally.
Now, while all of this is going on as inlet draft, you’re also supposed to be helping out in controlling the flow of all these gasses moving through the system and have enough left over for proper ventilation of the area the boiler, furnace or water heater is in. That ventilation requirement is not only required by code, but it also goes a long way to prevent all of your controls and wiring from acting crazy or even melting. Are you starting to see just how important proper draft is? Are you sitting there thinking, ‘Holy moly, there is a lot more to draft than I thought there was!” Are you still thinking that there’s nothing to draft and it’s very simple?
The No. 1 problem out there is not enough inlet draft. It happens in new homes, old homes, commercial and industrial applications, and it’s getting worse. It runs rampant on jobs with mechanical draft.
Whether you are using a powerventer, a draft booster, a chimney turbine or anything else, you can’t get it out if you can’t put it in. Remember the seesaw? Before you add a mechanical device to get rid of air, first make sure there is air to get rid of.
At this point I am just going to say a few words about powerventers. These things, without a doubt, are an essential part of this industry’s ability to stay competitive in the conversion market and in new homes. In my opinion we need them, but like everything else they have limitations, and must be sized and installed correctly. My experience is that 80 percent are installed wrong and the other 20 percent just won’t work on the job no matter what you do. If you’re not going to read the directions, you will not understand how they work or what they can do and trust me, if used properly that’s a lot. The biggest problem I see with powerventers goes right back to the front of this article, inlet draft. Everybody seems to agree with the following statement, but doesn’t do it: ‘Every powerventer installation should have outside air.”
Let’s get back to inlet draft. Back in the 1970s Beckett brought out the Outside Air Intake Kit. They have now improved it and have a different version. As a burner OEM they know the importance of inlet draft or combustion air. In fact, they even hit you over the head and emphasize their point by bringing the ability to vent right into the burner housing of the AFII. Carlin has also joined in with a ‘boot” and Riello uses a special burner model. After that, you can get aftermarket boots from several manufacturers.
More about those fans. Keep this simple rule in mind: ‘fans don’t suck, they blow.” Most fans that you work with need the air brought to them. Then they will blow the air as far as they can; that’s called fan drop. When they can’t throw the air any further, a chimney (thermal draft) or fan (mechanical draft) has to take over. Think of every fan that you work with as lazy. If you bring the air to them that’s okay, but they are not going to do it by themselves. Even in forced-draft applications, where the burner fan is supposed to provide the air (draft) to push the draft (air) all the way through, it has to be there to begin with. Bottom line again is: If you don’t have enough air coming in, you won’t have enough going out.
Now there’s another small problem that falls into the realm of overkill. You absolutely do not need outside air in a structure for venting; it’s in every burner code and manual in the world. You only go to 100 percent outside air if there is not enough air being delivered through infiltration, and that’s in the books too. Another problem that makes us think of outside air as dirty air is due to animal hair, dryer lint, sawdust and other heavier-than-air contaminants.
Outside air at the 40th parallel and above gets cold in the winter and when you work up at the 50th parallel or further north it gets very frigid. Before you go to outside draft, look at where else you can get your inlet air from. Can it be drawn from another room or from the rafters above? This may not only be clean air, but it’s going to be warmer than outside. Keep in mind that cold air means cold air over the fire and depending on how it’s brought into the burner may also mean cold oil. Well, that about does it for now. We’ll rap this up next month in Part 2.
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 St., Arlington, MA 02474-2756. His phone is (781) 646-2584, fax (781) 641-7099 and his e-mail is FiredragonEnt@comcast.net.