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NORA Oilheat Technicians Manual

Chapter 7 Combustion An important thing to remember when using a draft gauge is to “zero it out” before using it Chapter 7—Combustion 7-15 air flow will be reduced, causing excessive smoke. If the draft is too high, then too much excess air will be drawn into the unit and the combustion gases will be pulled through the heat exchanger too fast, lowering efficiency. During steady state operating conditions, the draft should be stable. Over-fire drafts of -.01" to -.02"wc water column, see below are generally recommended for residential units. Measurement and adjustment of draft are important because draft affects all other burner adjustments. Draft measurement To measure draft, we use a manometer, a U-tube or a gauge. The U-tube is a glass tube bent into the shape of a U. The tube is filled with water up to a zero mark on a scale etched on the tube. A sampling tube is attached to the right leg of the tube, and it is inserted into the unit to be tested. The left leg is left open to atmospheric pressure. The draft on the right leg causes a pressure drop on that side and pulls the water up in the tube, and the atmospheric pressure on the open side pushes the water down in the left leg. The difference between the water levels in the two legs is the draft in inches of water column. The suction from draft is so slight the difference is only a few hundredths of an inch. One one-hundredth of an inch of draft is expressed as -.01"wc. Since it is a vacuum and not a pressure, we call it negative draft. If the right leg measured a pressure higher than atmospheric the water would go up in the left leg and we would call it positive draft. Our modern draft gauges (manometers) are the mechanical equivalent of the Utube. An important thing to remember when using the draft gauge is you must “zero it out” before using it. Atmospheric pressure changes all the time, so we take this variable pressure out of the equation by adjusting the gauge reading to zero before we take each test. To zero out the draft gauge place it on a level surface near where you are going to test and slide the lever on the side of the unit until the arrow points to zero. You should also be sure the gauge is functioning properly by spinning the rubber sample tube or blowing gently across the end of it to be sure the needle is not stuck. It should move smoothly and return to zero. The most important draft reading on residential equipment is the over-the-fire draft. On most residential and light commercial negative draft units, this reading should be -.01" to -.02"wc. If upon reaching steady state you cannot obtain this reading, you should next check the draft at the breech, and adjust the draft regulator. If you still cannot achieve sufficient over-fire-draft, then you probably have one of the following problems. Common causes of poor draft • Chimney is too small for the load of the attached appliances • Chimney is too large in diameter or cross-section • Chimney too short or improperly constructed • Leakage of air into chimney, thimble, stack, or breeching


NORA Oilheat Technicians Manual
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