Just two things in your ducts are responsible for giving the blower in your furnace or air handler a hard time. They make the blower push against more pressure, thus reducing air flow or increasing energy use, depending on blower type. They cut the amount of air that gets delivered to the rooms. And they can be reduced but not eliminated. Do you know what they are? Maybe you're thinking it's flex duct that's not pulled tight or not using rigid elbows or maybe even the dreaded ductopus. Those things are related, but we need to go back further. We want the root causes. This is basic physics I'm talking about. Maybe looking at the image below, a view through a piece of flaccid flex duct liner, will give you an idea of what's to blame. The first cause of reduced air flow is friction. When air moving through a duct rubs against the inner surfaces of that duct, it loses energy. The more it rubs, the more those things happen. It's like walking down a busy sidewalk with your shoulder rubbing against the buildings.

The amount of friction depends on the nature of the material the duct is made of, how it was installed, how dirty it is, and how fast the air is moving. The photo above shows flex duct that's not pulled tight at all. Even though you can't see it all that well, you can tell that there's probably going to be a lot of rubbing when air moves through that duct. The same flex duct pulled tight is shown below. It still looks a bit rough but is much better than the one above. A piece of rigid metal duct would provide a much smoother surface. The other primary cause of reduced air flow is turbulence. This one is a kind of friction of the air rubbing against itself. The main cause of turbulence within ducts is turning the air. When you send air through a 90° turn, the type of fitting you use to do so can make a big difference. The diagram below is from ACCA's booklet Understanding the Friction Chart. In both of the 90° elbows, the air enters nice and smoothly. When it makes the turn, however, notice that the air in the elbow with the curved inside edge (the throat) results in less turbulence.

The elbow with the square throat produces more turbulence. auto glass repair in reno nvPick your fittings carefully!10 ton hvac rooftop unit Friction rates and pressure drops5000 btu window air conditioner power consumption The result of friction and turbulence, as I said above, is that you get a drop in the pressure. As air moves through a supply duct, the pressure created by the fan behind it keeps it moving. The farther it travels down the duct, though, the more that pressure is reduced by friction and turbulence. That's true in good duct systems as well as bad. Both of these causes, friction and turbulence, are included in the friction rates given for various types of ducts and fittings. As the word 'rate' indicates, the friction rate doesn't tell the whole story.

You've got to combine it with something else to figure out what the whole pressure drop is. That's where equivalent length comes in, and I'll save that for a future article. Or you can skip ahead and go read Manual D. When designing and installing ducts, you've got to know about this stuff. Friction and turbulence play a big role in whether a duct system does what it's supposed to or not. We've got this stuff quantified. If you're not using Manual D or a ductulator or some other method that quantifies these effects, you may well end up with a system that no amount of commissioning can save. How to Install Flex Duct Properly Keep Your Elbow Rigid — A Lesson in Flex Duct Installation Don't Kill Your Air Flow with This Flex Duct DiseaseCeiling fans are a great way to keep cool in the summer and warm in the winter if you know how to use them properly. Though there is much confusion on the subject, it’s not that hard as long as you remember two simple rules. Ceiling fans make you feel cooler in the summer by creating an artificial breeze that evaporates moisture from your skin.

This allows you to set your thermostat higher, saving money on air conditioning bills. When a ceiling fan rotates counterclockwise (while looking up at it), the slant of the blades pushes air down, causing a noticeable breeze. The faster the fan spins, the cooler you feel. This cooling effect doesn’t change the temperature of the air, it only makes you feel cooler. That is why you should turn the fan off when the room is empty. Otherwise, heat from the motor will actually increase the temperature in the room. Ceiling fans can produce the opposite effect in the winter by gently circulating hot air that is trapped near the ceiling. Since heat rises, the temperature near the ceiling is greater than at floor level. This is particularly true in houses with high ceilings or those with heating systems that do not circulate the air. The reason the fan is set to rotate clockwise on low is to keep from creating a strong draft, which would cool you down just like it does in summer.

Instead, the air near the ceiling is pushed up and outward, causing it to circulate down near the perimeter of the room. Since the air at floor level is warmer, the thermostat can be set lower, saving money and helping the environment at the same time. Any heat generated by the fan’s motor is a bonus in the winter, so you can keep it running whether you are present in the room or not. Follow these two simple rules, and your house will feel more comfortable while being a little greener at the same time.Adjustable pulleys are the most commonly used method of changing fan speed in light commercial balancing. Let’s take a look at how adjustable pulleys work on a fan and blower motor and learn how to adjust them. We’ll also learn how to use Fan Law One to calculate how to change the dimensions of a pulley. This will result in a predetermined change in the fan speed. Since adjustable pulleys are more expensive than fixed pulleys and since motor pulleys are smaller than fan pulleys, adjustable pulleys are usually used on the motor shaft, not on the fan shaft.

Adjustable pulleys are manufactured in two tapered sections. The two sections are connected as one half is screwed (turned) onto the other. This allows the diameter or pitch of the belt to vary, as the two halves of the pulley are turned together or apart. As the two sections are turned closer to each other, the belt is forced to the outside of the pulley causing the belt to travel a greater distance around the pulley. This increases the speed of the fan. Once the pulley, also called a sheave, is adjusted to the proper size, the set screw is tightened usually with an Allen wrench, to lock the pulley. Be sure to tighten the set screw onto the flat surface of the shaft. This assures it will not slip off the shaft during operation. Be certain to shut off and lock out power before working near fans and pulleys. No measurement or adjustment is worth the loss of a finger. Always use extreme caution when working around air moving equipment. To effectively measure the adjustable belt or pitch diameter (where the belt rides) on a pulley, remove the pulley and belt from the motor and grasp by the belt.

Using a caliper, measure the outside diameter of the belt, while it’s wrapped around the pulley. The diameter of the pulley is the physical outside dimension of the pulley. These are usually two different diameters. Open or close the pulley to increase or decrease the diameter to the calculated size. Then tightly wrap the belt around the pulley again to measure the diameter to assure the right pitch has been achieved. After you’ve measured and adjusted the diameter to the required size, replace the pulley on the shaft. Start the system and measure to verify expected airflow is being delivered. Align the Motor and Fan Pulleys When attaching pulleys to the shaft, care must be taken to align both pulleys in a straight line to each other. This will increase power transmission and reduce wear and tear on the belts and the pulleys, extending the life of both. Also make sure the shaft of the motor and the shaft of the fan are aligned with each other. Alignment can be done on smaller fans by simply laying a ruler or straightedge flat on the face of each pulley to assure both pulleys are in line with each other (Fig. 3 & 4).

More advanced technology such as laser belt pulley alignment instruments may be required to accurately align larger systems. Belt tension is normally adjusted by loosening the motor mount bolts and sliding the motor closer or farther away from the fan. Ideally when a 10 PSI pressure is placed between the pulleys on the belt, the belt deflection should equal the distance between the centerline of the pulleys divided by 64. For more detailed procedures see the manufacturer instructions. Use a belt tensioning tool to measure the 10 psi of pressure. Example: Two pulleys have a 24-in. center-line distance. Twenty-four inches divided by 64 equals an ideal deflection of .375-in. or .4 in. (Fig. 5) Fan laws are formulas that enable you to determine the outcome of the belt and pulley adjustment before the adjustment is actually made. Basically, they allow you to peek into the future and see what is going to happen before happens. Using Fan Law One, you can calculate the change in pulley diameter needed to increase or decrease fan speed until the fan is delivering the required airflow needed.

Begin by measuring the airflow at the fan. This can be done by traversing airflow near the fan. Fan airflow can also be plotted by measuring the fan speed in RPM and the fan operating total external static pressure, and by plotting the fan airflow in the manufacturer’s fan performance tables. Measure the outside diameter of the belt riding on the pulley as described above. Also measure the physical outside diameter of the pulley just to be sure the pulley is large enough to adjust to the required diameter. The math required for Fan Law One is quite simple. Simply divide once, and then multiply once to find the new pulley size needed for the fan to move the required airflow. Example: Suppose we have a 5.5-in. adjustable motor pulley and the fan is currently delivering 5100 CFM on a 15-ton system that requires 6000 CFM. Here’s what the raw formula looks like: (PD represents Pulley Diameter) Here’s the formula with the actual fan numbers poured into it: Divide the 6000 CFM by 5100 CFM to find the ratio of airflow increase.

Notice the airflow should increase 17% for the fan to deliver the required airflow. Then multiply the pulley belt diameter of 5.5-in. times 1.17 to find the new belt diameter of 6.44-in. Adjust the motor pulley to 6.44-in. (or as close as you can get it, 6.5-in. will do) to take the airflow up to 6000 CFM. Of course, this will only work if the pulley diameter is at least 6.5-in. or so. Also the fan and motor capacity must be at least 6000 CFM. Notice the airflow increased 17% and the pulley size also increased 17%. The idea that pulley diameter and airflow increase at the same rate is what we learn from Fan Law One. Learn to measure airflow and fan properties, and then use the fan laws and fan engineering data to calculate what the change in fan performance will be before you ever pick up a wrench. Fan laws can also calculate the changes in fan RPM, static pressure and motor amp draw. Give these measurements and calculations a try the next time you need to change fan airflow. Rob “Doc” Falke serves the industry as president of National Comfort Institute an HVAC based training company and membership organization.