Measuring System Airflow Using a Ductblaster

If you install systems in a state which requires duct leakage tests on all new installations, chances are you have a ductblaster which you use for that purpose. Your ductblaster is more accurate at measuring airflow than just about any tool you have. Afterall, the way it works is to pressurize the ductwork and measure the airflow required to maintain that pressure. You can use the ductblaster to measure system airflow using a procedure called pressure matching.

Operate the air conditioning system normally and use the ductblster manometer to measure the static pressure in the supply plenum or trunk. You should measure after the coil but before any takeoffs. Record this pressure. Now, turn the system off and connect the ductblaster to the blower on the return side. You will need to block the return air trunk off so air only goes through the unit and into the supply ducts.

Now turn on the ductblaster. Once again, measure the supply air static at the same location where you measured it with the unit operating.  Dial the ductblaster up until the measured supply air static equals the reading you took when operating the system. The amount of airflow the ductblaster is moving is the same as the airflow through the system when it was operating.

How does this work? You are matching the airflow required to create a supply static equal to the supply static created by the system blower. Note that this does not require any particular manufacturer’s data. This procedure allows you to make an accurate airflow measurement using a tool you may already have. It also allows you to get more from your investment in the ductblaster.

How Much Refrigerant Does it Take to Make the Fan Blow Harder?

My brother Richard sometimes finds himself helping younger technicians who are stumped and need the help of an experienced professional technician. However, they have to really need help because they know Richard is going to twist their tail a bit in the process.

One day a frustrated tech called and told Richard that he was working on a system with a low suction pressure that was frosting up. He further explained that his new digital gauges were telling him that the superheat was 0, so he added refrigerant. However, no matter how much refrigerant he added, the superheat would not increase.

Richard asked him, “how much refrigerant do you have to add to make the fan blow harder?” There was no response, so Richard asked again, “Tell me, I really don’t know. How much refrigerant does it take to make the fan blow harder?” Finally, the tech responds: “Your question makes no sense! There is no relationship between the amount of refrigerant in the system and how hard the fan blows.” Richard then replies “So why are you trying to fix an airflow problem by adding refrigerant?”

While it is true that an undercharge can cause an air conditioning system to frost, the most common cause of a frosting air conditioning coil is actually low airflow. Always look at airflow issues first when trying to remedy a frosting air conditioning system. Common airflow issues include a dirty air filter, a dirty evaporator coil (caused by dirty air filters), closed registers, and poor ductwork.

One tip-off is superheat.  A system with airflow issues will operate with a low superheat while an undercharged system will operate with a high superheat. Note that if the coil is frozen over it will need to be defrosted before any pressures or temperatures are checked. The ice covering the coil makes its own airflow restriction.

Checking Airflow is Not Just a Summer Thing

Checking for proper airflow is not just a summer thing. Most techs know that poor airflow is the first thing you should check in the case of freezing up air conditioning coils or lower than normal suction pressures in cooling. We need to be concerned with airflow in the heating season as well. In fact, in the case of heat pumps, airflow is arguably more important in the heating season because the indoor coil is now the condenser. Sure signs of an airflow problem in a heat pump are low suction pressures and icing in the summer and frequent high pressure switch trips in the winter. I have heard several stories about systems which techs “fixed” by adding charge in the summer and taking it out in the winter. In effect, they are overcharging the system in the summer and undercharging it in the winter. Of course this kills both system efficiency and the compressor. In the summer, the overcharge causes liquid dilution of the compressor oil, and in the winter the undercharge makes the compressor run hot. Other signs of poor airflow include open fuse links on strip heaters, open strip heaters, or open thermal limits which eventually fail from opening and closing repeatedly. In gas furnaces, poor airflow will cause a higher than normal temperature rise. In the case of gas furnaces, it is possible to have too much airflow. Too much airflow will reduce the temperature rise below the minimum, which can cause condensation in heat exchangers which are not designed for condensation. Typical temperature rise for mist furnaces is between 40°F and 70°F. However, check the data plate on the furnace for the exact specification.

Dryer Sheets In The Coil

Recently, two different techs have told me stories about finding dryer sheets used creatively in customers air conditioning systems.  When they asked their customer why there were bunches of dryer sheets stuck to the air filter, the customer said they liked the “clean, fresh scent.” Another story involved customers removing supply registers and stuffing  dryer sheets across the outlet of the register to “filter and freshen” the air. Naturally, some of these found their way to the coil where they clogged the coil and caused enough air flow restriction to freeze the system up. In all cases, dryer sheets stuffed into clever places only serve to restrict the airflow. Even if they did not, all you are accomplishing is introducing whatever chemical is on them into your system. They don’t remove odors – they just add more. Honeywell used to sell a system to do that called the “Scentrol.” Scentrols used little gel cans that looked like sterno. They system controlled how much air circulated over the can to regulate the scent. It even had a wall control. My father sold one to a customer who also bought an electronic air cleaner with an activated charcoal filter to remove smells. The customer insisted on having the Scentrol installed, even after dad explained that they were buying one machine to put smells in the air and another to take them out. Here is a link to an online forum that has a picture http://www.hvacproforums.com/threads/honeywell-scentrol.1148/

So what do you suggest to customers who want the air conditioner to improve their home’s odor? I don’t think you can buy a Scentrol anymore. First, try to find out if there are specific odors they are trying to get rid of. Finding and eliminating the source is really the best option. My brother once discovered that the kitchen sink drain leaving the disposal had never been connected on a new house in which the customers were complaining about dirty sock syndrome. If the odors are system related, look at the condition of the indoor coil and make sure there are not problems such as a leaky return in a crawl space. If they just like the idea of having a “fresh scent” system there are a number of filters that use activated charcoal to absorb odor. Just check that the pressure drop across the filter is not too high. Like the famous 1” pleated filters, some of the charcoal filters can add enough restriction to cause an airflow problem even when they are clean. Another option would be an air cleaner that uses titanium dioxide and UV light to eat up compounds that cause odors. Both Field Controls and Lennox make air cleaners that use this technology.  Now you are on the high end of the cost spectrum, but you are dealing with solid, reputable companies who sell things that work.

Check System External Static BEFORE Sealing Ducts

I recently read a thought provoking article by David Richardson in the July 7 Air Conditioning Heating and Refrigeration News. In it he argues that sealing an undersized duct system can cause problems. The system cooled BEFORE you sealed the ducts, and now that you have worked on it, the system cannot move enough air to stay operating. I know I have seen many systems with marginal ductwork that still managed to operate, even if inefficiently. The duct leaks could be allowing just enough extra air flow to keep the system operating. The combination of leaks on both the return and supply sides of the system serve to reduce the static pressure difference against which the fan must move the air. If the static pressure difference between the return and supply is already at or past the limit the manufacturer publishes for their system, sealing the ducts will increase the already high static pressure difference and push the system over the edge. This is why Mr. Richardson advises taking a reading of the total external static pressure difference BEFORE sealing the duct to avoid this trap. If it is too high, the duct system will need more attention than just sealing – it will need some duct modifications as well. Better to know before you do the job. What is too high? I don’t like to see anything higher than 0.8” wc – that is usually the top end on most residential systems. Most residential systems will operate comfortably around 0.5” wc. Some will go all the way to 1” wc, but at a cost. With ECM fan motors you are burning electricity to shove air through restrictive ductwork. With PSC blowers you lose airflow at high external static pressures. This costs electricity in another way. The suction pressure drops, the compression ratio increases, and system capacity decreases – causing extended run times and inefficient operation.

Sensible and Latent Cooling

When you look at manufacturer’s tables showing the capacity of their units, you will notice terms such as latent capacity, sensible capacity, or total capacity. The sensible capacity expresses the unit’s ability to reduce the air temperature. In most conditions, only part of the system cooling capacity goes into reducing the air temperature. The name comes from the idea that this change in heat can be sensed, or measured with a thermometer. I have had students tell me they remember it because it makes “sense,” and is therefore “sensible.” The word latent means hidden. Latent cooling capacity is used to take water out of the air. It is latent, or hidden, because you cannot measure it through temperature change. Latent changes involve a change of state. The water in the air is changing state from a vapor to a liquid. To accomplish this, the heat that went into the water to vaporize it must be removed. A system’s capacity is not fixed – it changes with the temperature, relative humidity, and volume of the air blowing across the evaporator coil. System capacity used for latent cooling is not available for sensible cooling. So as you increase the percentage of latent cooling a system performs, you decrease its sensible capacity. Here in the southeast, latent cooling is just about as important as sensible cooling. We MUST remove water from the air to be comfortable. In other parts of the country, such as the southwest, taking water out of the air may be undesirable. We can have some control over this by controlling the airflow. As a general rule, as you increase airflow across the evaporator coil, you increase sensible cooling and decrease latent cooling. In a humid area it makes sense to set the system airflow to a level that will increase latent cooling. For example, 350 CFM per ton rather than 400 CFM per ton. In a dry area, you may want to increase airflow to minimize latent cooling. For example, 450 CFM per ton. Although most systems will operate at 400 CFM per ton with no mechanical problems, that may not be the ideal airflow for your application.

Airflow in Heating

Technicians who know to check airflow in cooling sometimes neglect to check airflow in heating. But poor airflow can cause heating issues too. If you have a heat pump that is tripping out on high pressure – check for problems that can affect airflow. Dirty air filters, dirty indoor coils, or duct problems can all cause airflow problems. Since the indoor coil is the condenser in the heating cycle, poor indoor airflow will cause high discharge pressures and hot compressors. These can cause tripped high pressure switches or open compressor internal overloads. You would not want to condemn a compressor because the air filter was dirty!

Electric strips may cycle on the their thermal overloads, or the fusible links may open. Anytime you replace a fusible link in a strip, check the airflow and all problems that can cause poor airflow. A stopped up evaporator coil can cause a problem in the heating cycle, even if you do not have a heat pump because it is creating an airflow restriction. Another problem that can be easy to miss is an overly restrictive CLEAN air filer. Most 1" pleated air filters have a very high static pressure drop even when brand new. Homeowners often replace the lightweight filters with these and inadvertently cause airflow problems.

Gas and oil furnaces can cycle on the high limit due to poor airflow. This can be more difficult to catch because most high limits automatically reset. When a furnace cycles on the high limit, the blower keeps running and the burners cycle on and off. Furnaces often cycle on the high limit when they have an airflow restriction. This will shorten the life of the furnace, and is a dangerous situation because you are depending on the safety control to prevent overheating. If the limit would fail to open, the furnace could dangerously overheat. Remember, good airflow in the heating season is really just as important as in the cooling season.

RSES November 2013 Article

I want to brag a bit. The November 2013 issue of the RSES Journal has an article taken from one of my blogs. It discusses reading the external static pressure on a blower to determine the airflow. I get a kick out of seeing my stuff published in the RSES Journal because of the great respect I have for that organization. It is said that you can tell a lot about a person by the company they keep. I am happy to be in pretty good company this month. If you have a chance, you might want to check out the November 2013 issue - or better yet - subscribe. Looking for an easy way to pick up continuing education units?  Answer the questions at the end of the magazine.

Applying the Temperature Rise Airflow Formula

Many techs are familiar with the temperature rise formula for checking airflow. It is derived from the specific heat formula:

BTU = weight x ΔT x Specific Heat
(Note ΔT is simply shorthand for the change)

After rearranging the formula to solve for weight, changing the weight of air to a volume, and reconciling BTUs per hour with Cubic feet per minute you end up with

CFM = Btuh/(1.08 x ΔT)  

For heat pumps we get the Btuh by measuring both the voltage and current and multiplying them by 3.41. The formula becomes

CFM = (volts x amps x 3.41)/(1.08 x ΔT)

For furnaces we measure the firing rate in Btuh and multiply it by the furnace combustion efficiency. The formula becomes

CFM = (Btuh input x %Efficiency)/(1.08 x ΔT)    
 (Note %Efficiency is stated as a decimal in this formula.)

Did you ever wonder where the 1.08 comes from? The "magic number" 1.08 is a convenience constant. It is basically a bunch of math combined into one factor as a short cut. When you multiply the airflow by 60 to get airflow per hour, multiply by the density of air 0.075 pounds per cubic foot to convert volume to weight, and multiply by the specific heat of air 0.24, you end up with 1.08. The factor is often rounded to 1.1 because it makes the math easier.

The number is not really constant because the volume of the air varies with altitude, temperature, and humidity. A change in any of these variables changes the density of air, which in turn changes the "magic number." The factor 1.08 in this formula is only accurate for dry air at 70°F at sea level. For example, 1.08 really does not work with flue gas or airflow in freezers because the air volume has changed, which changes the convenience factor. Similarly, 1.08 does not work in Denver because the altitude changes the air pressure, changing the density. Even the relative humidity changes the factor. The ubiquitous 1.08 is for dry air at 0% relative humidity – a condition that is never seen in Georgia. Changing the relative humidity to 50% changes the air volume, which changes the factor.

Lets look at some examples. 0°F air at sea level has a density of 0.086 pounds per cubic foot, while 300°F air at sea level has a density of 0.052 pounds per cubic foot. Instead of the commonly quoted 1.08, these densities produce factors of 1.24 for 0°F air and 0.746 for 300°F air. If the air is 70°F but at 5000 feet elevation, the factor becomes 0.9 because the air density has changed due to the increased elevation. Even changing from 0% relative humidity to 50% relative humidity changes the density to 0.0741 instead of 0.0745 (the 0.075 for 70°F air is rounded). This changes our convenience factor to 1.07.

If you would like to play with different scenarios, there is an online air density calculator that takes all three factors into account at http://www.denysschen.com/catalogue/density.aspx
Just multiply the density times 14.4 to get your new magic number. What is 14.4? Oh, you get that by multiplying 0.24 times 60.

For more details on checking airflow using the temperature rise I recommend a great article by Norm Christopherson on the nuts and bolts of measuring airflow using temperature rise. You can find it on docstoc by clicking HERE.

Blower External Static vs CFM

  

It is possible to use a magnehelic gauge or a digital manometer and two static pressure probes to determine the amount of airflow a blower is moving. You can read the static pressure difference across the blower and compare it to the manufacturer’s data to determine the blower CFM. It does make a difference what type of blower you have and what type of motor the blower uses. A typical residential blower uses a forward curved centrifugal blower and a PSC motor. The airflow these blowers deliver decreases as the static pressure the blower is working against increases. You need the manufacturer’s data to compare the measured static pressure across the blower to the fan performance table or curve. Here is a table from a unit with an air handler with a PSC blower motor. Note that on high speed at a static pressure difference across the unit of 0.1” wc the airflow is 1150 CFM, while at 0.7” wc across the unit the airflow drops to 775 CFM.

PSC Blower	External Static
Motor Speed	0.1” wc	0.2” wc	0.3” wc	0.4” wc	0.5” wc	0.6” wc	0.7” wc
High	1,150	1,095	1,045	1,025	950	865	775
Medium	890	855	835	775	715	665	605
Low	640	605	565	530	485	440	360

With ECM motors, the airflow varies very little as the static pressure across the blower changes. That is the point of an ECM blower. It recognizes the amount of static pressure it is working against and adjusts the blower motor RPM and power output to keep the same programmed airflow – up to a point. ECM motors do have a programmed speed limit. When they hit their speed limit, they shut off. A key point here is that although the ECM motor can compensate for extra restriction, it does this by using more electricity – which can turn an energy efficient blower into an energy hog. It is far more cost effective to remove the restriction than to pay for enough electricity to shove the air through. At any rate, checking the static pressure across an ECM blower is done primarily to make sure it is operating within its design parameters and in an efficient manner. It does not tell you how much air the blower is moving. A table from an air handler similar to the one above, but with an ECM blower motor is listed below. Note that there is very little change in the CFM as the static pressure across the blower increases. The CFM delivered is determined by the program: A, B, C, D.

ECM Blower	External Static
Motor Program	0.1” wc	0.2” wc	0.3” wc	0.4” wc	0.5” wc	0.6” wc	0.7” wc
A	630	625	625	620	610	605	600
B	895	885	875	865	845	825	815
C	1030	1020	1005	995	970	945	935
D	1185	1175	1160	1145	1120	1090	1080

X13 motors are also electronically commutated, but they are programmed for a specific torque or power output, not a specific airflow. The airflow across an X13 motor does drop off as the static pressure across it increases, but not as dramatically as a PSC motor. They are considerably more efficient than a PSC motor and considerably cheaper than a fully programmable ECM. You can determine an airflow from the static pressure across the blower and the manufacturer’s specifications. Below is data from an air handler with an X13 blower motor. Note that its performance compared to external static pressure is somewhere between  a PSC motor and an ECM blower.  

X-13 Blower	External Static
Motor Program	0.1” wc	0.2” wc	0.3” wc	0.4” wc	0.5” wc	0.6” wc	0.7” wc
Tap 1	960	905	890	810	760	720	665
Tap 2	1,060	1,010	995	920	875	835	790
Tap 3	1,150	1,100	1,090	1,025	990	945	905
Tap 4	855	810	775	755	750	720	690
Tap 5	1,470	1,440	1,425	1,405	1,375	1,260	1,315

How's It Flowing?

When air, water, refrigerant, or any fluid flows through a pipe, the flow can be characterized as laminar or fluid. I can hear you saying “what does this have to do with HVACR?” It applies to how refrigerant flows through piping, how air flows through ducts, how air flows across coils, and how refrigerant flows through those same coils. In short – it applies to pretty much everything in HVACR!

Laminar means in layers. Laminated furniture like you find in most schools has a thin layer, or laminate, on the surface to make it pretty and usually cardboard or glued together sawdust underneath to make it appear substantial. In laminar flow, the stuff on the top stays on the top, the stuff on the bottom stays on the bottom: there is no mixing. In turbulent flow, the fluid swirls all around mixing everything together and creating all sorts of eddies and whirlpools. If you turn on a water faucet that does not have an aerator on it, the stream of water is laminar. The water falls straight down without much mixing. After it hits the sink you have turbulent flow – swirling all around and mixing. Laminar flow is preferred in refrigerant piping, water piping, and ductwork. This is because the eddies, whirlpools, and swirling found in turbulent flow add a great deal of resistance to flow.

For example, take a bunch of people trying to leave a crowded room. If they file out in an orderly line moving in the same direction, they can all get out faster than if there is a lot of pushing and shoving with people going off in all directions and running into each other. To get fluid through a pipe or duct with the least amount of energy loss, laminar flow is the way to go.

High velocities tend to create turbulent flow while low velocities create laminar flow. That is why the resistance to flow increases so dramatically as pipes get smaller. Velocities increase and the flow goes from laminar to turbulent, increasing resistance to flow within the fluid. Its not just that the cross sectional area is smaller - the fluid is getting in its own way as it tires to go through the pipe.

In heat transfer coils such as evaporators and condensers, we need turbulent flow. In heat transfer, all the swirling becomes an asset. Turbulence makes more of the fluid contact the surface of the pipe and slows down the fluid, which helps it transfer more energy. This is why coils have features such as rifling, twisted tubing, or turbulators to encourage swirling and mixing.

The Air First Pledge

I want you to take the “air first” pledge. “I solemnly swear to check the system airflow before I connect my gauges.” If the system airflow is off, the pressures are going to be wrong, so there is no point in connecting your gauges until you know that both the evaporator and condenser are clean and the correct amount of air is moving through each of them. No amount of refrigerant can correct for a dirty air filter, you have to change the filter. I understand that not everyone in the air conditioning business has a tool for measuring airflow. But we are in the AIR conditioning business! Would you trust an electrician who worked without a volt meter? You don’t have to have a flow hood to measure airflow. There are several tools under $300 that do a good job. You can get a Fieldpiece hot wire anemometer, several companies make reasonably priced digital manometers, there are many inexpensive rotary vane anemometers, or you can get a Magnehelic gauge for less than $100. Dwyer sells an airflow meter for less than $50 that reads both velocity and inches of water column pressure. It is not in the same class as the other tools mentioned, but it is a whole lot better than nothing. Why should you invest in a tool that you have been doing without? For one, customers notice when you use instruments instead of guessing. But the best reason is because it makes your job easier. It is always easier to solve problems if you have good data, which you can’t get by holding your hand over the register. How many times have you added refrigerant to a system only to discover later that the coil was plugged up with cat hair? Wouldn’t it have been easier to check the airflow first and correct the real problem? Even if I can’t convince you to start measuring airflow, please at least check the air  filter and check out the airflow with your handomometer before pumping refrigerant into a system that does not need it. Take the air first pledge! 

Its In the Bag!

I have had an eventful two weeks – first in Colorado Springs at the 2013 HVACR & Mechanical conference and next in Las Vegas at the 2013 HVAC Excellence Educators and Trainers Expo. Even though I am very glad to be home I always am energized by these events. The educational sessions are great, put on by industry professionals who are tops in their field. Although I love ogling the latest techie toys, there are often inexpensive ideas and tips that help demonstrate how systems work. For example: you can use a large plastic garbage bag and a stop watch to demonstrate airflow. You flatten the bag, place it over a register, and time how long it takes to fill up. Then a quick calculation gives an idea of the CFM. The formula works like this – there are approximately 7.5 gallons per cubic foot, so a 55 gallon trash bag = 55 gal/7.5 gal/ft³ = 7.33 ft³. The flow in cubic feet per second is determined by dividing 7.33 ft³ by the seconds it takes to fill the bag. That multiplied by 60 gives you CFM. Written out it looks something like (7.33 / seconds to fill bag) x 60 = CFM. You can even do return air by filling the bag and then holding it over a return grill so that it completely covers it. You time how long it takes to collapse the bag. Now I am not recommending this as a means of checking system performance, but it is great for demonstrating exactly what is meant by cubic feet per minute. What I love about this is it demonstrates both the volume by seeing the bag fill up, and the time by clocking how long it takes. After demonstrating the concept of air flow you can show how to measure it properly with accurate instruments. Hopefully, the students will have a better idea of exactly what is meant by CFM and the measurements will mean more to them.

Winter Airflow

Technicians often hear how critical airflow is in the cooling season, but we sometimes forget that it is just as important in heating as well. Just as poor airflow can be a major source of problems in the cooling season, poor airflow in the heating season can cause inefficient operation, high utility cost, and system failures.

Gas and Oil Furnaces
Gas and oil furnaces will operate with a higher than normal temperature rise if the airflow through the furnace is inadequate. While the air leaving the registers will be toasty, this can actually result in less heat being delivered into the house. The limit switch on a furnace with inadequate airflow will typically cycle the burners on and off  - reducing the amount of heat being produced. The cycling causes poorer operating efficiency, increasing the cost of operating the furnace.

Electric Strips
With electric furnaces and strip heaters, poor airflow will eventually result in open fusible links, bad thermal limits, and open heat strips. Like the gas and oil furnaces, the thermal limits may cycle the strips on and off, reducing the amount of heat.

Heat Pumps
Heat pumps with poor indoor airflow will operate at excessive head pressures because the indoor coil is the condenser in the heating cycle. This reduces system capacity by increasing the compression ratio while the compressor is operating.  Eventually, the compressor will cut off on the high pressure switch. Usually, the high pressure switch will need to be manually reset, meaning no compressor heat until the service tech arrives. Systems without a high pressure switch will shut off on the compressor internal overload. Once that happens, the compressor may not come back on for a couple of hours. Either way, the compressor is taking a beating and the system is using more electricity to produce less heat.

So if you run into furnaces that are cycling on the limit, electric strips with open fusible links, or heat pumps with tripped high pressure switches – check out the system airflow. Remember those are all symptoms – not the root cause. Fix the cause of the problem – poor airflow.

Centrifugal Blower Motors

Since air is what we work with it makes sense to insure that our students understand airflow and fan performance. Fan motor performance is one of the most often misunderstood aspects air conditioning systems. The amp draw on a centrifugal fan with a standard AC inductive motor goes down as resistance to airflow is increased. For most people this seems counterintuitive. It is easy to picture the fan motor pushing harder to overcome the resistance and increasing in amp draw. However, this is exactly backwards. Centrifugal fans move air by throwing the air outwards through centrifugal force. The amount of air the fan is moving decreases as the resistance to airflow increases. If the fan blades are moving less air, they can actually spin easier because there is less air to sling. This causes the motor RPM to increase and the motor amp draw to decrease.

The most convincing way to teach this concept is to have students figure it out for themselves using a centrifugal blower. Have them operate a centrifugal blower in free air with no restriction and measure both the amp draw and the fan RPM. Note that most centrifugal blowers cannot operate in free air for an extended time without overheating, so try and keep the free air operating time to a minimum. Next have them block one side of the air intake with a piece of cardboard and recheck the amp draw and RPM. Typically the increase in RPM is immediately obvious, but measurements prove the point. Have them slide the cardboard to block the intake only half way while watching the amp draw. A few minutes of experimentation will convince the students that blocking the intake actually causes an increase in RPM and a decrease in the motor amp draw. Next have them partially block the fan outlet while checking the amp draw. Once again, the amp draw will decrease. Allow them a few minutes of play time to convince themselves. This experiment does more to explain centrifugal blower motor performance than a week’s worth of lectures.

Although the noise and increased air velocity make it seem like the fan is actually moving more air, the truth is that it is moving less. The air that it IS moving is traveling at a very high velocity, or speed. This is what makes the increased noise. But since you are effectively making the hole that the air travels through smaller, less air is able to get through, even at higher velocity. This is more difficult to show. To get accurate readings, you really need the air to be moving through some ductwork.

Another point to discuss is the difference between a blower with a traditional PSC motor and one with an EVM blower. The behavior we have been discussing is typical of an AC induction motor, like the PSC motors that come standard on most blowers. However, an ECM blower motor senses the change in work and increases its speed enough to actually overcome the resistance, so that the fan moves the same amount of air even against increased resistance. Since this requires more electrical energy, the amp draw for an ECM blower will increase when the fan is restricted. The ECM technology solves one problem: losing airflow due to increased resistance. But is creates a new one: increased electricl use to overcome the resistance. 

To read more, check out Unit 41 Fundamentals of Psychrometrics and Airflow and Unit 75 Fans and Air Handling Units in Fundamentals of HVACR 2nd edition. You can find the blower labs in the new Lab Manual for Fundamentals of HVACR, 2nd edition. They are labs 75.3 AC Induction Motor Blower Properties and lab 75.4 ECM Blower Properties.

Desired Air Conditioning Temperature Drop

I was recently asked for a formula to determine the temperature drop between the return air and supply air of an air conditioning system. While it is logical to check the temperature drop, determining exactly what it should be is not as simple as plugging in readily available numbers into a formula. Two operating conditions can have a pronounced effect on the results: the relative humidity of the return air and the amount of airflow. Most air conditioning systems condition the air two ways. They cool the air, referred to as sensible cooling; and they take water out of the air, referred to latent cooling. Only sensible cooling creates a temperature drop. Removing water from the air takes system capacity. The more water the system removes from the air, the less capacity is left for reducing the air temperature. Standard airflow is 400 CFM per ton for most systems, but that does not mean your system is actually operating at 400 CFM per ton. If you move less air across the coil, the air will be cooled a little more. To determine the temperature drop you must know the outdoor ambient temperature, the return air dry bulb, the return air wet bulb, the CFM of airflow, and the system’s sensible cooling capacity at that condition.

Take for example, a system that is removing no water out of the air operating at 100% sensible cooling with a standard 400 CFM per ton of airflow and producing 12,000 Btuh per hour. The temperature difference is calculated as TD = 12000/(400 x 1.08) = 28°F TD. If the airflow is reduced, the TD becomes 12000/(350 x 1.08) = 32°F. Increasing the airflow would make the TD 12000/(450 x 1.08) = 25°F. In humid climates, the latent capacity can easily be as much as one third of the total capacity, reducing the sensible cooling capacity to 8,000 Btuh. These same airflows would then give TDs of: 8000/(400 x 1.08)=19°F, 8000/(350 x 1.08)=21°F,8000/(450 x 1.08)=16°. In reality, these TDs would be a little off because the overall system capacity would be a bit less with the decreased airflow and a bit more with increased airflow. The system capacity will also change depending upon the outdoor ambient. The 12.000 Btuh per ton is a nominal figure based on the AHRI rating condition of 95°F outdoor ambient, 80°F indoor dry bulb and 67°F wet bulb.

You can try to account for duct gain by reducing the expected TD by some amount: say 3°F - 5°F. However, it is really difficult to use TD at the registers because the duct gain from one system to another can vary a lot. Ducts in the attic will pick up more heat than ducts in a crawl space. Duct leakage also has a big effect. If 10% of the air entering the coil comes from a 150°F attic, that obviously will affect the delivered air temperature. The rule of thumb people have used for many years is a TD of 15°F to 20°F across the coil, not at the registers. Looking at the above calculations, you can see where this comes from. However, it is also easy to see how little you actually know if you don’t really know all the operating conditions, the system airflow, and the system capacity at those conditions. If all you do is measure the return and supply air temperatures at the registers, you don’t really know much.