What is Air-Free CO

If you have used a combustion analyzer or CO detector, you may have come across the term “air-free  CO reading.” A common question is “What is the difference between the regular CO reading and the air-free CO reading?” You may have noticed the air-free reading is always higher than the “regular” reading. Basically, the “air-free” reading is calculated to determine what the CO concentration in flue gas WOULD BE if all the excess air were removed. CO readings tell how many parts per million of the sampled gas are CO. If you add a bunch of gas that  was not part of the combustion process, it dilutes the CO reading because now all those other gas “parts” are being counted, even though they had nothing to do with the combustion processs. All gas appliances are designed to operate with some excess air. Excess air is left over in the flue gas after the combustion process – thus its name; excess air. The purpose of excess air is to insure adequate combustion air by providing more than is needed. Lack of combustion air leads to CO production, so having more than enough air helps reduce CO levels. Older, natural draft furnaces have higher levels of excess air than induced draft burners; but all should have some excess air in the flue gas. Excess air is necessary for safe operation. The problem is that the excess air dilutes the flue gas, lowering the CO reading. The air-free calculation corrects for this dilution effect.

Basically, the CO reading is multiplied by the ratio of the atmosphere’s oxygen percentage (20.9)  to the excess oxygen percentage. The formula is
Air-Free CO  ppm = Measured CO ppm x (20.9 / (20.9 – O2% in flue gas))
 So if the measured CO ppm is 50 and the measured oxygen in the flue gas is 10.5%
Air-Free CO = 50 x (20.9/(20.9 – 10.5)) = 100 ppm (approximately)
If the CO reading is exactly the same, but the O2 reading is 14%
Air-Free CO = 50 x (20.9/(20.9-14))= 150 ppm  (approximately)
The second furnace is producing much more CO than the first, but the CO meter reads the same because the extra excess air in the second furnace has diluted the flue gas. This is why you should use air-free readings whenever checking flue gas CO levels. Most digital flue gas analyzers will do this for you. If you are using the old hour glass bubblers, you will need to do the math yourself.  

For a more detailed look at air-free CO and carbon monoxide in combustion gas, take a look at this article by Richard Karg http://www.karg.com/pdf/coairfree_article.pdf

It is actually about ovens, but the processes and science are relevant. 

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.

HVACR Language

New students can easily be confused by HVACR techno-speak We have our fair share of acronyms in HVACR. Used properly; acronyms speed up communication by reducing long polysyllabic phrases to a few letters. For example, speakers would get a bit winded if they used the phrase “heating, ventilating, air conditioning, and refrigeration” repeatedly in a conversation. The abundance of industry specific acronyms and the use of more than one acronym for the same item can be truly bewildering to students. For example, one major valve manufacturer uses the acronym TEV to represent “Thermostatic Expansion Valve,” while another uses the acronym TXV for the same thing. They do not represent two different types of components, just two different ways to abbreviate the same words.

Try to avoid using acronyms when introducing new material. Teachers who have had years of experience with HVACR techno-speak often forget that the acronyms we casually throw around often have no meaning for our students. Take the following sentence for example: “The evaporator delta T is controlled by the TEV adjustment, the return air wb, and the CFM.” Now I believe that most any air conditioning instructor understands this sentence, but it is essentially unintelligible to many air conditioning students. It might as well be in Russian! Replacing the HVACR specific jargon with the wingdings font it looks like
“The evaporator deltaT is controlled by the TEV adjustment, the return air wb, and the CFM.”
THAT is what this sentence looks like to a new student!

In general it is a good idea to resist using an acronym for something until that item or process has been discussed. Otherwise, the acronym will appear to students to be a mysterious grouping of letters used by air conditioning shaman to communicate with each other. The acronyms and jargon become a secret language which they are not familiar with. I believe that acronym free language promotes better understanding.

An acronym should first be explained and defined before it is used. It is much easier to remember an acronym if you understand what the letters stand for. When using an acronym for the first time make sure and explicitly spell out what the letters represent, this will increase student’s understanding and retention of the term. It also helps for the students to have a good mental picture of the object or process being described. They are far more likely to remember what Delta T stands for if they have actually measured a temperature difference and you have discussed it in class. That way the abbreviation is not an esoteric piece of jargon attached to something they don’t understand, but a name for something they have done. In Fundamentals of HVACR, we always use the complete word or phrase before introducing an acronym. It helps to explain concepts plainly. After introducing the concept, we give the technical terminology that is used to refer to the concept. The students are more likely to remember the terminology if it is logically connected to something they understand.

Four Freedoms

I have been enjoying a thanksgiving weekend with my family. We spent a day looking at all the wonderful paintings in the Art Institute of Chicago. We saw the original Norman Rockwell painting titled “Freedom from Want.” I am sure you have seen it reproduced many places – a family is seated in anticipation of a thanksgiving meal and the turkey is just being placed on the table. It is one of a series of four paintings Rockwell did – each portraying an essential human freedom we often take for granted. They are Freedom of Speech, Freedom of Worship, Freedom from Want, and Freedom from Fear. I realized that I enjoy these on a daily basis, but seldom stop to give thanks or even realize how privileged I am to enjoy them without a second thought. So this Black Friday I plan to take some time out from chasing after the next great deal to give thanks for the basic human freedoms I enjoy daily – and thank God I have the freedom to do it.

Layered Learning

One common problem with teaching and learning HVACR is that there is just so much material to master. It is easy to drown the students in details if you are not careful. When I was using a text which presents enthalpy diagrams in its early chapters on basic refrigeration, students would come up to me and ask for drop slips after reading the chapter. They looked at the PH diagrams and concluded that they just were not smart enough for HVACR. It was just way too much information for them to comprehend while they were still wrestling with refrigerant boiling while it is cold! The real skill in presenting HVACR subject matter is to take inherently complex material and break it down into a series of comprehensible lessons, without leaving out material. Presenting information in stages, at a rate which most students can more easily digest is preferable to feeding them the whole enchilada at once and causing cognitive indigestion. You can still reach the peak, but you have to get to base camp first.

Electricity is a perfect example. We start with the basic concepts of voltage, current, resistance, and simple circuits. If you think of a simple balance beam with voltage in the middle – if resistance goes up, current goes down. Students can in fact understand how current, voltage and resistance are related without using any formulas. Similarly, you don’t have to bring in Ohm’s Law to understand the concepts of source, path, and load. In fact, I believe circuits are best approached at first without discussing Ohm’s Law. Students can build circuits and operate them to get a mental concept of how switches  and loads behave. They can even build series and parallel circuits to see how they differ: all without formulas. Then AFTER the students understand the basic concept of an electrical circuit, you introduce Ohm’s Law as a mathematical description of a circuit. Now they have something real to relate to the parts of the formula and it is less abstract. You can even build circuits with heaters and take measurements to demonstrate how Ohms Law works.

After students have learned to solve Ohm’s Law problems, then we tell them that Ohm’s Law does not work in most AC circuits. At first, we don’t discuss the effects of alternating current, inductive, and capacitive reactance. It is just too much to take in at once. However, since these concepts have far more to do with most air conditioning circuits than Ohm’s Law, we can’t really afford to ignore them. The effect of inductive reactance can be demonstrated by operating a small motor in a circuit and comparing its resistance, current, and voltage. You can have students build circuits that demonstrate the effect of inductive and capacitive reactance and how they relate to each other. Depending on how much AC electricity you teach, you can put an oscilloscope on the circuits to show what happens in capacitive and inductive circuits (the teacher would be doing this.) Finally, you can bring in reactance and impedance calculations, although I must admit we do not have our Air Conditioning classes at Athens Tech doing LCR calculations. It’s a little like boiling a frog – you do it a little at a time. Before the students know it, they have learned some complex subject matter that would have sent them running for the exits if it had been presented all at once.

Electricity for HVACR

Pearson has released a new Electricity book by Joe Moravek titled "Electricity for HVACR." It shares the same overall structure as my book - lots of pictures and drawings with easy to follow text. It has short units and easy to follow organization. I have not read it end to end yet, but I have looked through several chapters - especially the ones on motors. I am impressed with what I have seen. If you are looking for a text to cover the electrical aspects of HVACR, you might take a look at this new book. You can  learn more about it at the Pearson web site CLICK HERE

Furnace Drains

One important aspect of condensing furnace installation is sometimes overlooked – the furnace drain. A 90% condensing furnace makes a lot of water – comparable to an air conditioner evaporator. All that water needs somewhere to go. In most installations, there will also be an air conditioning evaporator and a drain for it. It is natural to just run the furnace drain into the air conditioning drain, but that can lead to problems. The positive air pressure inside the evaporator cabinet can create a slightly positive pressure on the evaporator drain. If the furnace drain is connected directly to it, this positive pressure can push back on the furnace drain. On some furnaces, this can create a positive pressure at the vent draft switch, causing it to open and shut off the furnace. One solution is to run completely separate drains for the furnace and air conditioner. If the air conditioner and furnace are connected to a common drain, you need to leave an air gap between the furnace drain and the common drain so that pressure cannot back up into the furnace drain and vent. If you are using a condensate pump, make sure it is rated to handle furnace condensate. Because furnace condensate is slightly acidic, some condensate pumps are specifically NOT approved for use with furnace condensate. In case you are concerned about the acidity, you can breathe easy. It is about the same as a carbonated beverage. However, some codes early on required that the condensate from condensing furnaces be neutralized before going into a sewer system. So manufacturers produced acid neutralizing filters which were basically large PVC containers filled with rocks (usually limestone or marble). The acid reacts with the rocks, which have a basic ph that counters the acid. I don’t believe most places require this any longer, but I am not certain. Another issue to watch is the drain outlet. If a gravity drain ices up or is blocked with snow, the draft switch can trip and shut off the furnace. Also, remember that the furnace really does make a lot of water. Try not to dump this water in a place that will cause problems – like right over a sidewalk.

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.

Gas Furnace Firing Rate

How do you know the rate at which your furnace is actually firing? You really can’t assume that it is firing at the manufacturer’s published rate just because the manifold pressure is at 3.5”, even if that is what is specified on the nameplate. To determine the Btuh firing rate of your furnace you must first determine the cubic feet of gas per hour being burned (CFH) and multiply that times the heat content of the gas. To determine the CFH, you need to clock the gas meter. First, you want to turn off all other gas appliances. Then clock the number of seconds it takes for the smallest dial on the gas meter to make one revolution. For more accurate results, clock the time for two or three revolutions. The CFH can be calculated by dividing the time clocked into 3600 (the number of seconds in an hour). Multiply this answer times the value of the dial. If you clocked for multiple revolutions, multiply by the number of revolutions. The formula looks something like this

(3600/time clocked) x dial value x revolutions = CFH
So if the a 1/2 ft3 dial took 90 seconds to make three revolutions
(3600/90) x 1/2 x 3 = 40 x 1/2 x 3 = 60 CFH

Next, you need to know the heat value of your gas. You get this from your local gas supplier. If you can’t get that you can get a good estimate from the EIA. The US Energy Information Administration (EIA) has a table online that shows historical values for every state. GAS HEATING VALUES.

In our example, if the furnace is in Mississippi in 2011, the gas heat content is somewhere around 1010 Btu/ft3. So our firing rate would be 60 ft3/hr x 1010 BTU/ft3 = 60,600 BTU/hr

The same results in West Virginia in 2011 would mean a firing rate of 60 ft3/hr x 1083 BTU/ft3 = 64,980 BTU/hr

If the furnace in West Virginia is rated for 60,000 Btuh, you would need to reduce the manifold pressure slightly to reduce the firing rate. Another complication is altitude. Furnaces need to be de-rated approximately 4% for every 1,000 ft of altitude. In West Virginia, it is quite possible to be 3,000 ft above sea level, necessitating a 12% reduction in firing rate. So the new firing rate should be 60,000 Btuh x 88% = 52,800 Btuh. Now you are looking at firing a furnace at 64,980 Btuh which should only be fired at 52,800 Btuh. You most likely will need to change orifices as well as adjust the manifold pressure. After making any changes to the manifold pressure or orifices, make certain to clock the meter again and recalculate the gas firing rate to insure your adjustments brought the firing rate into line. Over firing a furnace can increase the production of CO in the flue gas. In extreme examples, it can cause soot formation.  

Ghost Voltages

I know everyone who has used a digital meter has taken some measurements that don't always make sense. I believe we just learn to ignore them, recognizing the "real readings" from the junk. But when students see the odd readings, they become confused. Here is a great article by Fluke explaining what causes strange voltage readings when there really is no voltage.

GHOST READINGS

Natural Gas Furnace Firing Rate

Do you know the heat content of the natural gas supplied to your area? Although we often use the nominal value of 1000 Btuh per cubic foot for natural gas, the heating value is really a bit higher in most places. A look at a table by the U.S. Energy Information Administration, EIA, shows that only a few states have natural gas with a heating value close to 1000.Gas Heating Values In 2011, average gas heating values ranged from 1004 in Nebraska to 1076 in West Virginia. Furnace manufacturers must choose a heating value when they select the orifices to put in the furnace. Typically, they use a value on the higher end of the range, 1075 BTUs/ft3. A furnace with the factory supplied orifices set up at the same manifold pressure in each of these states would deliver a different amount of heat. Assuming they were all installed at sea level in their respective states, they would burn the same volume of gas because they would have the same manifold pressure and the same orifice size. A furnace that burns 100,000 Btuh in West Virginia would only be firing at 93,000 Btuh in Nebraska. Small adjustments in the firing rate can be made by adjusting the manifold pressure. However, it is often necessary to change the burner orifices to get the manufacturer’s listed firing rate.

Higher altitudes can make de-rating a furnace necessary because the lower pressure, less dense air just does not have enough oxygen in it to support the full capacity of the unit. In general, furnaces are de-rated 4% for each 1,000 ft of altitude. Some manufacturers provide tables showing what orifices and manifold pressure should be used depending upon the gas heating value and altitude. The point is that setting up a furnace for the correct firing rate involves a little more than simply adjusting the manifold pressure to 3.5" wc using the manufacturer supplied orifices. You may need to change the orifices and/or adjust the manifold pressure to something other than 3.5" wc. Below is an example from one manufacturer. Note that this is an example - it does NOT apply to all furnaces.

Gas Heating Value	Sea level - 2000		2001 - 3000		3001 - 4000		4001 - 5000
	Orifice	Man press	Orifice	Man press	Orifice	Man press	Orifice	Man press
975	44	3.3” wc	44	2.8” wc	44	2.6” wc	47	3.5” wc
1000	44	3.2” wc	44	2.7” wc	44	2.5” wc	47	3.3” wc
1050	44	2.9” wc	44	2.5” wc	48	3.7” wc	48	3.4” wc
1100	46	3.3” wc	48	3.7” wc	48	3.4” wc	48	3.7” wc

Checking Gas Pressure

Heating season is upon us and it is time to get out your gas pressure measuring instruments. What do you use to check gas pressure on natural gas furnaces? A popular tool that I see a lot is probably the least accurate available. Many techs use the diaphragm gauge gas pressure test kit which comes in the blue plastic box. But these can’t read anything below 2” wc, making them useless for the low fire stage of a two stage furnace. Even 3.5” is in the first part of the scale, not in the sweet spot. If you are testing natural gas and feel that you must use one of the diaphragm pressure gauge gas test kits, at least get the model that puts 3.5” wc more towards the center of the scale. Then you can actually read a pressure of 1” wc. The reason many people don’t choose the gauge which is more appropriate for natural gas is that it does not read high enough for propane. So they buy the gauge that was really designed to read propane gas pressure and use it for both. That sounds reasonable, until you try to read gas pressure on a two stage natural gas furnace using one of those gauges and cannot get a reading.

For an inexpensive gas manometer, an actual water column manometer is a far better choice. I like the straight tube water manometers that Yellow Jacket sells. They are actually cheaper than their gauge style gas pressure test kits and more accurate. No, they are not quite as easy to use – you have to put water in them and learn to read the bubble in the center tube, but that is not too difficult. For a little more money you can buy a U-tube manometer. If you want both convenience and accuracy, a digital manometer is the thing to get. They are considerably more expensive, but also considerably more accurate and convenient. An added bonus is that you can also use most digital manometers to read duct pressures – so the tool can serve more than one function.

Why should you worry about how accurately your manometer reads the gas pressure? Because you cannot insure the furnace is operating safely at the correct firing rate and efficiency if you don’t really know what the manifold pressure is. Checking furnace operation by just seeing blue flames is similar to checking the charge of an air conditioning system by just feeling to see that the suction line is cold. You can see that it is operating, but you really don’t know it is operating as designed. Incorrect gas manifold pressure can lower system operating efficiency and increase the operating cost. Under fired furnaces will lose capacity, over-fired furnaces can be a safety risk due to increased CO output. So do yourself and your customers a favor and get a tool that will allow you to take an accurate manifold pressure, even on low fire.

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.

2013 RSES Annual Conference and Technology Exo

Are you planning to attend the 2013 RSES Annual Conference and Technology Exo? 

It takes place October 23 – 26, 2013 at the Sheraton Station Square Hotel in Pittsburgh, Pennsylvania. The Refrigeration Service Engineer’s Society is dedicated to advancing the knowledge and skills of the HVACR professional.  And professional is definitely the correct word to describe the people you will meet there. The conference is a wonderful opportunity to network with industry professionals. You will also meet many instructors and trainers. After all, RSES is all about advancing skills and knowledge, which is what teachers do. There are more educational sessions than you can attend – so you have choices of which ones to see. Here is a link to the educational sessions. 

When you go, be sure to see David Skaves, my writing partner and co-author of “Fundamentals of HVACR” published by Pearson. He will be discussing how the emphasis on STEM education can benefit the HVACR industry. In case you are not familiar with the acronym STEM, it stands for Science Technology Engineering and Math. HVACR fits in quite nicely. (No, David will not be giving a two hour lecture on valve stems). 

There is also a great Technology Expo where companies display their latest and greatest and you have an opportunity to see all their great products. It is really fun to go to, wander around and talk with people. Of course, David will be there at the Pearson booth. Stop by the booth and say hello. David is a great guy with a wealth of interesting experiences. Truthfully, that is typical of the people there: professionals with a wealth of experience in the HVACR industry. 

Want to find out exactly how much you know about HVACR? You can take a whole range of professional exams at the RSES conference. I believe the  RSES CM and CMS exams are the most difficult professional exams I have ever taken - they will challenge you. They are very reasonably priced - so you can afford to take several if you like. 

Do yourself a favor – take in the 2013 RSES Annual Conference and Technology Exo and see what RSES has to offer. You belong. 

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