Author: Bryan Orr

Bryan Orr is a lifelong learner, proud technician and advocate for the HVAC/R Trade

For those of you who use the MeasureQuick app for system diagnosis and performance testing, you may have noticed the “fan efficacy” results and wondered what it is.

It is simply the CFM output of the system divided by the wattage used by the blower. It is only for the blower motor and has nothing to do with the other components when done properly.

Fan (blower) efficacy is called out in various codes and standards such as California Energy Commissions requirement that all blowers perform at or below a 0.58 fan efficacy. This means a blower that is moving 1000 CFM cannot use more than 580 Watts of power to do so.


The tricky part is measuring fan efficacy is getting accurate measurements of system CFM and blower amperage. Equipment manufacturer fan charts can be used along with an accurate TESP (total external static pressure) measurement to figure out the CFM when the system is new and clean. When using these charts it’s important that the system is setup and run according to what is shown on the chart, one wrong pin setting or input can lead to vastly different airflow than the chart shows resulting in a fan efficacy that is way off.

Other options like measuring airflow at the return with a hood, anemometer duct traverse or the Trueflow from TEC can be used for measuring system CFM, but all have their own challenges.

Blower Wattage

When measuring blower amperage the panels must be in place which can be difficult to accomplish on some system types making a wireless connected ammeter very handy where the meter can be put in place and the panels put back on for testing.

Traditionally techs calculate wattage by measuring voltage and amperage and multiplying them together. This is actually VA not Wattage becasue it does not account for power factor. The only way to accurately measure wattage is to use a watt or power quality meter like the Redfish IDVM550 which calculates wattage by multiplying the VA by the power factor for the final wattage.

ECM Motors

ECM (electronically commutated motor) motors are more efficient than traditional PSC motors but their efficacy will generally vary based on the static pressure they are subjected to. Becasue most ECM motors are either constant airflow or constant torque rather than constant speed they will increase in wattage as the static pressure increases. This means that the fan efficacy will decrease on these motors as filters and coils become dirtier.

— Bryan



I’ve been reading a book called “Cool, How Air Conditioning Changed Everything” and it got me interested once again in the history of air conditioning and refrigeration. Like many things the people who are credited with “inventing” are the ones dogged enough to make an idea commercially successful, not the idealists forever tucked away in the lab.

I bought a 1921 version of the periodical “Ice and Refrigeration” and mixed in with the ads for absorption ice machines and “mineral wool” insulation was the advertisement shown above. Willis Carrier understood how to connect ideas and make sense of emerging technology, first to keep paper dry in a factory and later to cool the world with “Manufactured Weather”. Look carefully at the ad, you will notice it mentions many things… but not cooling, the ad is in ICE AND REFRIGERATION and the ad doesn’t mention COOLING.

Many of you know that in 1906 Willis Carrier patented what is now referred to as the “First Air Conditioning System” but do you know what it was that he actually invented?

You may be led to believe that Willis Carrier invented compression refrigeration? Nope, the first commercial attempts at compression refrigeration began in the 1830’s and the patent above actually has no compression refrigeration in it whatsoever. Many will say that he was the first to dehumidify the air, this is also false, there had been compression refrigerated cooling coils in use that dehumidified the air before Willis came along they just didn’t do it on purpose.

What Willis Carrier understood better than anyone else in his day was the RELATIONSHIP between humidity, temperature and saturated air or “dew point” and how to manipulate water temperature, water volume and air volume to produce a CONTROLLED humidity environment first and later a controlled temperature, humidity, and ventilation environment.

The Carrier “Air Washer” was nothing more than water pumped through nozzles that produced a mist of water. The air would blow through the water mist and it would clean the air, drop it to dew point (100% RH) and then continue to sensibly cool the air. Willis worked in northern states with cold groundwater at a time before water use restrictions so the cold water would serve to cool AND dehumidify the air. At the time it seemed like black magic that running air over water could REMOVE water from the air, but so long as the water temperature was below the dew point temperature of the air that is exactly what would happen. All Willis had to do to change the dehumidifier to humidifier was to increase the water temperature or change the dehumidifier to a sensible cooling machine was to use cold water and give the air more dwell time or passes through the water to decrease the sensible temperature.

In the process Carrier and his team made many discoveries about air and in 1911 Carrier presented possibly his greatest work which he called the “psychrometric formulae” which is the founding document on which all of current understanding of psychrometrics is built. Carrier took a VERY SIMPLE idea and pursued it and understood better than the others around him and because of that, we remember him today. He thought about cooling, heating, ventilation, humidity and air cleanliness and combined them together into one machine that controlled it all.

Later on, Carrier would begin actively “cooling” the air with compression refrigeration and replaced water sprays with refrigerant evaporator coils to leverage the latent capacity of refrigerants, but it all started with a mist of water an understanding of dewpoint, some dogged determination and some clever marketing for his “manufactured weather”.

— Bryan

To find the catalog where I found some of this information I created a link to the national archives at




Photo Courtesy of Emerson

What is Cascade refrigeration?

Cascade refrigeration is a term you will hear more and more over the coming years, and while some of the systems may be very complex, the concept is actually pretty simple.

Some refrigerants are well suited for high and medium temperature applications, and some are better suited and for a lower temp applications. In a cascade system the high/medium temp refrigerant circuit is used to cool the condenser of the low temp circuit by way of a heat exchanger.

In essence, the condenser for the low temp system is also the evaporator or part of the evaporator of the high/medium temp system.

In the diagram above the medium temp circuit is used in the medium temp cases and is ALSO used in the heat exchanger to condense the refrigerant in the low temp circuit.

There are many reasons for this type of system but one of the big reasons is it is a practical solution for using CO2 (R744) as a low temp refrigerant.

— Bryan


Airflow, Airflow, Airflow…. when we setup and commission comfort cooling and heating systems we need to pay more attention to airflow before we worry about the fancy controls or the refrigerant circuit.

So as a thought exercise let’s consider a typical 2-ton, straight cool, TXV, residential system and think through what happens when we alter airflow and what impacts that has on the system.

Rather than talk in terms of advanced psychrometric math we will keep the math to a minimum and focus on “If this than that” relationships between airflow and system function

Mass vs. Volume

First let’s establish that it is the molecules or “stuff” that makes up air that contains and can move heat energy. While we often talk in terms of CFM (Cubic Feet Per Minute) that is a measurement of volume rather than mass. The air conditioner cares about the mass flow of air over the coil not the volume flow which is why more airflow in CFM is required in high altitudes where air density is lower.

In other words…

Mass flow is what matters and when air get’s less dense we need more air volume to move the same amount of heat

So when we speak in terms of CFM/ton (Cubic feet of air per ton of cooling) that is referring to typical air at sea level and needs to be adjusted as air density changes.

400 CFM/Ton 

The 400 CFM/ton design has been used for years and it is an adequate baseline airflow for many types of equipment and in many moderate climate zones. There are several issues with the 400 CFM/ton rule where it needs to be adjusted.

  • Higher altitudes where air is less dense and therefore more air is required to maintain the same mass flow rate over the coil
  • The nominal or listed tonnage on a piece of equipment is often NOT what the equipment produces at current load conditions. A 2-ton system that is designed for AHRI conditions (95° outdoor and 80° indoor return temperature) could easily produce under 20K btu/hr at 73° indoor and 97° outdoor temperatures, so 800 CFM would be well over 400 CFM/ton in that scenario.
  • Areas with higher latent (humidity) load will run lower than 400 CFM/ton on purpose to remove more moisture from the air and areas with arid (dry) climates will often run higher than 400 CFM/ton to remove less or no moisture from the air.

How The Evaporator “Absorbs” Heat

In my refrigeration circuit basics training I call the evaporator coil the “heat absorber” because its end goal is to take heat from where you don’t want it and move it somewhere else.

The heat gained in the evaporator in this scenario comes from the indoor air being moved over the evaporator coil. The air is warmer than the refrigerant so heat leaves the air as it impacts the tubing and fins of the coil because “hot goes to cold”.

The heat is transferred from the air though the walls of the copper tubing and into the refrigerant via conduction while the heat is transferred through the air and refrigerant itself via convection because they are both dynamic (moving) fluids.

The air temperature is decreased because heat is removed from it into the refrigerant. The refrigerant in the evaporator coil is at saturation (boiling) so the coil temperature doesn’t change directly as heat is added to the refrigerant but it does begin to increase indirectly because as the total heat energy in the evaporator increases so does the coil pressure and vice versa. This is similar to the pressure cooker effect where as the water boils in the pressure cooker the pressure increases and so does the boiling temperature of the water.

When the temperature of the coil is below the dew-point of the air moving over it there is also a transfer of latent energy from the air as some of the water vapor in the air condenses to liquid water (condensate) on the evaporator coil. This latent heat transfer does not result in colder air but rather lower moisture content in the air, this heat does impact the evaporator in the same way as sensible heat as it is added to total heat picked up in the evaporator.

Evaporator Coil TD

We use the term “coil TD” a bit differently in different parts of the industry but in air conditioning it is the difference between the air temperature of the return air entering the evaporator coil and the saturated suction temperature often called the “coil temperature”. In typical 400 CFM/ton applications this difference will be around 35° with a higher number meaning a colder coil and a lower number meaning a warmer coil. There are several things that can impact coil TD including refrigerant mass flow rate (how much refrigerant the compressor is moving), metering device performance, return air dew point (moisture content) and most commonly…. airflow.

What Happens When Airflow is Decreased?

In this theoretical system when the airflow is decreased and all else stays the same the following things will occur –

  • Mass airflow will decrease, meaning there are fewer molecules moving across the coil
  • Air velocity will decrease, meaning the air is moving over the fins and tubing more slowly
  • Bypass factor decreases, this means more of the air molecules will be touching the metal as a ratio
  • Air temperature decreases (to a point) due to the air moving more slowly across the coil with less bypass factor
  • Coil temperature decreases because less overall heat is being picked from the air
  • Coil drops further below dewpoint, causing more moisture to be removed from the air increasing dehumidification
  • Suction pressure decreases because less heat energy being picked up means less pressure and as the superheat falls the TXV also futher throttles the flow of refrigerant through the coil
  • Compression ratio increases as the suction pressure drops meaning the compressor moves less refrigerant as the refrigerant density entering the compressor falls
  • Coil TD increases as indicated by the colder coil in relationship to the return air

We all know that if you have far too little airflow a system can freeze up when the coil temperature drops below 32°F. The other consequence of dropping airflow is lower overall sensible capacity and therefore a drop in EER and SEER rating. On the positive side in humid climates, a system with lower airflow will remove more water from the air which can be desirable.

The lesson is, sometimes you need more airflow and sometimes you need less but no matter what, changing airflow changes a lot about how the system operates and should be done carefully and thoughtfully.

— Bryan



Connecting more than one wire on or under a single lug or connection point is called “double lugging” and it is ONLY allowed in line voltage wiring under one condition according to NEC 110.14

If the terminal, lug or connector is specifically rated for more than one wire

In the case of a conductor splice like a wire nut or a split bolt, they are only designed for 2 wires unless they specifically state otherwise on the box on the connector itself or in the instructions / product data.

This means that wiring in a surge protector under the same lugs as the main, or jamming as many wires as you can make fit under a split bolt or wire may be common, but it is not allowable according to NEC 110.14

HVAC techs and installers will often double lug contactors when making a repair, or they will connect to the closest, easiest point when installing a 120v or 240v accessories like a UV light , humidifier or air purifier.

In all of these cases is is best to take a few minutes and find an approved and permanent method of making the connection instead of taking the easy way out.

It is also worthwhile to mention that some connections are rated for copper only and will be marked CU while others designed for aluminum will be marked AL or ALR. Some will be marked as CU / AL which means that either copper or aluminum may be used but not necessarily that copper and aluminum may be MIXED.

There are very few connection that allow the mixing of copper and aluminum and if they do they must be specifically listed for that purpose.

— Bryan


This article is written by my good friend Neil Comparetto, one of the all-around best dudes in the industry and a guy who practices what he preaches on duct design. Thanks Neil!

These are some fundamentals for designing and installing duct systems that I’ve learned over the years. Included are links to some great resources if you choose to dig a little deeper.

#1. Lower the air velocity and static pressure will follow. 

The total equivalent lengths (TEL) for fittings in Manual D is based on 900 feet per minute air velocity. When the velocity is lowered, friction is lowered, hence TEL is lowered. This means you can “get away” with some lower-performing fittings if the velocity is low.

In ACCA’s Manual D appendix 15 in the conclusion it states “There are scores of things to worry about when designing and installing a comfort system. Low velocity through a duct system is not one of them.”

John Semmelhack, owner of the building science firm Think Little in Charlottesville, VA, proved this to me when I installed one of his designs. We conditioned a large area with a small low static ducted mini-split without any “high performance” fittings. Basically, he increased the duct sizes to above what’s “normal” to make sure velocity doesn’t go above a set value. (We’re not talking about increasing the duct sizes a lot, one or two sizes bigger than you’re used to.) The total external static pressure was .17” WC. When we commissioned the system every register delivered design airflow.

This article by Allison Bailes goes into way more detail on the topic. https://

#2. The largest “duct” in the house is the house itself. 

What connects the air coming out of the supply registers to the air going into the return grille? The house. It’s a big duct. Another reason why it’s difficult to provide comfort solutions without looking at the house as a system. In a battle between HVAC and leaky poorly insulated house, the house always wins. (And the people lose.)

This article by David Richardson is one of the reasons I started to take the “house is a system” approach.

#3. The closer the duct is to the blower the more important it is. 

This is where pressures in the duct system are the highest. Low-performance fittings create higher pressure drops (compared to lower pressure parts of the duct system), “system effect” can come into play, and any duct leakage will be intensified.

It’s important to have larger, high-performance fittings, with as much of a straight section as possible entering and leaving the equipment. I touch on this again in rule #13.

#4. Return location doesn’t matter as long as pressure imbalances aren’t created. 

“Add a return” is the go-to move for some to solve comfort issues. But if the equipment is moving the design airflow and the room does not have a pressure imbalance adding a return will not change anything. Supply registers condition air and create room air currents, not returns. What’s important is that return air has a free path back to the return grille, not the grille’s location.  

#5. Air is a fluid, ducts should not leak. 

Duct leaks in unconditioned spaces will cause outside air to enter into the conditioned space. This is an energy and IAQ penalty, I’ve heard Nate Adams call it the “double whammy”, basically you pay for it twice. (Once to condition the air, then again to re-condition the outside air.) More on duct leakage in this tech tip

Even if the ducts are in conditioned space they still shouldn’t leak. It is not efficient or high performance having an air leak in unintended spaces. You will not be able match room-by-room design airflow if x amount of air is leaking behind walls.

This study by Comfort Institute (owned by Aeroseal) was conducted to prove leaky ducts in conditioned space matter.

#6. Install balancing dampers to regulate airflow. 

Without balancing dampers it’s difficult to balance airflow… Some rely on design software to create a “self-balancing” system. Personally, I haven’t seen this work other than accidentally.

Our preference is to use registers with opposable blade dampers. This makes balancing a lot faster and easier. On top of that, they are always accessible, and they don’t leak air like typical takeoff manual dampers do.  

#7. Increase filter surface area to lower pressure drop. 

When designing to a static pressure of 0.50 inches water column (in.w.c.), a good rule of thumb is to keep the pressure drop across the air filter at or below 0.10in.w.c. This will not happen with a standard 1” filter if you plan on using anything other than a see-through “rock-catcher”. If you want a high-performance filter (you should) the 4” media type have low-pressure drops, but you might have to use two for anything above a 3 ton. Our preference is 2” filter grilles with MERV 13 filters. Easy to accommodate on new construction, and retrofits with multiple returns.

Keep in mind that the MERV rating of filters is typically at a low velocity, say 300 FPM. Performance of the filter drops when velocity is increased.

Allison Bailes (yes, him again) of Energy Vanguard recently blogged on this topic. https://

#8. Avoid installing ducts in unconditioned spaces. 

Vented attics are the worst possible place for the ducts. It’s the hottest part of the house in the summer, and the coldest in the winter.  Ducts in vented attics come with a large energy penalty. Vented crawl spaces do not have as heavy of an energy penalty, but because of high humidity, present other challenges.

Supply duct leakage will cause outside air infiltration due to depressurizing the living space. Any return leakage will directly bring in attic or crawl space air.

Here’s a great article from one of the best building science resources. https://

#9. Select registers with enough throw to provide adequate air mixing. 

This helps with providing even room temperatures, prevents stratification, and reduces the number of registers needed per square foot.

We use a lot curved blade ceiling registers located close to an interior wall, pointed toward the exterior. In addition to good air mixing interior high wall or ceiling register placement also uses less materials when compared to an exterior location. Simple, small duct systems cost less, leak less, and have less thermal transfer.

This is one of my favorite free duct design resources.


#10. Test your duct system to verify design. 

Feedback may be the single most important thing when it comes to your process and improving it. Whether it’s from a duct leakage test (even when it’s not required), or measuring airflow, feedback accelerates the learning process and allows you to make adjustments to your design on the fly.  

#11. Flex duct is high performance if installed straight and tight. 

Don’t believe me? Try it. Because of rule #10, I know this to be true.

Yes, I probably could have linked Energy Vanguard to every rule. https://

#12. Don’t blow air on people. 

In my opinion this is why heat pumps get a bad rap. It’s not comfortable having 85° air blowing on you. At the same time that 85° air may be enough to maintain a 70° room temperature. Keep this in mind when selecting register type and location.  

#13. Use elbows with radius throats or turning vanes. 

In ACCA’s Manual D, Appendix 3, you will see how much of an impact radius throats and turning vanes have in comparison to square throats, and elbows without turning vanes. You will also see that radius heel vs. square heel makes little difference.

Piggybacking on rule #3, high-efficiency fittings have a greater impact the closer they are to the equipment. A lot of times when doing a retrofit, I will install turning vanes in the existing fittings that are close to the equipment. This is a good way to make measurable improvements without reinventing the wheel.

#14. Design and build quiet duct systems. 

Noise can come from several sources, vibration, turbulence, high velocity, and the equipment.

Depending on the type of equipment and its orientation, there are several ways to isolate it from the structure to reduce vibrations. I always use canvas connectors as close to the equipment as possible so ducts can be supported while being decoupled from the equipment. Vibration pads under the equipment help if it is installed on a platform or stand. Hanging equipment using spring or neoprene isolators provides better isolation, but is more time consuming and costly.

Air noise from high velocity and turbulence can be reduced by fittings with sweeping turns and lowering the velocity. In addition to that, silencers (such as Fantech’s LD line) are effective, as well as flexible duct. Some use an acoustical liner (which works) but we choose not to because of IAQ concerns. (Potential deterioration and releasing fiberglass into the airstream, also tends to promote mold growth on the supply side).

One technique for addressing equipment noise is reducing line-of-site. Most of the time this can be done with a few turns in the ducts between the equipment and any registers and grilles.

Neil Comparetto,
Co-owner of Comparetto Comfort Solutions in Virginia

We all know (or should know) that venting refrigerant is a big no-no and can result in trouble from the EPA.

There are many other potential violations, but two that can easily occur if you aren’t thinking ahead are the disposal of mercury and oil.

Mercury is found in fairly large quantities in the bulbs of old thermostats. Instead of ditching these stats, gather them up and return them to an A/C supply house for proper recycling. Most supply houses offer this service.

Refrigerant and Vacuum pump oil are both oils that we often need to drain for one reason or another. Make sure to capture the oil in a pan (or an oil coil cleaner jug) and turn it in to an oil recycler. Many auto parts stores and auto mechanics will have no problem taking it off your hands.

Simple… but easy to get wrong if you don’t pay attention.

No matter what you believe politically we all agree that taking care of the environment is worth doing and something that all responsible adults consider.

— Bryan

There was a story that came out recently based on an ASHRAE study performed by David Yuill from University of Nebraska that appeared to indicate that cleaning condenser coils makes no difference on system performance and efficiency.

Those of us who have worked in the field know that coil cleaning matters because most of us have had a system that wasn’t working well, or possibly even cutting out on high head pressure. We cleaned the coil and the system started working properly… over and over again

But as an exercise… a thought experiment… let’s work through this and see some possible reasons why this conclusion may have been reached.

The job of an air cooled condensing coil is to reject heat from the refrigerant to the air. The rate at which it does this is a function of contact time, temperature differential, the thermal conductivity of the material through which heat is being transferred and turbulence of both fluids (refrigerant on the inside of the tubing and the air on the outside).

You may have noticed that modern condenser coils are larger than they used to be, the reason for this is simple, the larger the surface area of the coil, the more heat can be transferred from the refrigerant to the air resulting in a lower required condensing temperature and lower head pressure. In other words, by increasing the contact time we don’t need as great of temperature difference between the the refrigerant in the tubing and air passing over it to accomplish the same amount of heat transfer.

Engineers have also learned that by changing the design of coils we can get greater contact surface area with less refrigerant with coils such as micro-channel or they can get greater internal turbulence by adding grooves or rifling in the tubes of better external turbulence by adding little kinks to the fins of the coil. They do all of this to attempt and move heat from the refrigerant in the most efficient way possible and I applaud them for their efforts.

So how could a “dirtier” coil ever be more efficient? it is at least theoretically possible that certain types of surface fouling might act to create more air turbulence and actually increase heat transfer… and if you tested 100 systems in field conditions you may find a few that exhibit this undesigned behavior depending on the type of coil and the type of soil.

In the field we know this isn’t normal…

How many of us who do small kitchen refrigeration have gone out to a freezer not keeping temp or an ice machine making ice like it once did, only to clean the condenser and everything starts working properly again?

In our minds we imagine that the dirt or grease is acting like an insulating “blanket” preventing heat transfer, and that is certainly one factor, but it isn’t the only thing going on.

Condenser Fan Efficacy 

Condenser fans are prop fans, more technically known as “axial” fans as opposed to blower wheels which are known as radial or centrifugal. Axial fans are good at moving a lot of air against very low pressure, but as soon as the pressure starts to build their performance drops off REALLY quick. We have all walked up to a condenser fan where the air was just sort of beating out of the side instead of really pushing out the top like it’s supposed to. Once you clean the coil it starts moving air again and you can really tell the difference.

So much of the decreased heat transfer comes from the fact that dirt blocks the airflow causing less air to move over the coils which drives up the condensing temperature and head pressure.

Compression Ratio 

As the head pressure and condensing temperature increase the compression ratio increases (absolute head divided by absolute suction) which causes the amount of refrigerant the compressor moves to decrease resulting in both higher compressor amperage and lower system capacity. This effect is greater with TXV/EEV systems because the valve will tend to throttle down as the head pressure increases to maintain superheat further increasing the compression ratio.

Evaporator Temperature

On fixed metering device systems higher head pressure will also drive up suction pressure which will tend to keep the compression ratio slightly lower but will result in higher coil temperature and poor latent (humidity) control.

So to put my money where my mouth is we picked a nice dirty coil and ran a full, white paper style test. For the sake of complete disclosure we used the fan curve charts to come up with evaporator air flow, which is fine because it was a before / after test. I used MeasureQuick for the calculations and my phone was giving me trouble and kept losing my manually entered data so I realized later that in my AFTER report (that some of you may have seen in my group) the airflow was set to 750 and before was set to 700 so I went back in and changed the math so everything was apples to apples. Either way… the results are pretty self evident. You will notice that the “official” results below are slightly different than those in the screenshots at the top, and that math change is the reason.

Equipment Cleaned

2-ton 1999 Trane R22 10 SEER “Spine Fin” Heat Pump Split system with a direct return operating and 0.4” WC total external static pressure on a PSC blower and a fixed piston type metering device.

Test Process

I Allowed the system to run 20 minutes continuously and took detailed measurements sufficient to compare wattage, total BTU/H removal and therefore the EER of the system using wireless connected digital instruments and the MeasureQuick app.

We cleaned the condenser coil only while performing this test no other cleaning or servicing and making no adjustments to refrigerant charge.

We then allowed the system to run continuously for another 20 minutes to ensure the coil is completely dry while confirming by measuring condenser air dew point entering and leaving. Retake the same measurements and compare the results.

Cleaning Method

CoilJet using Refrigeration Technologies Viper cleaner and then rinse working inside out


The before results showed clearly that the head pressure and liquid line temperature were both high with a low subcooling and superheat. The measured system performance was poor even though the evaporator coil, air filter and blower wheel were quite clean considering the age of the system.

After cleaning the head pressure and suction pressure dropped, the subcooling and superheat increased and the compressor amperage dropped. It became clear after the cleaning that the system was slightly low on refrigerant because it maintained a stable 31° superheat.

The system performed significantly better in terms of decreased wattage and increased BTU removal after the cleaning.

Suction Pressure / Evaporator Temp75.967.9
Liquid Pressure / Condensing Temp278.6216.5
Outdoor Air DB 89.091.0
Airflow CFM750*750
Condenser Voltage245244
Condenser Amperage 11.410.3
Total BTU Capacity19,37220,992
Total Wattage 2,6442,367

After this test was complete we added 9 oz of R-22 to achieve the factory required superheat. Following the adjustment the EER and total system capacity improved even further.

This illustrates that cleaning this condenser indisputably improved –

System Capacity
Compressor Longevity


I wrote to David asking him to come on the podcast and explain his findings a few moths ago and he responded to that via email with this –

“At some point I’d like to set everybody straight in one fell swoop, and maybe your HVACrSchool is the venue for that, but I haven’t decided yet.”

I don’t think David’s research is “wrong”, I’m sure they got the results they said they got, the issue must be a disconnect in the way the tests were performed and the way many systems perform in the field. I do think the conclusion the article came to was incorrect


The point of the study was all about heat transfer and in real life if we control for ambient conditions all we would need to do it measure head pressure, clean the coil, let it dry and measure head pressure again. If it goes down then more heat transfer is occurring (again, controlling for changes in ambient conditions and indoor load).

For fun, I would encourage you to try the same tests and let me know your findings. I used MeasureQuick, a Redfish meter and Fieldpiece Joblink probes to collect the data but you could do it with any accurate modern digital instruments. Just make 100% sure the coil is dry after cleaning or you will get false measurements. If you find a system that doesn’t improve, or gets worse it would be great to know the “why” behind that example by reviewing the application and data.

If you want to come to your own conclusions as to why the research came to the findings it did… the test apparatus is shown below.

This is the peer reviewed article

Image shown under Creative Commons from –

Mehdi Mehrabi & David Yuill (2019) Fouling and Its Effects on Air-cooled Condensers in Split System Air Conditioners (RP-1705), Science and Technology for the Built Environment, 25:6, 784-793, DOI: 10.1080/23744731.2019.1605197

— Bryan

P.S. – Out full report on coil cleaning will be available on in a few weeks so keep your eyes peeled.


This photo is of a blower door test we performed at my own house using a Retrotec blower door. You will notice the 15 passenger van outside the window which is a dead giveaway.

A blower door is used to measure the tightness of a building and is often discussed in terms of an ACH50 number that many will simply refer to as the “blower door number”.

What is this secret number and what does it mean?

In the HVAC and building performance industry, you will hear the terms CFM (Cubic Feet per Minute), ACH (Air Changes per Hour) and ACH50 (Air Changes per Hour at 50 Pascals) thrown around a lot and it’s important that you understand the differences.


CFM is a measurement of volume (not mass) flow rate, the cubic feet of air moving per minute. To convert CFM to cubic feet per hour you simply multiply by 60 and to convert cubic feet per hour to CFM you divide by 60. When measuring air flow we generally convert air velocity (speed) to CFM by simply measuring the velocity  (FPM) and multiplying that by the opening we are measuring.

For example, if we are measuring an air velocity in a duct of 700 FPM (feet per minute) and the duct is 24″ x 24″ (2’x 2′) that would be a square footage of 4 x 700 = 2,800 CFM (Cubic Feet Per minute). If we needed to calculate cubic feet per hour we would multiply that by 60 (minutes in an hour) and the result would be 2,800 CFM x 60 = 168,000 CFH (Cubic Feet per Hour)


Like we talked about in a recent article air changes per hour is often used to calculate ventilation requirements for a room or structure.  Let’s imagine there was a dining room that required 5 air changes per hour (CFH) for proper ventilation. If that dining room was 10′ x 10′ x 10′ that would equal 1,000 cubic feet of internal volume in the space. This means that we would need to provide that room with 5,000 cubic feet of “new” air every hour to hit that number. 5,000 CFH ÷ 60 minutes per hour = 83.33 CFM of airflow to hit that target.

When discussing ACH within a space for ventilation it can often get confused with discussing ACH with outdoor ventilation air for healthy air dilution via mechanical ventilation or with undesigned infiltration through a loose envelope shell.

ACH for ventilation of specific rooms within a structure, ACH for designed outdoor air ventilation and ACH50 for envelope infiltration testing should not be mixed up or you will find yourself being very confused   

All of these ACH measurements are simply a calculation of the cubic volume of a space dived by air volume moving in and out of a space in an hour. The question is, is the air from inside the structure or outside of the structure and when/how is it being brought in.


A blower door is used primarily to measure infiltration of outdoor air into a structure. In order to compare the “tightness” one structure to another. The common standard for measurement is 50 pascals of negative pressure in the structure in reference to the outside.

In other words, you use a big fan placed in an exterior door with a fine-tuned manometer and you run the fan at a rate that gets the building down to 50 pascals of negative pressure.  You then calculate how much air is moving through the blower (in CFM) and convert CFM to CFH by multiplying by 60. You calculate the internal cubic feet of the structure and divide that number into the calculated CFH to come up with the ACH50 number. This is simply the air changes per hour of the structure at 50 pascals of negative pressure. A higher number means more infiltration (loose)  and a lower number means less infiltration (tight).

This ACH50 number ONLY applies when the house is under a pretty strong negative pressure, it doesn’t actually tell you how much air is moving in and out of the space under normal operating conditions.

— Bryan

P.S. – For those of you wondering my house has an ACH50 # of 3.2, while this isn’t great by building performance practitioner standards,  I’m happy with it…. and as the great philosopher Cheryl Crow said, “If it makes me happy, it can’t be that bad”



The tech tip today is a video put out by my friend Brad Hicks from the HVAC in SC YouTube Channel. Thanks Brad!

Seal boots to prevent raccoon leaks

Ok, so this has nothing to do with raccoons but I like that photo.

Whenever you are installing duct boxes (also called boots or cans) in an aftermarket application, make sure to place a bead of sealant like mastic or silicone on the flange so that as it presses against the substrate it will seal against leaks to and from the unconditioned space. When installing in a new construction environment where the boxes / boots / cans go in before the substrate you will either want to use boots that already have gaskets or you will want to add a gasket to the flange such as foam tape.  In these cases, it is still a good idea to seal the edge further from the inside once the drywall (or similar) is in place and before the grilles and registers are installed.



Video Transcript

What’s going on guys? here’s another 60-second tech tip, this is on supply and return grills and properly sealing them. As you can see this return grill that I have pulled down was not properly sealed. No silicone or mastic, so basically what’s happening you can see a little bit of wood here when the blower comes on it pulls air that’s pulling unconditioned air from between the sheetrock and the wood that’s framing this box out of the attic and into our Airstream. Since our air filter goes here as well most of this isn’t being filtered, it’s just passing right into the system. As you can see that return is fairly dirty so all of this should be sealed with mastic and usually we just silicone or you can mastic this as well. Same thing with supply grilles, so if you ever have customers that are dealing with dust issues or units getting dirty but the filters aren’t that dirty this could be your culprit. Make sure you’re paying attention to the supplier return grilles and look out for this kind of stuff so hope that helps thanks for watching.
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