There was a story that came out recently based on an ASHRAE study performed by David Yuill from the University of Nebraska. The study appeared to indicate that condenser coil cleanliness 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 a temperature difference between 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. That is the case with coils such as micro-channel. Also, the engineers can design the coils to 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. If you tested 100 systems in field conditions, you might 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 certainly is 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 instead of 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 REAL quickly. We have all walked up to a condenser fan where the air was just 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 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 airflow, which is fine because it was before/after the 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 that 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

A 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 for 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. We also made no adjustments to the 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 the condenser air dew point entering and leaving. Retake the same measurements and compare the results.

Cleaning Method

CoilJet using Refrigeration Technologies Viper Heavy Duty cleaner and then rinse—working inside out.

Results

The “before” results clearly showed that the head pressure and liquid line temperature were 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.

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
• Performance
• Compressor Longevity

I wrote to David asking him to come on the podcast and explain his findings a few months 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 how the tests were performed and how many systems perform in the field. I do think the conclusion the article came to was incorrect.

No… I KNOW IT IS 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 is 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 worsens, 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. – Our full report on coil cleaning is available on Speedclean.com, and you can access the PDF either from the website or from this link right HERE.