How HVAC Coils Transfer Heat, and What Degrades It

Key takeaways

• Heat passes through several resistances in series: the air or water film, any fouling, the metal wall, any oil film, and the refrigerant film.
• When any resistance grows, the refrigerant must run colder in the evaporator or hotter in the condenser, increasing the compressor's temperature lift and energy use.
• FEMP guidance: raising chilled water 2 to 3°F or lowering condenser water 2 to 3°F can improve chiller efficiency by up to 3 to 5% and 2 to 3% respectively, showing how sensitive efficiency is to temperature lift.
• Air-side dirt mostly hurts through reduced airflow; water-side scale and refrigerant-side oil act as insulating layers.
• Approach temperature, coil temperature difference, superheat and subcooling are the practical gauges of heat exchanger health.

An air conditioner or chiller does not create cold. It moves heat from where you do not want it (inside the building, or inside a refrigerated case) to where it does not matter (outdoors, or into cooling tower water). The compressor provides the push, but the actual hand-off of heat happens in two heat exchangers: the evaporator and the condenser. How well those two surfaces pass heat largely decides how hard the compressor must work. This page explains the basics in plain terms and shows what degrades heat transfer over time.

The refrigeration cycle in one paragraph

Cold, low-pressure liquid refrigerant enters the evaporator, where it boils by absorbing heat from the air or water passing over the other side of the tubes. The compressor draws in the resulting vapor and squeezes it to a high pressure and temperature. In the condenser, that hot vapor gives up its heat to outdoor air or condenser water, condensing back into liquid. An expansion device then drops the pressure, and the cycle repeats. The difference between the condensing and evaporating temperatures is the lift, and the compressor's energy use rises with it.

Heat transfer as a chain of resistances

Think of heat flowing through a heat exchanger like current through resistors in series, or warmth escaping through layers of clothing. Between the air (or water) on one side and the refrigerant on the other, heat must cross:

1. The air-side or water-side boundary layer: a thin, slow-moving film of air or water hugging the surface. Faster flow thins it.
2. Any external fouling: dust on fins, or scale and biofilm inside chiller tubes.
3. The fins and tube wall: copper and aluminum conduct heat very well, so this is rarely the limiting layer when the fins are intact and well bonded to the tubes.
4. Any refrigerant-side film: an oil-rich layer on the inside of the tube.
5. The refrigerant-side boundary layer: governed by how the refrigerant flows and boils or condenses.

Engineers summarize the chain as an overall heat transfer coefficient, often written U. Heat moved equals U, times the surface area, times the temperature difference between the two fluids. Add a resistance anywhere in the chain and U falls. Since the coil area is fixed, the only way to move the same heat is a larger temperature difference, which means the refrigerant must run colder in the evaporator or hotter in the condenser. Either way, lift increases and so does compressor power.

Why lift matters so much

The Department of Energy's Federal Energy Management Program (FEMP) O&M guide gives a sense of the sensitivity for centrifugal chillers: raising chilled water temperature by 2 to 3°F can increase system efficiency by as much as 3% to 5%, and lowering condenser water temperature by 2 to 3°F can increase it by as much as 2% to 3%. Fouling works in the opposite direction. FEMP notes that scale and sludge buildup "insulates the tubes," requiring a larger temperature difference between water and refrigerant, and that air trapped in a condenser has the same effect as scale. A few degrees of extra lift, caused by a thin layer of deposit, shows up directly on the electric bill.

The evaporator: sensible and latent cooling

An air-side evaporator coil does two jobs. It lowers air temperature (sensible cooling) and, when the coil surface is below the air's dew point, condenses moisture out of it (latent cooling, or dehumidification). The balance between the two depends on coil temperature, airflow and run time. A NIST study of a residential heat pump with deliberately imposed faults found sensible capacity fell for every fault tested, while low indoor airflow actually increased latent capacity because the colder coil removed more moisture. That illustrates why symptoms can be confusing: a fault can make a space feel clammy, or unusually dry, depending on what changed.

The condenser: rejecting the heat plus the compressor's work

The condenser has to reject all the heat picked up in the evaporator plus the energy the compressor added. In an air-cooled unit, the key indicator is how far the condensing temperature sits above the outdoor air temperature. In a water-cooled chiller, it is the condenser approach: the difference between the refrigerant condensing temperature and the leaving condenser water temperature. On the chilled water side, the evaporator approach is the difference between leaving chilled water temperature and the refrigerant evaporating temperature. A clean heat exchanger has a small approach; as fouling builds, the approach widens. Tracking approach temperatures against the values recorded when the chiller was clean or new is one of the most useful and least expensive diagnostics a plant can run.

NIST's fault testing showed the condenser side clearly: blocking part of an air-cooled condenser's face area raised condensing temperature and compressor discharge temperature, and compressor work increased with the fault level even as refrigerant flow fell.

What degrades heat transfer

Each row has its own remedy. External cleaning addresses the first two; airflow work addresses the third; tube cleaning and water treatment the fourth; purging the fifth. The last row sits on the inside of the tubes and is covered in what is oil fouling. For which cleaning fixes what, see does coil cleaning restore efficiency.

Worked example: reading a chiller approach trend

The following is illustrative, not data from any particular chiller. A water-cooled chiller's log shows a condenser approach of 2°F after a tube cleaning in spring. By late summer, at similar load and condenser water temperature, it reads 5°F. The refrigerant is now condensing about 3°F hotter than it needs to for the same water conditions. Using the FEMP sensitivity above in reverse (2 to 3°F of condenser temperature worth as much as 2% to 3% of efficiency), that drift could plausibly be costing a few percent of chiller energy. The next questions are which layer changed:

• Did condenser water flow drop (strainer, pump, valve)? Check flow and pressure drop across the bundle.
• Is water treatment on target? Check conductivity and treatment logs.
• Is there air in the condenser? Low-pressure machines have purge units; check purge run time.
• If water-side conditions are clean and normal and the approach is still wide, refrigerant-side causes (charge, oil) move up the list.

How to measure heat exchanger health

• Approach temperatures (water-cooled chillers): trend daily from the chiller controller or building automation system.
• Air temperature difference across the coil at known airflow, compared with the manufacturer's expected range.
• Superheat and subcooling: indicators of charge and of how much of the evaporator and condenser is doing useful work. FEMP's chiller maintenance list includes using superheat and subcooling readings to obtain maximum efficiency.
• Pressure drop across coils (air side) and bundles (water side).
• Efficiency metrics such as kW per ton, where metering allows. See kW per ton, EER and COP explained.

Where CryogenX4 fits

Most maintenance attention goes to the outside of the coil and the water side of the chiller, because those are the surfaces a technician can reach. CryogenX4 addresses the refrigerant side. The company describes its treatment as lifting the insulating oil film from internal coil surfaces, returning it to the compressor sump and conditioning the metal for better heat transfer, and reports energy savings of up to 30% with results varying by equipment condition. The company's description of the chemistry is on the role of advanced chemistry.

Next step

If you operate water-cooled chillers, start trending condenser and evaporator approach temperatures this week and record the values right after the next tube cleaning as your baseline. For air-cooled equipment, ask your service provider to record coil temperature difference, superheat and subcooling at each visit so you can see drift instead of guessing at it.

Frequently asked questions

What is a good approach temperature for a chiller?

It depends on the chiller design and load, so the best reference is the value the manufacturer specifies and the value your chiller showed when it was new or freshly cleaned. What matters most is the trend: a steadily widening approach at similar conditions points to fouling or another heat transfer problem.

Why does a dirty coil increase energy use if the compressor is the same?

A dirty coil makes the refrigerant run hotter in the condenser or colder in the evaporator to move the same heat. That increases the pressure the compressor must create, so it draws more power for every ton of cooling, and it may run longer as well.

Are microchannel coils more sensitive to fouling?

Microchannel coils have small internal passages and tightly packed fins, so both air-side debris and internal oil retention can matter. Laboratory work on microchannel evaporators has measured oil occupying about 10% of internal volume at typical oil concentrations. Follow the manufacturer's cleaning instructions, which often differ from those for tube-and-fin coils.

Does refrigerant type change how coils transfer heat?

Yes. Different refrigerants boil and condense with different pressures, densities and heat transfer properties, and each is paired with particular lubricants. That is why retrofitting a refrigerant can change capacity and why oil behavior differs between systems.

Sources

1. O&M Best Practices: A Guide to Achieving Operational Efficiency, Release 2.0 (PNNL-14788) — Pacific Northwest National Laboratory for DOE Federal Energy Management Program
2. Performance of a Residential Heat Pump Operating in the Cooling Mode with Single Faults Imposed (NISTIR 7350, 2006) — National Institute of Standards and Technology
3. The impact of evaporator fouling and filtration on the performance of packaged air conditioners (International Journal of Refrigeration 30(3), 2007) — Yang, Braun & Groll, via International Institute of Refrigeration FRIDOC
4. Upstream Solutions to Downstream Problems: Improving Field Performance of Small Commercial Rooftop Units (ACEEE Summer Study 2004) — Jacobs, Higgins & Shwom, ACEEE
5. Experimental study of oil retention in microchannel type evaporators of air-source heat pump systems (International Journal of Refrigeration 91, 2018) — Cremaschi, Yatim & Mulugurthi, via International Institute of Refrigeration FRIDOC
6. Guideline E: Fouling Factors: A Survey of Their Application in Today's Air-Conditioning and Refrigeration Industry — AHRI