Chiller approach temperature is the difference in temperature between two fluids exchanging heat inside a chiller. It tells you how efficiently that heat transfer is actually happening. A lower approach means the chiller is transferring heat more effectively; a higher approach signals that something is reducing performance, often fouling or scaling on heat exchanger surfaces. Monitoring approach temperature is one of the simplest and most reliable ways to track chiller health over time.
How Approach Temperature Works
Every chiller has two main heat exchangers: an evaporator (where the chiller cools your building’s water) and a condenser (where it rejects heat to the outside). In both, two fluids flow near each other separated by tube walls, and heat moves from the warmer fluid to the cooler one. If those tube walls are perfectly clean and heat transfer is ideal, the two fluid temperatures would nearly converge. In reality, they never quite match, and that remaining gap is the approach temperature.
Approach is measured in degrees Fahrenheit or Celsius. You calculate it by subtracting the colder fluid’s temperature from the warmer fluid’s temperature at the point where heat transfer is complete.
Evaporator Approach vs. Condenser Approach
There are two approach temperatures to track on a typical water-cooled chiller, and they tell you different things.
Evaporator approach is the difference between the leaving chilled water temperature and the refrigerant temperature inside the evaporator. For example, if chilled water leaves at 44°F and the refrigerant evaporates at 41°F, the evaporator approach is 3°F. A well-maintained chiller typically runs with an evaporator approach between 1°F and 3°F. Values above 5°F usually indicate a problem.
Condenser approach is the difference between the refrigerant condensing temperature and the temperature of the condenser water as it leaves the condenser. If refrigerant condenses at 95°F and condenser water leaves at 90°F, the condenser approach is 5°F. Healthy condenser approach values generally fall between 1°F and 3°F for new or well-maintained equipment, though slightly higher values are common as equipment ages.
Why Approach Temperature Matters
Approach temperature is essentially an early warning system. When tubes inside a heat exchanger accumulate scale, biological growth, mud, or corrosion, they create an insulating layer that makes heat transfer harder. The chiller compensates by working harder to maintain the same output, which raises energy consumption. The approach temperature rises before you notice any change in comfort or water temperature, making it a leading indicator of efficiency loss.
A chiller with a fouled evaporator, for instance, might show an approach that has crept from 2°F to 6°F over a cooling season. That increase forces the compressor to run at a lower suction pressure to maintain the same leaving water temperature, consuming significantly more electricity. Studies of commercial chiller plants have found that every degree of increased approach on the evaporator side can raise energy use by roughly 1.5% to 2%. Over a full cooling season on a large building, that translates to thousands of dollars in wasted electricity.
What Causes High Approach Temperatures
The most common cause is tube fouling. Mineral scale from hard water, biological slime, sediment, and corrosion byproducts all coat heat exchanger tubes over time. The condenser side is especially vulnerable because it uses open cooling tower water, which introduces airborne contaminants, dissolved minerals, and microorganisms into the loop. Evaporators foul less frequently because they use a closed loop, but glycol degradation, corrosion, or poor water treatment can still cause buildup.
Other causes include low refrigerant charge, which reduces the effective heat transfer area in the evaporator; excess non-condensable gases (like air) trapped in the refrigerant circuit; and reduced water flow through the heat exchangers. Low water flow means the fluid spends more time in the tubes but at lower velocity, which reduces the turbulence needed for efficient heat exchange and can also accelerate fouling.
How to Track and Use Approach Data
Most modern chillers with digital controls display refrigerant and water temperatures in real time, making approach calculation straightforward. The key is not a single reading but the trend. Record approach values at consistent operating conditions, ideally at or near full load, on a weekly or monthly basis. Comparing approach at the start of a cooling season to the end reveals how quickly fouling is progressing and whether your water treatment program is working.
Many building operators set action thresholds. A common rule of thumb is to schedule tube cleaning when approach temperature rises 2°F to 3°F above the baseline established after the last cleaning or commissioning. Waiting too long costs more in energy than the cleaning itself. Some facilities with building automation systems log approach continuously and trigger automated alerts when the value exceeds a preset limit.
Approach Temperature at Design vs. In Operation
Chiller manufacturers specify a design approach temperature based on clean, new equipment running at rated conditions. Real-world approach is almost always slightly higher than the catalog value because operating conditions rarely match the test bench exactly. Variations in entering water temperature, load percentage, and water flow rate all shift the actual approach. This is normal. What matters is whether your approach is stable at a reasonable value and not trending upward over weeks or months.
At part load, approach temperatures can behave differently depending on chiller type. Some centrifugal chillers see approach drop at part load because the refrigerant-side conditions change favorably. Others, especially older or fixed-speed machines, may see approach hold steady or even rise slightly. Understanding your specific chiller’s behavior at part load helps you avoid false alarms when comparing readings taken at different building loads.
Approach Temperature vs. Overall Efficiency
Approach is just one piece of the efficiency picture, but it’s uniquely useful because it isolates heat exchanger performance from other variables. Chiller efficiency measured as kW per ton reflects everything at once: compressor condition, refrigerant charge, water temperatures, load, and heat transfer. Approach lets you pinpoint whether the heat exchangers specifically are degrading, even when overall kW per ton hasn’t shifted enough to notice yet. Pairing approach trending with regular kW-per-ton tracking gives you a complete view of chiller health without waiting for your utility bill to deliver the bad news.