Approach temperature is one of the most useful numbers you can track on a chiller, but it doesn't get talked about enough. I've been maintaining chiller plants across New York City for over a decade as a Local 30 Operating Engineer, and this is the first thing I check every single time I walk into a plant.
What You're Seeing
You're doing your rounds and the chiller seems to be running fine. Setpoints are being met, no alarms, building is comfortable. But the kW/ton is creeping up. The chiller is working harder than it should. The condenser water is coming back warmer than expected. Something is off, but nothing is "broken."
This is where approach temperature tells you the story that the alarms won't.
What It Usually Means
Evaporator Approach = Leaving Chilled Water Temp − Refrigerant Saturated Suction Temp
Condenser Approach = Refrigerant Saturated Discharge Temp − Leaving Condenser Water Temp
A healthy centrifugal chiller should show 1-3°F on both sides. Screw chillers run a bit higher — 2-5°F is normal. When those numbers start climbing, your heat exchangers are losing efficiency.
Rising evaporator approach usually means tube fouling on the waterside (scale, sediment) or low chilled water flow. The refrigerant is colder than it should need to be to make the water temperature, which means the compressor is working harder.
Rising condenser approach is almost always condenser tube fouling. Cooling tower water is dirty — that's just the nature of open systems. Scale, biological growth, and debris coat the tubes and act as insulation. The refrigerant can't reject heat efficiently, so head pressure climbs.
Both approaches rising together — check for non-condensables. Air or moisture in the refrigerant circuit is an insulator on the condenser side and can also affect evaporator performance. Run a purge cycle and see if the numbers improve.
What to Check
Get yourself a good set of calibrated temperature sensors. I use pipe-strap thermocouples and verify against the chiller's onboard readings. Don't just trust the BAS — verify with your own instruments.
Log it. I keep a simple spreadsheet for every plant I manage: date, load percentage, evaporator approach, condenser approach, kW/ton. Takes five minutes during your PM visit. But when you look at six months of data and see that condenser approach went from 2°F to 6°F, you know exactly when to schedule tube cleaning — before it becomes an emergency.
Compare at consistent conditions. Approach temps change with load. A chiller at 30% load will have different approaches than at 80% load. Always compare readings taken at similar load conditions. I try to log at 60-80% load whenever possible.
Common Mistakes
Ignoring it because the chiller is "running fine." The chiller will run fine with a 10°F condenser approach. It'll just use 20% more electricity doing it. Your building owner won't notice until the utility bill shows up.
Cleaning tubes on a schedule instead of on condition. Some guys clean tubes every year whether they need it or not. Others never clean them. Both are wrong. Trend your approach temps and clean when the data tells you to. Some plants need annual cleaning, some can go three years. Let the numbers decide.
Not checking for non-condensables. If your approaches are high and tube cleaning didn't help, you probably have air in the system. Run the purge unit and check. I've seen chillers with pounds of non-condensables that nobody noticed because they just kept blaming the tubes.
Field Notes
I had a building where the chief engineer was convinced his chiller was failing. Condenser approach was 12°F — terrible. He wanted to bring in the chiller manufacturer for a major overhaul. I asked when the tubes were last cleaned. "Oh, the water treatment company handles that." Called the water treatment company. They hadn't cleaned the tubes in four years. Four years of cooling tower water running through those tubes.
We cleaned the tubes. Approach dropped to 2.5°F. The chiller wasn't failing — it was suffocating.
Cost of tube cleaning: about $3,000. Cost of the major overhaul he was about to approve: $85,000.
That's why you trend your approach temps, people. The data doesn't lie, and it's a lot cheaper than guessing.
Frequently Asked Questions
What is a good approach temperature for a chiller?
A well-maintained centrifugal chiller should have evaporator and condenser approach temperatures of 1-3°F. Screw chillers typically run 2-5°F. Anything above these ranges indicates fouling, flow issues, or non-condensables.
How often should chiller tubes be cleaned?
Clean based on condition, not schedule. Trend approach temperatures monthly and schedule cleaning when condenser approach rises 2-3°F above baseline. Some plants need annual cleaning, others can go 2-3 years depending on water treatment quality.
