How do I tell if my building's chiller is actually performing — what are delta-T and approach temperature?

Question

I manage a building on a water-cooled chiller and I want to know whether the plant is genuinely running well or quietly wasting energy. People mention "delta-T" and "approach temperature" as the numbers that actually tell you. - What are delta-T and approach temperature on a chiller? - What does good vs bad look like? - How do they reveal a chiller that's underperforming or fouled before the energy bill blows up? Trying to move from "it seems to be cooling" to actually knowing if it's performing. What should I be reading, and what's a red flag? (This is the performance side of the water-treatment upkeep.)

Sam D. · Accepted Answer

Short answer: two numbers tell you most of it. DELTA-T is the temperature difference between the chilled water leaving the plant and the water coming back — it should be close to the system's design (often around 5-6°C for chilled water). APPROACH TEMPERATURE is how close the water gets to the refrigerant temperature in the heat exchangers — a small approach = clean, efficient; a growing approach = fouling/scale. Watch both over time: a falling delta-T or a rising approach is a chiller quietly losing efficiency.

Delta-T (chilled-water): - It's supply-water temp vs return-water temp. The system is designed for a certain delta-T (commonly ~5-6°C / ~10°F for chilled water). - "Low delta-T syndrome" is the classic building-plant problem: the water comes back too cold (small delta-T), so the pumps have to push far more water to move the same cooling — pumping energy soars and the plant runs inefficiently. Causes: fouled/bypassing FCU coils, stuck valves, poor balancing. A chronically low delta-T is a real, expensive red flag.

Approach temperature: - It's how closely the water approaches the refrigerant's temperature in the evaporator/condenser. A small approach means the heat exchanger is clean and transferring heat well. As scale/fouling build (exactly what water treatment prevents), the approach widens — the chiller has to work harder for the same result, so efficiency (kW/ton) drops.

How to use them: - Trend them, don't just spot-read. A gradually rising condenser approach = fouling building up (clean/treat). A falling delta-T = a distribution/coil/valve problem starving the return. - Together they catch a chiller silently losing efficiency long before it "breaks" — which on a big plant is a large, invisible chunk of the bill.

So: know your design delta-T and baseline approach, log them, and act when they drift. That performance monitoring plus the plant upkeep is core to a proper chiller service programme. FMs — anyone tracking delta-T and caught a low-delta-T problem?

DubaiFix Team · Answer

The trending point is the FM gold here. A single reading tells you little; the same numbers logged weekly tell you everything — a slowly widening approach is fouling you can schedule a clean for, a dropping delta-T is a distribution problem to chase. Most plants that are "quietly expensive" are ones nobody's baselined, so nobody notices the slow drift. Write down design delta-T and a clean-plant approach when it's healthy, then watch for deviation. It turns the chiller from a black box into something you can actually manage.

Omar K. · Answer

Even as a non-engineer the low-delta-T thing is intuitive once explained: if the water comes back nearly as cold as it went out, you're circulating loads of water for very little actual cooling delivered — so you're paying to pump water around for nothing. Makes sense that it's a big hidden cost on a building. Sounds like the takeaway is: someone should actually be reading and recording these numbers, because a chiller can waste money for ages while looking like it's "working fine."

How do I tell if my building's chiller is actually performing — what are delta-T and approach temperature?

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