In a chilled water system, delta-T is the temperature difference between return water (warm) and supply water (cold). When delta-T drops, the chiller delivers less capacity — even at full compressor speed.
The capacity formula
Q = 500 × GPM × Delta-T
Where:
Q = capacity (BTU/hr)
GPM = water flow rate (gallons per minute)
500 = constant (8.33 lb/gal × 60 min × 1.0 BTU/lb·°F)
Delta-T = return temp - supply temp (°F)
Design vs actual
300-ton chiller, 600 GPM:
Design:
Supply: 44°F, Return: 56°F
ΔT = 12°F
Q = 500 × 600 × 12 = 3,600,000 BTU/hr = 300 tons ✓
Actual (low delta-T):
Supply: 44°F, Return: 49°F
ΔT = 5°F
Q = 500 × 600 × 5 = 1,500,000 BTU/hr = 125 tons ✗
Same chiller. Same flow. 58% capacity loss.
Why delta-T drops
Three-way valve bypass
Coil control valve bypasses water around the coil.
Bypass water returns at supply temperature (44°F).
Mixes with coil leaving water (56°F).
Mixed return: lower than 56°F → delta-T drops.
Fix: convert to two-way valves (no bypass path)
Fouled coils
Dirt, scale, or biofilm on waterside of AHU coils.
Reduces heat transfer coefficient.
Less heat moves from air to water per pass.
Water returns cooler than design.
Fix: chemical cleaning or mechanical brushing
Excessive pump speed
Variable-speed pump set to low DP setpoint.
More GPM than coils need at partial load.
Water moves through coils too fast.
Not enough contact time for full heat absorption.
Fix: reset pump DP setpoint based on valve positions
The pump energy penalty
When delta-T drops, you need more GPM for the same cooling:
Design: 600 GPM at 12°F ΔT = 300 tons
Low ΔT: 1,440 GPM at 5°F ΔT = 300 tons
Flow increase: 2.4×
Pump power increase: 2.4³ = 13.8×
Pump power follows the cube law. Small delta-T reductions cause massive pump energy increases.
In practice, the pump maxes out before reaching 1,440 GPM. The system starts a second chiller instead — doubling chiller, condenser pump, and cooling tower energy.
Real case: hospital
Problem: two 300-ton chillers both at 90% load
(three years ago, one chiller handled it)
Investigation:
ΔT = 5°F (design = 12°F)
14 of 22 AHUs had three-way bypass valves
Several coils not cleaned in 4 years
Fix:
Converted to two-way valves
Chemically cleaned coils
ΔT recovered: 5°F → 10°F
Result:
One chiller handles the load
Second chiller = peak days only
Annual savings: $47,000
The check
1. Measure supply water temp (at chiller leaving)
2. Measure return water temp (at chiller entering)
3. Subtract: ΔT = return - supply
4. Compare to design ΔT (typically 10-14°F)
ΔT within 2°F of design → system OK
ΔT 50%+ below design → investigate valves, coils, pump speed
Two thermometers on the chilled water pipes. The number between them tells you whether your chiller plant is working — or wasting energy moving water that is not absorbing heat.
Calculate your system's delta-T and capacity with the delta T calculator on CalcEngineer.
