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Starter question — a frequently asked question posted by the MEP Center team to open this forum. Share your own experience below.
For a new office building with about 500 kW of cooling, we are choosing the design temperature difference for the chilled water system. The traditional choice is 7/12 °C (5 K), but some consultants now design for 6/13 °C (7 K) or more.
What are the real effects on flow rate, pipe size and pumping energy? Does the chiller lose efficiency, and what has to change in the coil and fan coil selection? Are there situations where 5 K is still the better choice?
A larger ΔT reduces flow in direct proportion, which cuts pipe sizes and pumping energy substantially. The price is larger coils or a lower supply temperature, which costs some chiller efficiency. For most new buildings 6 to 8 K is the better balance, provided every coil is actually selected for that ΔT.
1. Flow rate
q = Q / (c · ΔT), with c = 4.19 kJ/kg·K:
- ΔT 5 K: 500 / (4.19 × 5) = 23.9 l/s (86 m³/h).
- ΔT 7 K: 500 / (4.19 × 7) = 17.0 l/s (61 m³/h), 29 % less.
2. Pumping energy (worked example)
Assume the 5 K design needs 25 m head and pump efficiency 0.70:
P = ρ·g·q·H / η = 9.81 × 23.9 × 25 / 0.70 ≈ 8.4 kW.
Keeping the same pipes at 7 K, friction falls roughly with the square of flow: H ≈ 25 × (17.0 / 23.9)² ≈ 12.8 m.
P ≈ 9.81 × 17.0 × 12.8 / 0.70 ≈ 3.0 kW, about 64 % less. In practice designers take part of the gain as smaller pipes and valves (lower capital cost) and part as lower pump energy. Coils selected for higher ΔT can have a higher water-side pressure drop, so take the actual coil and valve losses from selections.
3. Chiller and coils
- To get 7 K you either lower the supply (6/13 °C) or raise the return (7/14 °C). Lowering supply temperature costs roughly 2 to 3 % chiller efficiency per kelvin; raising the return costs nothing at the chiller but needs deeper coils.
- Coils and fan coils must be selected at the new water temperatures: more rows or larger units for the same sensible and latent duty. Check dehumidification at the higher mean water temperature.
- Many chillers have a minimum evaporator flow; check it against the reduced flow at part load.
4. When 5 K still makes sense
- Existing systems where coils cannot be changed.
- Process loads with tight supply temperature limits.
- Small systems where pump energy is minor and standard fan coil selections are at 7/12 °C.
5. Keep the ΔT in operation
Designing for 7 K is worthless if the plant runs at 3 K (“low ΔT syndrome”). Use two-way pressure-independent control valves, no three-way bypasses at terminals, clean coils, correct supply setpoint and variable speed pumping with separate duty and standby pumps controlled on differential pressure at the index circuit.
Common mistakes
- Changing the plant ΔT on paper while fan coils are still selected at 7/12 °C.
- Three-way valves at terminals, which return supply water and collapse the ΔT.
- Ignoring chiller minimum flow at part load.
- Oversized control valves with poor authority.
Related: Chilled Water Pump Head: A Quick Estimate, Fan Coil Unit Selection Step by Step.
Check the edition of the code that applies to your project.
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