Low Delta-T Syndrome on a 400-Ton Plant: The Wrong 100 Tons to Add
A 400-ton chilled-water plant serves three air handlers through a variable-flow secondary loop. Design is 44°F supply, 58°F return, 14°F delta-T. Trend data at 2 p.m. on a 95°F day: chiller supply 44.5°F, return 51°F, plant loading 62%, all three AHU cooling valves 100% open, secondary pump at 60 Hz (100% speed), and an ultrasonic meter reads about 30% of design flow through a 3-way bypass at the far end of the loop. No valve or pump alarms. Perimeter zones sit at 58–62% relative humidity against a 50% design, and occupants report the space is 'cool but clammy' at 74°F. The owner's engineer proposes adding a 100-ton modular chiller before the summer peak and asks you to confirm the scope.
1. Compute the delivered cooling and the implied secondary flow. What do the numbers say about where the problem is NOT?
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Measured delta-T = 51.0 - 44.5 = 6.5°F (design 14°F) Load = 0.62 x 400 = 248 tons Implied flow = 248 tons x 24 gpm/ton / 6.5°F = 916 gpm Design flow = 400 x 24 / 14 = 686 gpm Flow is 916 / 686 = 133% of design.
What this tells you:
- The plant is moving a third more water than design to deliver 62% of its capacity. The cost of that is pump and chiller auxiliary energy at full speed with nothing to show for it.
- The chillers are not the constraint. They are at 248 of 400 tons and have 152 tons of unused capacity sitting there.
- The building is uncomfortable while the plant is unloaded, which can only mean the cooling is not reaching the coils.
- Therefore: do not add 100 tons. The problem is in the distribution and terminal side, not in plant capacity. A new modular chiller would add flow at the same 6.5°F delta-T, add condensing and auxiliary load, and leave the space exactly as clammy as it is now.
2. The bypass carries about 30% of design flow. Estimate what that does to the return temperature, and lay out the sequence you would follow to fix the plant.
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Bypass effect. 30% of 686 gpm ≈ 206 gpm of 44.5°F supply water returning directly to the plant. If measured return is 51.0°F and 916 gpm total is returning:
51.0 = (710/916) x T_coil + (206/916) x 44.5 → T_coil ≈ 52.9°F
So the coils themselves are only achieving 8.4°F delta-T versus 14°F design. There are two defects, not one: a short-circuiting bypass and coils that cannot produce design delta-T.
Sequence (cheapest and most diagnostic first):
- Close the bypass. A 3-way bypass at the end of a loop should only open to hold minimum evaporator flow; a leaking or stuck-open valve steals flow from every coil and dilutes the return. This is free.
- Verify every AHU valve actuator actually strokes and that valves do not leak by in the closed position (a leaking valve looks exactly like a stuck-open bypass at the plant).
- Measure each coil's water-side delta-T and air-side approach at design flow; check for fouled tubes, air-side fouling, and control-valve sizing.
- Rebalance to design flow at each AHU, then reset the secondary pump differential pressure off the most-open valve rather than running at 60 Hz.
- Calibrate the supply/return temperature sensors - confirm the readings that started this whole analysis.
Expected result after step 1–2: return water warms toward 54–56°F, coils get their design flow back, zone valves begin to modulate below 100%, and total flow falls. The chiller loading rises (it is doing more useful work) while pump and auxiliary kW fall.
3. Explain to the owner why the 100-ton module will not fix the humidity complaint, and give the metrics you would track for 30 days to prove the diagnosis.
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Why more capacity makes it worse:
- Space latent removal depends on coil leaving-air conditions and airflow, not on installed tonnage. The coils are already flooded with 44.5°F water and saturated at 100% open; they are flow-starved and under-performing, so more plant capacity changes nothing at the coil face.
- Adding a chiller to a 6.5°F delta-T loop adds another machine into a system that will now run three or four chillers at light load - worse part-load efficiency, more cycling, higher kW/ton.
- Humidity is the tell: 'cool but clammy' at 74°F with 58–62% RH means the space sensible load is being met at high airflow with a coil that is not achieving its design moisture removal. Fixing delta-T and coil performance is the latent fix.
30-day proof plan - track these:
| Metric | What it proves |
|---|---|
| Per-AHU water delta-T (hourly) | Coil performance and whether closing the bypass restored it |
| Valve command vs. actual position | Whether valves are stuck, leaking, or overridden |
| Plant kW/ton and tons | Real efficiency, not just loading |
| Space RH in perimeter zones | The complaint metric that started this |
| Secondary pump kW and Hz | Whether flow can be reduced after rebalance |
If delta-T recovers to 11–12°F and RH falls below 55% after the bypass and valve work, the 100-ton module is dead - and the owner has saved roughly $250,000–$400,000 of installed cost plus electrical service work.