Hotel Chilled-Water Plant Held at 25 °C / 66% RH in Coastal Summer
A 96-room beach hotel in Alexandria runs a two-chiller plant (2 × 350 kW screw chillers, chilled water 7/12 °C) serving four AHUs. Since June, complaints about stuffiness and condensation on ceiling diffusers have risen; handheld meters show 25 °C and 66% RH in guest rooms against a design of 24 °C and 50% RH. Each AHU delivers 3,600 m³/h, measured at 13.5 °C off the coil with a moisture content of 13.0 g/kg. Measured room sensible load is 14 kW and latent load about 4.5 kW. Chilled-water flow is 20% above design because two balancing valves were left fully open, and the chilled-water pumps run at fixed speed. The operator proposes dropping the chilled-water supply to 5 °C 'to dry the air'. Outside design is 35 °C DB / 27 °C WB and outdoor air is about 25% of the mixture. The BMS has no humidity sensor, and each AHU's cooling valve modulates on return-air temperature only. The client wants a written diagnosis before agreeing to any capital spend.
1. Does the air side actually deliver enough dehumidification at the measured operating point? Work the numbers and explain why the rooms settle at 66% RH rather than the design 50%.
Read the worked answer
Sensible check. 3,600 m³/h ≈ 1.0 m³/s ≈ 1.2 kg/s. Sensible removal = 1.2 × 1.006 × (25 − 13.5) ≈ 13.9 kW against a 14 kW sensible load - temperature control is working, which is why nobody complains about being too warm.
Latent check. Required moisture removal = 4.5 kW ÷ 2,450 kJ/kg ≈ 0.00184 kg/s. Per kg of supply air: 0.00184/1.2 ≈ 1.53 g/kg. Room air at 25 °C/66% carries roughly 13.2 g/kg, so the supply must be at or below about 11.7 g/kg. The measured 13.0 g/kg means the coil is removing only ~0.2 g/kg - roughly 0.6 kW of the 4.5 kW latent load, about 13%.
Consequence. The space humidity rises until the room-to-supply moisture difference closes the latent balance, which is exactly the 66–70% band observed; it will go higher with full occupancy and bathroom/kitchen extract imbalance.
Root cause. The coil is running effectively dry. Because the valve is driven by return-air temperature and there is 20% excess chilled-water capacity, the valve throttles, coil water flow falls, and the mean coil surface temperature rises above the entering air dew point (mixed air ≈ 14.3 g/kg, dew point ≈ 19.5 °C). No condensation, no latent removal. Return-air temperature control is structurally blind to humidity, so it happily delivers 25 °C at any RH.
2. Evaluate the operator's proposal to reset the chilled-water supply to 5 °C, and set out the control and balancing measures you would specify instead.
Read the worked answer
The 5 °C proposal is the wrong lever. A 2–3 K lower evaporating temperature costs roughly 2.5–3.5% compressor power per K, i.e. 6–10% of the running load - on an average 250 kW that is 15–25 kW, or 100–150 MWh a year - with freeze protection, low evaporator pressure limits and reduced chiller turndown to manage. It also deepens the low-delta-T problem rather than curing it: the coil is already over-supplied with water, so the extra capacity simply gets throttled away.
What to specify instead.
- Rebalance the chilled-water circuits to design flow, restore valve authority, and confirm design ΔT at the coil (low ΔT is diagnostic of the bypassing you already have).
- Change AHU control from return-air temperature to supply-air temperature (about 13 °C) with PI action on the supply sensor, so the valve opens and the coil stays flooded and wet at part load.
- Add return-air RH sensors plus an override that drives the valve open whenever RH exceeds 55%, and cap any supply-air temperature reset so the coil leaving dew point never rises above about 10.5 °C.
- For peak latent load (25% outdoor air at 27 °C WB), verify the coil can reach ≤11 g/kg at design water flow; if not, a dedicated outdoor-air unit or a deeper coil for one AHU is the justified capital spend.
- Add a 7-day trend of RH, valve position, coil ΔT and chiller kW/TR to prove the fix.
3. Before signing the commissioning/design certificate, what verification would you require and what would you refuse to certify?
Read the worked answer
Require: a minimum 7-day continuous trend per guest floor of dry-bulb, RH, supply and return air temperature, and cooling-valve position; a pitot or balometer traverse proving each AHU's 3,600 m³/h in the field, not just at the fan curve; coil air-side and water-side ΔT at stable load; chiller kW/TR and evaporator/condenser approach; and a fresh load calculation using the actual outdoor-air fraction and occupancy rather than the original design assumption.
Test the design, not just the installation. If a 4-row coil at design flow cannot reach ≤11 g/kg at 25% outdoor air, the design is short of latent capacity and no amount of commissioning will fix it - that becomes a design change with cost and programme consequences, which must be raised formally rather than buried in a commissioning report.
Refuse:
- Certifying compliance from a single spot reading taken at part load.
- Endorsing the 5 °C chilled-water reset without the chiller manufacturer's written confirmation of minimum evaporating temperature, freeze protection and warranty position.
- Accepting the humidity problem as an occupancy/behaviour issue while the control system has no humidity input at all.
Advise the client in writing that the root cause is control and balancing, that the capital spend proposed is not the remedy, and that performance-based acceptance be tied to measured RH at design conditions.