Unreasonable BAS control strategies for the hotel's air-conditioning system lead to high energy consumption and poor comfort.

Unreasonable BAS control strategies for the hotel's air-conditioning system lead to high energy consumption and poor comfort.

As a hotel MEP expert, in response to the typical problem you raised—"unreasonable BAS control strategy for the hotel air-conditioning system leading to high energy consumption and poor comfort"—I will provide a complete, point-by-point detailed answer from four dimensions: problem diagnosis, standard reference, control strategy optimization, and case review, based on current national standards (GB) and actual engineering cases.

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I. Problem Diagnosis: Typical Manifestations of Unreasonable BAS Control Strategies

In actual hotel O&M, defects in BAS control strategies usually manifest as the following "three highs and two lows" phenomena:

1. Chilled water supply-return temperature difference too small ("large flow, small temperature difference")

• Manifestation: Chilled water pump frequency is constant or pumps only start/stop by number, with the temperature difference at 2-3℃ year-round (design should be 5-7℃). This causes pump energy consumption to account for 25%-35% of total air-conditioning energy consumption, far exceeding the reasonable value.
• Cause: BAS does not dynamically adjust pump frequency according to terminal load changes (such as guest room occupancy rate and banquet hall usage), or the differential pressure setpoint (DP setpoint) is set too high.

• Manifestation: Cooling tower fans start/stop frequently or run at full speed for long periods, causing the cooling water temperature to fail to approach the wet-bulb temperature (WB), the condenser pressure of the chiller to rise, and the energy efficiency ratio (EER) to decrease.
• Cause: BAS only adopts simple "return water temperature + temperature difference" control, does not adopt the "Approach" control strategy, and does not consider the free-cooling potential of the cooling tower under low load at night.

• Manifestation: Even when the guest room vacancy rate is as high as 40%, the primary air handling unit (PAU) still operates at the design maximum air volume, causing the fresh air load to account for more than 30%-50% of the total air-conditioning load.
• Cause: BAS is not connected to the hotel PMS (property management system) or guest room status signals and cannot achieve "demand-controlled ventilation" (DCV).

• Manifestation: Guest room fan coil units (FCUs) simultaneously produce "cold air supply" and "reheating" in summer, causing the cooling and heating sources to work at the same time and serious energy waste.
• Cause: The supply air temperature set by BAS is too low (e.g., 12℃), while the room thermostat is set at 22℃, causing the terminal to require reheating to offset overcooling.

II. Standard Reference and Target Value Citation (GB Standards)

To address the above problems, the following national standards must be strictly followed as the optimization baseline:

1. "Design Standard for Energy Efficiency of Public Buildings"GB 50189-2015

• Article 4.2.5: The supply-return water temperature difference of the chilled water system shall not be less than 5℃. When a secondary pump system is adopted, variable-frequency speed control devices shall be provided.
• Article 4.3.7: The selection of cooling towers shall make the cooling water inlet temperature 3-5℃ higher than the local summer air-conditioning outdoor design wet-bulb temperature (i.e., approach of 3-5℃). BAS shall be able to automatically adjust the cooling tower fan speed according to the wet-bulb temperature.
• Article 4.5.8: Fresh air volume control devices shall be provided to adjust the fresh air volume according to indoor CO₂ concentration or occupant density.

• Article 11.3.2: BAS shall have the full-condition energy-saving optimization control function for the air-conditioning system, including: chiller plant group control, pump variable-frequency control, cooling tower optimization, fresh air enthalpy control, etc.

• Article 4.2.3: During the operation of the air-conditioning system, the inlet and outlet water temperature differences of the evaporator and condenser of the chiller shall be regularly tested and adjusted to ensure they are within the design value range.
• Article 4.3.4: According to the actual load demand, the speed of pumps and fans shall be adjusted by variable frequency to avoid "oversized equipment operating at partial load".

III. BAS Control Strategy Optimization Plan (Detailed Point by Point)

Based on the above standards, the following 6 core modifications to the BAS control logic are recommended:

1. Chilled Water System: Variable Flow Differential Pressure Reset + Temperature Difference Control

• Strategy:
• Cancel the fixed differential pressure setpoint and adopt the "most unfavorable terminal differential pressure reset" algorithm. BAS scans the opening of all terminal valves (e.g., 80%-95%) and dynamically reduces the pump frequency until the valve at the most unfavorable terminal is nearly fully open (e.g., 95%).
• Simultaneously monitor the supply-return water temperature difference. When the temperature difference is <4℃, forcibly reduce the pump frequency; when the temperature difference is >6℃, appropriately increase the frequency.
• Expected Effect: Pump energy consumption reduced by 40%-60%, and the temperature difference stabilized at 5-6℃.

2. Cooling Tower: Approach Control + Dry/Wet-Bulb Temperature Linkage

• Strategy:
• Install an outdoor wet-bulb temperature sensor near the cooling tower.
• Control target: cooling water outlet temperature = outdoor wet-bulb temperature + 3℃ (approach setpoint).
• BAS adjusts the fan speed by PID according to the deviation value (actual outlet temperature - target temperature). When the approach is <2℃, reduce the fan speed or shut down; when the approach is >4℃, increase the speed.
• Expected Effect: Cooling tower fan energy consumption reduced by 30%, chiller condensing temperature reduced by 1℃, and chiller COP improved by about 3%.

3. Primary Air Handling Unit: Dual Control by CO₂ Concentration + Guest Room Status

• Strategy:
• Connect to PMS: When the guest room shows "unoccupied/checked out", BAS automatically closes the supply air valve of the PAU on that floor or reduces the air volume to 20%.
• CO₂ concentration feedback: In areas such as banquet halls and lobbies, set CO₂ sensors (≤800ppm). When the concentration is below 600ppm, gradually reduce the fresh air valve opening to the minimum fresh air ratio (according to the requirements of GB 50189).
• Expected Effect: Fresh air load reduced by 50%-70%, with particularly significant effects at night when the vacancy rate is high.

4. Chiller Plant Group Control: Sequence Control Based on Load Rate

• Strategy:
• BAS calculates the total cooling load in real time (through chilled water flow × temperature difference).
• When the load rate is <50%: give priority to operating 1 large chiller to avoid two chillers operating inefficiently at the same time.
• When the load rate is >80%: start the second chiller in the order of "optimal energy efficiency ratio" (give priority to starting the chiller with less operating time and high energy efficiency).
• Expected Effect: The integrated part-load value (IPLV) of the chiller improved by 10%-15%.

5. Eliminating Terminal Reheat: Supply Air Temperature Reset

• Strategy:
• Replace the fixed supply air temperature (e.g., 12℃) with an "outdoor temperature compensation" algorithm.
• Summer: When outdoor temperature > 35℃, set supply air temperature to 14℃; when outdoor temperature < 28℃, set supply air temperature to 16℃.
• Meanwhile, the BAS monitors terminal valve opening. If a zone's valve opening is < 30% and the reheat valve is open, automatically raise that zone's supply air temperature setpoint.
• Expected Effect: Eliminate cooling-heating cancellation, reducing terminal energy consumption by 20%.

6. Nighttime Free Cooling Mode

• Strategy:
• When outdoor temperature < 18℃ and there is a cooling demand indoors, the BAS automatically switches to "free cooling" mode: shut down chillers, operate only cooling towers and plate heat exchangers, and directly cool AHU terminals via cooling water.
• Bypass piping and motorized valves must be installed.
• Expected Effect: In transition seasons (spring, autumn), save 50%-80% of chiller plant energy consumption.

IV. Case Review (Five-Star Hotel Retrofit)

Project Background: A five-star hotel in a coastal city, with a building area of 50,000 ㎡. The air-conditioning system accounts for up to 58% of annual energy consumption. Complaints focused on: guest rooms fluctuating between hot and cold, and a stuffy lobby in summer.

Pre-Retrofit BAS Issues:

• Chilled water pumps ran at fixed frequency, with a temperature difference of only 2.5℃.
• Cooling tower fans ran at full speed, with an approach of up to 8℃ (i.e., outlet water temperature 8℃ above wet-bulb temperature).
• The fresh air system ran at full load 24 hours a day, and the PMS was not integrated.

Post-Retrofit Results (6-Month Measured Data):
| Metric | Before Retrofit | After Retrofit | Improvement |
| :--- | :--- | :--- | :--- |
| Chilled water supply-return temperature difference | 2.5℃ | 5.8℃ | +132% |
| Cooling water approach | 8℃ | 3.5℃ | -56% |
| Pump energy consumption (kWh/year) | 2.8 million | 1.12 million | -60% |
| Chiller COP | 4.2 | 5.1 | +21% |
| Guest room complaints/month | 15 | 2 | -87% |
| Overall air-conditioning energy consumption | Baseline | Reduced by 32% | Annual electricity savings of approximately 1.2 million yuan |

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V. Implementation Recommendations and Precautions

1. Hardware First: Before optimizing BAS strategies, ensure sensor accuracy (temperature, pressure, CO₂, air velocity) meets requirements (e.g., temperature sensor accuracy ±0.2℃); otherwise, the algorithm cannot converge.
2. Step-by-Step Commissioning: Do not modify all logic at once. It is recommended to first implement "pump variable frequency" and "cooling tower approach control," observe for 1 month, then add "fresh air DCV" and "supply air temperature reset."
3. Personnel Training: After BAS optimization, the engineering department must shift from "manual intervention" to "automatic monitoring." Establish an alarm mechanism: if the temperature difference remains < 3℃ for more than 30 minutes, the system automatically sends an SMS to the engineer.
4. Compliance Check: All control parameter adjustments must comply with mandatory provisions in GB 50189-2015 regarding minimum fresh air volume and indoor temperature (24-26℃ in summer), to avoid hygiene non-compliance caused by excessive energy saving.

Summary: The essence of optimizing hotel air-conditioning BAS control strategies is the shift from "extensive start-stop" to "refined prediction + dynamic adjustment." By strictly referencing standards such as GB 50189 and GB 50314, and combining practical strategies such as "differential pressure reset," "approach control," and "PMS linkage," it is possible to reduce energy consumption by 30%-40% while keeping guest room temperature fluctuation within ±1℃, completely resolving the contradiction of "high energy consumption and poor comfort."