The hotel's underground garage ventilation system (CO concentration control) has long been running "always on at full speed," resulting in high energy consumption. How can it be optimized to "demand-controlled ventilation"?

The hotel's underground garage ventilation system (CO concentration control) has long been running "always on at full speed," resulting in high energy consumption. How can it be optimized to "demand-controlled ventilation"?

The energy consumption of the underground garage ventilation system accounts for approximately 5–10% of the hotel's total energy consumption (annual electricity cost of RMB 100,000–300,000), and the energy-saving potential of "demand-controlled ventilation" retrofits is enormous. The following is a systematic technical analysis.

I. Basis of Codes and Standards
Article 8.2.1 of the Code for Fire Protection Design of Garages, Repair Garages and Parking Lots (GB 50067-2014) requires that underground garages be provided with a smoke exhaust system, and Article 8.2.2 sets explicit requirements for the smoke exhaust rate (calculated based on storey height and area). Article 7.3.4 of the Code for Design of Garage Buildings (JGJ 100-2015) specifies the ventilation requirements for underground garages—"underground garages shall be provided with independent supply and exhaust air systems, with an air change rate of not less than 6 times/h." However, the explanatory notes to this clause also state that "the air change rate may be automatically controlled based on the detected value of diluted CO concentration." Article 6.4.7 of the Code for Design of Heating, Ventilation and Air Conditioning of Industrial Buildings (GB 50019-2015) requires that "underground garages shall be provided with carbon monoxide concentration detection devices interlocked with the ventilation system." Article 6.2.10 of the Assessment Standard for Green Building (GB/T 50378-2019) includes bonus-point requirements for the energy-saving measure of "CO concentration monitoring and interlocked ventilation." With regard to local standards, both the Beijing Design Standard for Energy Efficiency of Residential Buildings (DB11/891-2020) and the Shanghai Design Standard for Energy Efficiency of Public Buildings (DGJ08-107-2015) impose stricter requirements on CO interlocked control. The national standard GBZ 2.1-2019, Occupational Exposure Limits for Hazardous Agents in the Workplace, specifies a time-weighted average concentration limit for CO of 20 mg/m³ (approximately 17.5 ppm)—hotel garages may refer to and apply this limit.

II. Impact on Hotel Operations (Pros and Cons)
Advantages: During peak vehicle entry and exit periods (7:00–9:00 in the morning and 14:00–16:00 in the afternoon, when CO concentration is highest), the "demand-controlled ventilation" system operates at full speed to ensure air quality; late at night (22:00–7:00, when there is almost no vehicle entry or exit), it operates at low speed or stops (while maintaining a "5–10%" baseline airflow)—thereby keeping garage air quality consistently compliant (CO ≤ 30 ppm) while achieving significant energy savings (annual electricity savings of 50–80%). The use of a "jet fan" system can improve ventilation effectiveness in garage "dead corner" areas (corners far from exhaust outlets), making CO concentration more uniform (the concentration difference between the worst point and the lowest point drops from 100–200% under "conventional ventilation" to 20–50%).

Disadvantages: "Accuracy drift" of CO sensors (zero-point drift of ±5–10 ppm after 1–2 years of operation) may lead to "control failure" ("positive drift" → CO falsely deemed excessive → fans kept running continuously, no energy savings; "negative drift" → fans not started when CO is actually excessive → safety hazard). The jet noise generated by "jet fans" (approximately 55–65 dB per unit)—when a large number of jet fans operate simultaneously (approximately 40–60 jet fans in a 5,000 m² garage)—the "superimposed noise" may reach 65–75 dB, affecting the "quiet" perception in the garage (however, jet fans can be "operated in rotation"—each group of fans alternately starts and stops, with each run lasting 30 minutes, which does not affect the overall airflow and reduces noise by 10–15 dB).

III. Pros and Cons of Post-Installation Maintenance
Advantages: The efficient speed regulation of variable-frequency fans significantly reduces the mechanical impact on fan motors and bearings, extending fan service life by 30–50%. The "self-diagnosis" function of CO sensors (some brands—e.g., Siemens SSC series CO sensors have a self-check cycle of 30 days, automatically detecting "zero-point drift" and "response time" and automatically alarming when "drift exceeds the limit") reduces maintenance difficulty.
Disadvantages: CO sensors require relatively frequent maintenance—every six months they must be "calibrated" with standard gas (a 250 ppm CO standard gas cylinder costs approximately RMB 500–1,000 per cylinder) (RMB 150–300 per point per calibration), and sensor life is approximately 3–5 years (replacement cost RMB 800–2,000 per unit). Jet fans require annual maintenance once (cleaning impellers + bearing lubrication, RMB 50–80 per unit × 50 units = RMB 2,500–4,000 per year). Inverter maintenance (cleaning heat sinks, checking IGBT modules and cooling fans) is required once every 2 years at approximately RMB 500–1,000 per service. The CO control "thresholds" of the system's "self-learning" function need to be fine-tuned after "one month of accumulated operating data."

IV. Cost Pros and Cons
Short-term investment: The "CO interlocked control" system adds investment—CO sensors approximately RMB 800–2,000 per unit × 15–20 units per 5,000 m² = RMB 12,000–40,000; inverter retrofit (if the fans already include inverters, only control cables and PLC modules need to be added) approximately RMB 2,000–5,000 per unit × 4–6 units = RMB 8,000–30,000; jet fan system (including jet fans + mounting brackets + wiring, approximately RMB 500–800 per parking space, approximately 120 parking spaces per 5,000 m²) approximately RMB 60,000–100,000. Total investment is approximately RMB 80,000–170,000 (the "CO interlocked" type) or RMB 150,000–250,000 (the "CO interlocked + jet fan" type).
Long-term benefits: For a "continuously running at full speed" system, the annual electricity cost is approximately RMB 210,000 based on 30 kW × 24 h × 365 days × RMB 0.8/kWh; for a "demand-controlled ventilation" system, the annual electricity cost is approximately RMB 80,000 based on 30 kW × 8 h (peak periods) + 20 kW × 4 h (medium-speed periods) + 10 kW × 12 h (low-speed periods) × 365 days × RMB 0.8/kWh (annual savings of RMB 130,000). The comprehensive energy-saving rate of the jet fan + CO interlocked system reaches 60–75%. The investment payback period is approximately 1.5–2 years.

V. Specific Technical Details and Engineering Experience
1) CO sensor selection and installation specifications: Electrochemical CO sensors (superior to semiconductor type—longer lifespan, higher accuracy, better interference resistance). Installation height: CO is slightly lighter than air (density 0.967 kg/m³ vs. air 1.2 kg/m³); in garages, CO concentration is highest within the height range of "1.2–1.6 m above floor" (vehicle exhaust outlets are approximately 0.3–0.5 m above floor, but hot exhaust rises and diffuses); recommended installation at 1.5 m above floor—the "personnel breathing zone" height. Installation location selection: ① Lane "mid-section" (neither "ventilation dead zone" nor "air inlet"); at least 1 per fire compartment + ensure "coverage" of 400–600 m²/unit; ② ≥5 m from fan suction inlet (to prevent "fresh air" dilution causing low readings); ③ Horizontal distance from "vehicle entry/exit lane centerline" ≤5 m.
2) "Jet Fan" system design and layout: Jet Fans are small high-speed axial fans (throw distance 10–30 m, outlet velocity 15–25 m/s, motor power 0.1–0.5 kW/unit), arranged by the "airflow path method" (not the "equal spacing method")—design the "main flow direction" based on the garage lane layout ("straight-through" to avoid "vortices"), installing one jet fan every 8–15 m along the "main flow direction" ("relay" propulsion of airflow). Jet direction toward the "central exhaust outlet" (the "air curtain" generated by jet fans → "pushes" CO-contaminated air "toward" the exhaust outlet, without creating "dead zones" or "air short-circuiting"). In the "parking space" areas on both sides of the "main flow direction," supplement with "induction" fans ("V" arrangement—two jet fans at 15–30° angle directed toward the "main flow direction").
3) Ventilation fan "minimum speed" "anti-cycling" ("Anti-Swinging") control strategy: VFD minimum frequency set at 10 Hz (corresponding to 10% speed)—the minimum speed to "maintain airflow" (air volume approximately 10–15% of rated value), rather than "zero speed." Logic: CO concentration >30 ppm begins speed increase → CO <20 ppm with 30-minute delay reduces speed to 10%. The dual safeguard of "deceleration delay" and "minimum speed" prevents "excessive start-stop cycling." Energy consumption at 10% speed is approximately 5% of rated value (fan power is proportional to the cube of speed)—negligible.
4) "Jet Fan + CO interlock" "failure mode" design: When a CO sensor fails ("self-diagnostic" alarm + communication timeout + data anomaly), the system automatically switches to "safe mode"—"assume CO in that zone exceeds limit" → "fans in the corresponding zone run continuously at medium speed (60% speed)" until "manual confirmation of CO sensor replacement/repair." "Safe mode" + "voice alarm" (BA system issues fault alarm → notifies engineering department for maintenance).
5) Application of "rotary wheel" or "plate" heat recovery in ventilation systems: For underground garage makeup air systems with "large fresh air volume (fresh air introduced at 80% of exhaust air volume)" ("makeup air" treatment—cooling/heating required in summer/winter introduces a "huge" fresh air thermal load), "heat recovery" schemes can be considered: "exhaust air → heat recovery core → outdoor fresh air" (pre-heating/pre-cooling), with "recovery efficiency" of approximately 50–70% ("rotary wheel"/"plate"). However, the "makeup air" volume for "hotel underground garages" is typically "80% of exhaust air volume"; calculated at "6 ACH + makeup air 4.8 ACH," the "makeup air" thermal load is enormous → "heat recovery" "payback period" is approximately 3–5 years (applicable in both cold northern regions and southern "dual heating/cooling conditions").
6) "Construction" experience: "CO sensor" "fit-out coordination"—"cables" need to be "embedded" in structural walls/columns/slabs (accurately positioned and conduit secured before "concrete pouring") → "junction box" size ≥100×100×60 mm ("reserved" space) + "waterproofing" treatment → "decorative" cover plate ("coated"/"masked" to "match the interior design style" → reserve "ventilation grille").