Office air quality: ventilation, HVAC and purification

The regulations set the minimum fresh air rate at 25 m³/h per occupant in non-smoking offices, and 30 m³/h in meeting or dining rooms. The departmental health regulations may tighten these thresholds in dense urban areas. At the same time, a 40% reduction in energy consumption is mandated by 2030 compared to a reference year after 2010 for the relevant tertiary sector building stock.

Kytom incorporates three sizing margins drawn from its field experience: an occupancy margin to absorb peaks (meetings, training sessions); a densification margin anticipating the gradual reduction of space per workstation; a filtration margin to absorb progressive clogging without a drop in airflow.

On a 500 to 1,000 sqm floor, fresh air distribution relies on 1 to 3 air handling units with an energy recovery unit at an efficiency of >= 75%. The terminals selected at the detailed design phase are 4-pipe fan coil units or VRV/VRF systems depending on the winter/summer scenario.

Kytom’s position, contrary to common practice: the total margin must not exceed 20%, never 40%. The industry’s conventional wisdom adds 30 to 40% of cumulative margin by default for safety. Across 18 audits of existing AHUs carried out by our thermal engineers between 2022 and 2024, 11 floors were over-ventilated by 25 to 38%, resulting in excess fan consumption of EUR 1,200 to 2,800/year and parasitic thermal mixing. The useful margin is capped at 15% occupancy and 5% filtration, i.e. 20% maximum. Beyond that, the CFO is paying for an AHU that only serves to cool the technical room.

When oversizing is NOT justified. Below 200 sqm of floor area or for continuous occupancy of fewer than 15 people, adding a dedicated AHU with a high margin becomes counterproductive: excess fan consumption, residual noise and a significant CAPEX surcharge rarely justified. A compact dual-flow MVHR unit with a single CO2 sensor is sufficient. Likewise, if the building is set to be vacated within 5 years (precarious lease, announced restructuring), the ROI of a new high-efficiency AHU is not reached: opt for corrective maintenance of the existing system instead.

ePM1 filtration and CO2 control below 1000 ppm

For the thermal engineer and the CFO: the CO2 sensor is the most cost-effective OPEX lever, ahead of the high-efficiency filter. On an 800 sqm floor instrumented with VAV-CO2 (Kytom project 2023), fan consumption dropped by 22% and the heating bill by 14% over the first heating season, for a sensors and controller CAPEX of EUR 6,500 to 9,000. Measured ROI: 2.8 years. Over-investment in HEPA filtration, by contrast, yields no economic return in a standard office: it degrades the ventilation PUE. To capitalise on reduced consumption within the framework of the tertiary sector’s regulatory obligations, the priority remains CO2 control before filtration.

Dual-flow heat recovery: 75 to 90% efficiency

Offices over-insulated to current thermal standards combine a risk of summer overheating with an increased need for controlled ventilation. Kytom incorporates dual-flow units with heat recovery from the extracted air, with a thermal efficiency of 75 to 90% depending on the technology selected, measured under standardised conditions.

Three families of exchangers are deployed depending on the context:

• counterflow plate exchanger: efficiency up to 90%, selected for standard open spaces;
• crossflow exchanger: 60 to 70%, compact format for cramped technical rooms;
• rotary exchanger: humidity transfer possible, reserved for projects above 1,500 sqm.

Latent recovery remains prohibited on sanitary, kitchen and shower extractions to avoid any odour transfer: a pure sensible exchanger is then sized. On 1,000 sqm of offices, heat recovery from extracted air significantly reduces heating consumption, with a return on investment generally estimated at between 4 and 7 years in renovation.

In new construction, the installation becomes immediately cost-effective because the dual-flow AHU is mandatory under the environmental regulations in force. Note: manufacturer efficiencies stated under laboratory conditions degrade significantly in real-world operation, particularly after progressive clogging of the exchangers.

Method

1. Initial IAQ audit
   Installation of CO2/VOC/PM2.5 sensors over 10 to 15 days in representative areas. Data analysis and identification of trouble spots. Deliverable: a quantified report with prioritisation of priorities.
2. Airflow diagnosis
   Measurement of actual supply and return airflows, filter checks and duct inspection. Comparison with regulatory requirements and the current occupancy rate. Identification of the discrepancies to correct.
3. Technical study and costing
   Design of the appropriate solution: rebalancing, filtration replacement, addition of AHUs or purifiers. design and build costing with a detailed schedule for the occupied site and zone-by-zone phasing.
4. Coordinated works on an occupied site
   Phased interventions in the evening and at weekends for critical connections. Daily coordination with the Office Manager, communication to users and maintenance of the minimum regulatory airflow throughout the project.
5. Commissioning and after-sales service
   Final balancing, IAQ acceptance measurements, training of the internal team and installation of connected monitoring. Predictive maintenance contract with quarterly reporting and on-call response within 4 hours.