Tertiary heat recovery ventilation: IAQ and energy recovery
Regulatory framework: 25 m3/h/person (R4214-2) and CO2 threshold at 1000 ppm
Recovering 78% of the heat from extracted air cuts the heating bill by 15 to 25 kWh/m²/year, meaning a 4 to 7 year payback with energy savings certificates (CEE). Regulations require 25 m3/h/person in offices, and the ambient CO2 target is set at 1000 ppm. Kytom operational feedback from 100+ tertiary sites places the actual efficiency of heat exchangers between 75 and 88%, far from the 92% stated on manufacturer datasheets.
Tertiary buildings (ERT) are governed by the Labour Code (Decree 92-332 recodified and the order of 5 August 1992 as amended), which sets office ventilation at 25 m3/h/person without specific pollution, 30 m3/h/person in meeting rooms and 45 m3/h in smoking rooms. RE2020 introduces a carbon requirement that must be met to launch the construction of a building, and has tightened requirements on HVAC consumption, which accounts for 30 to 50% of the energy consumption of a tertiary building. Tertiary regulations impose a stepwise trajectory over 30 years.
- Minimum flow rate: 25 m3/h/person (offices), 30 m3/h/person (meeting rooms)
- Target CO2 threshold: 1000 ppm at full occupancy (NF EN 16798-1)
- Pollutants measured: VOCs, formaldehyde, PM2.5 (standardised indoor air sampling protocol)
- Reduction: -40% of final energy consumption in 2030, -50% in 2040, -60% in 2050
IAQ also affects productivity: a loss of 6 to 9% is observed in under-ventilated spaces. For public buildings (ERP), the formaldehyde investigation threshold is set at 30 µg/m3, with notification of the prefect above 100 µg/m3, a benchmark that Kytom systematically integrates into its tertiary audits.
Technical principle: plate or rotary exchanger, actual efficiency 75 to 88%
A dual-flow air handling unit (AHU) captures heat from stale air via a static plate exchanger or a rotary exchanger, then preheats the fresh air supplied. Our reading differs here from manufacturer orthodoxy: datasheets claim 92 to 95%, but field measurements across 1200+ Kytom projects cap out at 88%, a gap explained by coil fouling and internal leaks after 18 months.
| Exchanger type | Actual measured efficiency | Typical tertiary use |
|---|---|---|
| Counter-flow plates | 75-85% | 300 to 800 m² floors, enclosed offices |
| Rotary | 80-90% | >1000 m² floors, dense open space |
| Glycol water coil | 50-65% | Recovery on constrained sites, remote AHUs |
On a 500 to 1000 m² floor, distribution relies on 1 to 3 AHUs with a recovery unit of efficiency >= 75%. The terminals are 4-pipe fan coil units or VRV/VRF systems depending on the winter-summer scenario. Heating consumption related to air renewal drops by 65 to 80% compared with a single-flow system.
Kytom’s 5-step method for 850 m² delivered in 12 weeks on average
The intervention is divided into 5 phases over the average Kytom timeframe, applied across the 11 France-Spain agencies.
- Airflow audit: survey of existing flow rates, CO2 measurement over 7 days during occupancy, mapping of pollutants (VOCs, PM2.5, formaldehyde) according to the Kytom internal indoor air quality framework.
- Sizing: calculation of flow rates per zone, choice of the dual-flow unit with a plate or rotary exchanger.
- Architectural integration: routing networks in the false ceiling while maintaining a minimum clear height of 2.50 m, supply and extraction vents positioned according to the layout plan.
- Installation and commissioning: teams, airflow balancing within a tolerance of plus or minus 10%, pressure tests according to the protocols in force.
- Commissioning: CO2, temperature and humidity measurements over 4 weeks, adjustment of setpoints according to the Kytom internal indoor comfort framework, handover to the Office Manager.
The procedure governs the whole process, with IAQ monitoring over the agreed warranty period.
Measured benefits: -65 to -80% heating consumption and CO2 reduced below 850 ppm
Operational feedback from our tertiary projects confirms significant heat recovery from extracted air, with higher performance on new rotary exchangers. Heating consumption related to air renewal falls by 65 to 80% compared with a single-flow system.
| Indicator | Before works | After works |
|---|---|---|
| CO2 at full occupancy | 1400-1800 ppm | 600-850 ppm |
| Heat recovery | 0% (single flow) | 75-88% |
| Air renewal heating consumption | 100% (base) | 20-35% |
| AHU acoustic level in enclosed office | > 42 dB(A) | < 38 dB(A) |
On a 1200 m² site in Lyon, reported absenteeism fell by 11% over 12 months and employee satisfaction gained 22 points. On the Paris market, rents for second-hand offices range between €544 and €584/m²/year, up 7%, which illustrates the value appreciation of quality tertiary assets.
Cost, ROI and CEE: payback reduced to 4-7 years with the BAT-TH-125 form
The budget for a tertiary heat recovery system covers the AHU, the airflow networks, the terminals, the insulation and the commissioning. The gross payback is between 6 and 9 years excluding subsidies, based on a gain of 15 to 25 kWh/m²/year, reduced to 4 to 7 years with the CEE (BAT-TH-125 form) which cover 15 to 25% of the cost depending on the efficiency of the selected exchanger.
The structuring economic levers:
- CEE BAT-TH-125: valuation of the cumac kWh saved over 15 years
- Eligibility for the Heat Fund on projects coupled with a geothermal water loop
- Tertiary regulatory obligation in absolute value or relative trajectory: heat recovery contributes up to 35% of the HVAC effort
- Rental premium associated with a high-level environmental certification: 4 to 7% of the headline rent
Kytom builds the economic scenario during the audit phase, with an IRR calculation over 10 years and a modelling of subsidies updated at signing.
3 constraints to arbitrate: height, acoustics under 38 dB(A), 5-year maintenance
Three constraints weigh on the feasibility of a heat recovery system in an existing building.
Ceiling height and false-ceiling allowance. A dual-flow network requires 35 to 45 cm in the false ceiling (supply and return ducts plus insulation), incompatible with a clear height below 2.40 m without structural rework. Kytom checks the height under the slab from the audit stage, and proposes compact decentralised AHUs (25 to 30 cm thick) on constrained floors.
AHU acoustics under 38 dB(A) in enclosed offices. Technical equipment must remain under 38 dB(A) in enclosed offices, with an NR 30 to 35 target in open space. Sound attenuators, anti-vibration mounts and acoustic enclosures are systematic on units in the false ceiling.
5-year preventive maintenance. Without a maintenance plan, exchanger efficiency drops by 15 to 30% over 3 years, due to filter clogging and airflow drift.
Preventive maintenance plan: replacement of F7/F9 filters and cleaning of airflow networks according to the requirements applicable to the cleanliness of ventilation installations.
Contrary to the practice of pushing network cleaning back to 10 years, the applicable framework sets the interval at 5 to 7 years for offices, failing which efficiency falls by 15 to 30% over 3 years. Filter clogging and airflow imbalance account for most of the loss. The Kytom contract covers the 11 agencies with intervention within 48 working hours and commits to a measured IAQ trajectory.
- F7 filter replacement every 6 months, F9 every 12 months
- Annual inspection of the exchanger, motors and belts
- Airflow re-balancing every 24 months (tolerance plus or minus 10%)
- Network cleaning every 5 to 7 years
- Annual verification of setpoints
Frequently asked questions
What regulatory fresh air flow rate applies in tertiary offices?
The regulatory ventilation thresholds require 25 m3/h/person in offices without specific pollution, 30 m3/h/person in meeting rooms and 45 m3/h in smoking rooms. The target CO2 concentration adopted by indoor air quality frameworks is set at 1000 ppm at full occupancy.
What efficiency should you expect from a heat recovery exchanger in real operation?
KYTOM operational data from over 100 commercial sites places the real-world efficiency of dual-flow heat exchangers between 75 and 88%, below the 92% quoted in laboratory conditions. Recovering 78% of the heat from extracted air cuts heating bills by 15 to 25 kWh/m²/year. Return on investment falls between 4 and 7 years with energy savings certificates (CEE).
