Ventilation technician is one of the plumbing and energy trade's main courses, and healthy indoor climate is a BR18 requirement. Here we cover air exchange and indoor climate requirements (CO2, humidity), balanced ventilation with heat recovery, duct sizing and commissioning, and energy consumption for air transport (SEL) — with the figures you size for in Denmark.
Introduction
People, cooking, bathing and building materials constantly give off moisture, CO2, odour and pollutants to indoor air. Ventilation's task is to lead used air out and fresh air in at a pace that keeps indoor climate healthy — without throwing unnecessary heat and electricity out the window. In modern, tight buildings it cannot be left to leaks and open windows; it requires a designed ventilation system. The requirements are in BR18 §§ 420–452, and the technical execution follows DS 447:2021.
How much air — basic air change and indoor climate requirements
The basic requirement for a residence is a continuous air change of at least 0.3 l/s per m² heated floor area, which at normal room height corresponds to approximately 0.5 air changes per hour. In addition to the basic air change, there are increased requirements in rooms that put extra strain on the air: kitchen, bathroom, WC and utility room.
| Requirements (BR18, dwellings) | Value |
|---|---|
| Baseline air exchange | mindst 0,3 l/s pr. m² etageareal (≈ 0,5 h⁻¹) |
| Kitchen — extraction over hobs (cooker hood) | at least 20 l/s, discharge to outside |
| Bathroom — extraction | minimum 15 l/s |
| Separate WC room — suction | minimum 10 l/s |
| CO2 in occupancy space (dimensioning) | must not exceed 1000 ppm |
CO2 concentration is the classic indicator of whether enough is being ventilated out: people exhale CO2, so if the level rises there is too little fresh air. The limit of 1000 ppm (0.1 %) at design conditions is BR18's target for acceptable indoor climate. Beyond CO2, indoor climate is about keeping relative air humidity down so that moisture, condensation and mould do not develop — particularly in bathrooms, kitchens and bedrooms. (Source: BR18 §§ 420–452; bygningsreglementet.dk.)
Balanced ventilation with heat recovery
A balanced system has both a supply fan (fresh air) and an extract fan (used air) that move equal volumes of air — hence 'balanced'. Fresh air is blown into living spaces (living room, bedroom), and used air is extracted from humid and smelly rooms (kitchen, bathroom, toilet). Along the way the two air streams meet in a heat exchanger, where the warm outgoing air transfers its heat to the cold incoming air — without the two air streams mixing.
- 01Counter-flow plate heat exchanger: high temperature efficiency (typically around 80–90 %); no moisture transfer; condensate must be drained and protected against frost.
- 02Cross-flow exchanger: slightly lower efficiency than counter-flow; simple and common in small systems.
- 03Rotating heat exchanger: also transfers some humidity back, which can be an advantage in winter; requires that some air may recirculate.
- 04Fluid-coupled battery: used when supply and exhaust air are physically separate; lower efficiency.
Channel sizing
The ducts must carry air volumes forward without noise and without wasting ventilator energy. Like in water pipes there is a balance: too-small ducts give high air velocity, pressure drop and whistling; too-large ducts take up space and cost more. Each duct section is therefore dimensioned according to the air volume it must carry, a chosen air velocity and the resulting pressure drop, and the pressure drops are summed towards the critical fitting, which the ventilator must be able to overcome.
- 01Low air velocity in ducts near occupancy spaces and at valves keeps noise down; main ducts can run faster.
- 02Bends, transitions, dampers and silencers contribute with individual resistances, just like fittings in water pipes.
- 03Specific recommended air velocities and pressure drops are design and comfort-dependent — they are taken from DS 447 and the industry's sizing tools, not from a guess.
- 04Sound attenuators are placed where duct noise would otherwise follow the air into occupancy.
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