How many decentralized ventilation units do I need?

There is no reliable rule such as “one decentralized ventilation unit per room”. Floor area matters, but so do room type, ceiling height, occupancy, moisture load, building airtightness and the paths through which air can move between rooms.

Why floor area alone is not enough

A bedroom for two people has a different demand from a rarely used office. Bathrooms, kitchens, utility rooms and damp basements also need a different strategy from dry living areas.

• Record room type, area and ceiling height.
• Consider occupancy and typical duration of use.
• Allow for moisture from sleeping, showering, cooking and drying laundry.
• Check suitable external walls, wall thickness and core-drilling positions.
• Define transfer-air paths through door undercuts or transfer elements.
• Decide whether the units will operate with heat recovery, supply only or extract only.

JESSP planning airflow values

In alternating heat-recovery mode, a unit regularly switches between supply and extract air. Its maximum fan rating is therefore not the same as the average fresh-air flow used for planning.

Why paired operation is often useful

With two synchronized units, one supplies air while the other extracts it. They then reverse together. This creates a more balanced air exchange and reduces pressure fluctuations. The units do not necessarily have to sit next to one another, but the intended airflow path must be clear.

Typical mistakes

• Selecting units only by square metres.
• Confusing maximum fan output with fresh-air planning flow.
• Treating bedrooms and bathrooms identically.
• Ignoring closed doors and missing transfer-air paths.
• Choosing only by core-hole diameter.
• Providing no reserve, so the system must always run at maximum speed.

A practical planning sequence

1. Record every room and its use.
2. Mark moisture and extract-air areas.
3. Check external walls and possible installation points.
4. Define airflow paths between rooms.
5. Determine the required volume flow.
6. Select the unit, filter and control concept.
7. Have complex layouts checked individually.

This guide supports initial selection. It does not replace project-specific professional planning or the assessment of fireplaces, internal extract rooms, acoustic requirements or complex dwellings.