• C system: you exhaust air mechanically (bathroom/kitchen). Fresh air comes uncontrolled in through window grilles or gaps. No heat recovery.
• D-system (WTW).: you feed and blow mechanically, with heat recovery. Incoming air is pre-heated by the heat of the exhaust air. In decentralized version this is done per room, without ductwork.

In short: you invest in insulation and airtightness, and a C system lets you blow those gains away.

Disadvantages of C in practice:

1. Energy loss: cold (or hot) outside air comes in directly through grilles. The energy gain from your renovation evaporates.
2. Comfort loss: draft in winter, hot influx in summer; additional street noise due to open grilles.
3. Unpredictable airflow: wind determines your supply, not you. Result: peaks/decreases in air quality.
4. No filtration: outside air enters unprocessed; dust/polls/smoke follow along.
5. Difficult control per room: you cannot finely control rooms. Night comfort or targeted boost is limited.
6. Aesthetics & maintenance: window grilles are visible, require cleaning and affect your window frames.

Per-room control, WTW comfort, no ductwork.

• Energy efficient: heat recovery keeps heat inside; lower heating costs.
• Comfortable: pre-heated supply, less draft. Windows stay closed → less noise and fine dust.
• Individually adjustable: each room its own setting (night mode bedroom, boost in kitchen). Control on Moisture/VOC.
• Little breaking work: one core bore per unit; ideal in renovations and apartments.
• Fast and scalable: place by room and phase over time; perfect for occupied renovations.
• Easy maintenance: filters 1-2×/year; done.

The classic price difference between C and D barely plays anymoreespecially with decentralized D solutions:

• You save ductwork, assembly hours and finishing.
• You avoid post-renovation breaking and visible window grilles.
• You gain energy through WTW (Total Cost of Ownership decreases).
• You get room-by-room control, which with central D systems is often only possible with more expensive control technology.

Renovate with a focus on energy and comfort, then a decentralized D-solution is the logical choice. You keep your heat in, you control room by room and you avoid major works-precisely what a renovation needs.