Push-pull ventilation: paired operation explained simply

Many decentralized heat-recovery units use the push-pull principle. The fan does not move air permanently in one direction; it alternates between extract and supply air. This reversal allows a ceramic core to store heat from the outgoing air and return much of it to the incoming fresh air.

The cycle step by step

1. Extract phase: stale, warm and possibly humid air is moved outside.
2. Heat storage: the outgoing air warms the ceramic core.
3. Reversal: the fan changes direction after the defined interval.
4. Supply phase: fresh outdoor air is drawn through the warm core.
5. Heat return: stored heat is transferred to the incoming air.

What paired operation means

Two units work in opposite directions and reverse together. While unit A extracts air, unit B supplies it. In the next phase they swap roles. This balances supply and extract air better than unrelated units operating in the same direction.

Why 50 or 60 m³/h fan output is not always 50 or 60 m³/h of fresh air

In heat-recovery mode each unit spends part of the cycle extracting air. For planning, use the average fresh-air flow: 25 m³/h per DZ or Easy+ unit, 30 m³/h per Vario 160 and 12.5 m³/h per Vario 110. Two synchronized units provide 50, 50, 60 and 25 m³/h respectively.

One unit or a pair?

A single unit can be suitable for limited airflow or supply-only/extract-only operation. A synchronized pair usually offers simultaneous supply and extract in the zone, smaller pressure fluctuations, better distribution and often quieter everyday operation because maximum speed is needed less frequently.

Installation and airflow paths

The two units do not always have to be in the same room, but doors and transfer paths must support the intended flow. Closed doors without undercuts or transfer elements can interrupt the concept. Controls must also be compatible so both units reverse in synchronization.

Common mistakes

• Using maximum fan output as the heat-recovery planning value.
• Operating two units without synchronization.
• Ignoring pressure-sensitive fireplaces or other extract-air systems.
• Assuming air will pass through closed rooms automatically.
• Selecting too little reserve for quiet continuous operation.

Special requirements for fireplaces, pressure balance, fire protection and internal extract rooms require project-specific assessment.