Decentralized ventilation in winter: leave it running or switch it off?

In brief: Decentralized ventilation should generally not be switched off completely in winter. Quiet, continuous operation at a setting suited to occupancy is usually more useful. Moisture and stale air are removed regularly while heat recovery transfers some exhaust-air heat to incoming outdoor air.

Why ventilation matters in winter

Windows stay closed longer, but showers, cooking, laundry, breathing and sleeping still release moisture. In airtight buildings this moisture does not disappear by itself. Warm humid air can condense on cold window reveals, corners and external walls.

The German Environment Agency gives 40–60% relative humidity as general guidance. Assess readings alongside room temperature, outdoor temperature and building condition. Poorly insulated surfaces may be at risk even when a room reading looks unremarkable. Ventilation is not a reason to switch off heating or ignore building defects.

Continuous operation or switching off?

Everyday use: steady, low or medium speed

Normally occupied living rooms and bedrooms usually benefit from continuous operation at low or medium speed. Towels, bedding, furniture and surfaces continue releasing stored moisture after the immediate source has stopped. Repeated on/off switching gives less consistent air exchange.

In paired push-pull operation, units reverse in coordination: one extracts and charges its heat store while the other supplies outdoor air, then they swap. Do not arbitrarily switch off one member of a pair.

Moisture peaks: temporarily increase ventilation

After showering, during cooking or with many occupants, temporarily raise the setting. Humidity or CO₂ sensors can assist, or adjust it manually. The sensor comparison explains which measurement suits each room. A residential unit does not replace a cooker hood, and one room's unit does not automatically ventilate the whole home. Doors, transfer paths and unit positions matter.

Absence: reduce rather than stop

A weekend away produces less occupant-related moisture, but plants, residual moisture, damp basement areas and building materials can still release it. Use a low setting where the instructions and building conditions allow. After a longer absence, check temperature, humidity and odour before resuming normal settings.

What heat recovery can do

Instead of simply losing warm air through a window, a heat recovery unit stores some heat from extraction and uses it to prewarm incoming air, reducing ventilation heat loss. It does not replace space heating.

Published JESSP shop data states heat recovery of up to 97%. This is an upper product specification, not a guaranteed operating value. Actual performance depends on speed, temperature conditions, installation and maintenance.

This table is not a design for a specific building. Room size, use, unit count and positions and available external walls must be considered. Start with the model comparison.

Finding a suitable winter setting

No single setting suits every home or winter day. Use measurements and observations:

• Normal occupancy: suitable continuous low or medium setting.
• Showers, cooking or guests: increase temporarily and observe the humidity trend.
• Several days away: reduce to low rather than automatically switching off.
• Persistently dry air: check readings, adjust airflow to need and assess moisture sources.
• Condensation or damp corners: increase air exchange, heat adequately and investigate thermal bridges.
• Unusual ice, noise or faults: follow the unit instructions and contact JESSP or a specialist.

A hygrometer helps track whether a moisture peak falls again or remains high for hours. The German consumer advice service stresses that building condition and outside temperature affect interpretation; a fixed target without considering cold surfaces may mislead.

Dry air: is ventilation to blame?

Cold outdoor air often contains little water vapour. Warming it indoors lowers relative humidity. Both window ventilation and mechanical ventilation can therefore dry indoor air. Simply switching off also reduces removal of CO₂, odours and moisture peaks.

Check the hygrometer and fan setting first. If the air stays very dry, a lower demand-based setting may help. Avoid unnecessarily high room temperatures and very long window ventilation. Choose sensing appropriate to room use.

Five common winter mistakes

1. Switching off every night, precisely when bedrooms accumulate moisture and CO₂.
2. Running continuously at maximum speed, causing unnecessary noise and drying.
3. Neglecting dirty filters, which restrict airflow. See filter maintenance.
4. Leaving windows tilted open, cooling nearby building surfaces and losing heat. Prefer short, intensive airing when extra ventilation is needed; see opening windows with ventilation.
5. Overlooking leaking pipes, failed waterproofing or rising damp: ventilation cannot repair these.

Frequently asked questions

Does it have to run 24 hours?

Continuous background ventilation is generally useful in occupied rooms. Reduce during longer absences as appropriate, following the instructions, occupancy and measured humidity.

May I still open windows?

Yes, for odours, exceptional moisture peaks or personal preference. Germany's environment ministry gives five to ten minutes as indicative winter airing guidance. Avoid permanently tilted windows.

Will the supply air feel cold?

Heat recovery prewarms it, but it is normally cooler than room air. For unusual draughts, check operating mode, pairing, installation and filters.

What about ice or fault messages?

Follow the exact model's instructions. Do not remove or block components or make arbitrary adjustments. Seek help for unusual icing, noises or repeated faults.

Steady ventilation rather than constant switching

Maintain suitable background ventilation, raise output for moisture peaks and reduce it with lower occupancy. Monitor humidity, temperature and filters, and investigate building-related causes of damp. Use the JESSP comparison or ask for advice about your installation.