Seasonal changes affect humidity, heating, window opening and how comfortable a ventilated home feels. This reusable checklist explains how to review MVHR summer bypass settings, PIV output, trickle ventilation and extractor fan controls without switching off essential airflow.
Overview
Most domestic ventilation systems should run continuously or be available when needed, even when the weather changes. Seasonal adjustment is usually about using the controls provided by the manufacturer, checking that air can move freely and responding to changes in occupancy or moisture—not about closing every vent in winter or turning the system off in summer.
The right setting depends on the system, the property and the conditions inside it. A well-insulated home with several occupants may need a different approach from a lightly occupied flat. Use the instructions for your specific unit as the primary reference, particularly for filter replacement, fan speeds, frost protection and electrical isolation.
Before making changes, record a simple baseline:
- Indoor temperature in the main living area and bedrooms.
- Relative humidity, ideally measured in more than one room.
- Visible condensation, musty smells or areas where mould returns.
- Whether fans are noisy, vents are dirty or airflow feels unusually weak.
- Any recent changes to occupancy, heating, insulation, windows or room use.
For a more detailed room-by-room airflow check, see How to Test Ventilation Airflow at Home. If indoor pollutants, humidity or carbon dioxide are a concern, the guidance in Indoor Air Quality at Home can help you decide what to measure.
Checklist by scenario
Use the table as a starting point, then confirm the available controls and recommended settings in your system manual. Some systems use automatic sensors or schedules, so a manual seasonal change may not be necessary.
| System | Winter and colder weather | Summer and warmer weather | Shoulder seasons |
|---|---|---|---|
| MVHR | Keep normal background ventilation operating. Check filters, condensate drainage and frost-protection information. Use boost during cooking, showering and other high-moisture activities if the system requires manual activation. | Use summer bypass only when appropriate for the indoor and outdoor conditions and when your manufacturer supports it. Keep background airflow available; bypass does not mean switching ventilation off. | Leave the normal programme in place unless comfort or humidity readings show a reason to adjust it. Review whether boost controls respond correctly. |
| PIV | Keep the unit operating at its normal setting. Check that the loft or hallway intake is unobstructed and that supply air is not creating an uncomfortable draught. Follow the manual for any heater or frost setting. | Maintain background supply airflow. If the system has temperature or speed controls, avoid reducing them solely because windows are open; use the specified adjustment range and monitor comfort. | Check for changes in condensation, odours and humidity as heating patterns change. Confirm that air can escape through rooms and undercut doors where designed. |
| Bathroom extractor | Use the fan during bathing or showering and for the manufacturer’s recommended run-on period. Keep the door closed while moisture is being removed, where practical. | Continue using the fan for moisture control. Opening a window may help in suitable weather, but it should not replace a correctly operating extractor. | Test the switch, timer or humidistat after periods of low use. Clean the accessible grille and check that external termination is clear. |
| Kitchen extractor | Use the appropriate setting while cooking and allow the fan to run afterwards if it has a timer or manual run-on control. Keep grease filters clean according to the appliance instructions. | Use extraction consistently rather than relying on open windows to disperse cooking moisture and odours. Check that warm air is not being recirculated when an external extraction system is intended. | Review whether the fan starts reliably and whether steam or odours linger after cooking. Arrange inspection if airflow has noticeably declined. |
| Trickle vents and passive openings | Keep designed background openings open and unobstructed unless there is a specific, professional reason not to. Do not seal them to stop ordinary draughts without checking the ventilation strategy. | Keep them available for background airflow, while using windows sensibly for purge ventilation when conditions allow. | Remove dust and confirm that blinds, curtains or furniture are not blocking the airflow path. |
MVHR summer bypass settings
A summer bypass changes how incoming air passes through the heat exchanger. It may be useful when indoor air is warmer than the incoming outdoor air, but it is not a substitute for all other cooling measures. Use the automatic bypass or manual control as described by the manufacturer. If the home remains too warm, review solar shading, window use, appliance heat and occupancy as well as the MVHR setting.
PIV settings through the year
PIV systems introduce filtered air into the property so that stale, humid air can leave through existing gaps and ventilation routes. Avoid reducing the supply so far that moisture or odours build up. Conversely, an excessive setting may cause draughts or discomfort. Make small changes only where the controls permit them, then observe humidity and comfort over several days rather than judging the result immediately.
Extractor fan run-on controls
Run-on timers and humidistat controls are intended to keep removing moisture after a shower or cooking session. Do not disconnect the timer because the fan continues after the light is switched off. If the fan runs constantly, never starts or appears unable to clear steam, investigate the sensor, switch, duct route and external grille rather than simply lowering the setting.
What to double-check
- Filters: Check MVHR and PIV filters at the interval given in the manual. A loaded filter can affect airflow and noise. Isolate power only as instructed and fit the correct replacement.
- Air routes: Make sure supply valves, extract grilles, trickle vents and external terminals are not covered by dust, storage, curtains, insulation or nesting material.
- Condensate: For MVHR, inspect the accessible condensate route for signs of blockage or leakage. Do not dismantle sealed components unless qualified to do so.
- Boost operation: Test bathroom and kitchen boost controls, pull cords, timers, humidistats and remote switches. A control that appears to work may not be producing useful airflow.
- Humidity trend: Take readings at similar times for several days. A single reading can be misleading after cooking, showering, drying clothes or opening windows.
- Comfort: Check for persistent draughts, excessive noise, cold surfaces or stale air. These may indicate a balancing, ductwork or commissioning issue rather than a seasonal setting problem.
- Maintenance records: Note the date, filter condition, readings and changes made. This makes it easier to identify gradual deterioration and explain the issue to a ventilation professional.
If condensation or mould continues despite sensible ventilation use, consider the wider causes, including cold surfaces, leaks, drying laundry indoors and inadequate extraction. The room-by-room condensation and mould guide provides a broader troubleshooting route.
Common mistakes
- Turning ventilation off to save energy: This can allow moisture and pollutants to accumulate. Use the normal or low setting provided by the system instead.
- Closing every vent in winter: Reducing draughts without preserving the intended airflow path can worsen condensation and stale air.
- Leaving boost on permanently: Continuous high speed may increase noise and energy use without solving the underlying fault. Use boost for the activity or period it is designed to cover.
- Cleaning only the visible grille: Dust may also affect filters, valves, sensors, ducts or the external terminal. Clean only accessible parts unless the manual says otherwise.
- Changing several controls at once: If fan speed, heating, window use and room occupancy all change together, it becomes difficult to tell what improved the result.
- Assuming an open window fixes poor extraction: Natural ventilation can be useful, but it does not repair a failed fan, blocked duct or incorrectly configured system.
- Ignoring a new noise or smell: These can signal a dirty filter, obstruction, worn component or electrical problem. Arrange a qualified inspection if basic checks do not resolve it.
When to revisit
Repeat this checklist at the start of winter, summer and the milder periods between them. Revisit it sooner after moving home, adding occupants, changing a bedroom into another type of room, installing new windows or insulation, or noticing condensation, mould, odours, noise or uncomfortable draughts.
At each review, record the date, indoor humidity readings, system setting, filter condition and any symptoms. Make one reasonable adjustment at a time, allow several days for the home to settle and then compare the result. If airflow remains weak, humidity stays high, the system leaks, or controls do not behave as expected, seek a ventilation maintenance service or qualified installer rather than repeatedly changing settings.
For decisions about upgrading or replacing equipment, compare the system’s design, ductwork, controls and maintenance needs—not just the season. The guides to MVHR versus PIV and home ventilation costs can help structure that assessment. A seasonal check should leave you with reliable background airflow, effective moisture extraction and a written record of what changed.
