Natural ventilation shafts do not operate like clockwork. Air movement obvious at grilles in winter may nearly disappear on a warm still day. That alone does not prove blockage, but 'normal in summer' does not establish sufficient room ventilation. Distinguish seasonal draught changes from duct defects and plan for every season.
More than shaft height matters
Warmer air has a lower density. When it is warmer inside a building than outdoors and suitable air entry and exit routes exist, an upward movement can develop. Its strength depends on height and temperature differences, as well as resistance. The PNNL passive-cooling description discusses this as one driver of natural air movement.
In summer, temperatures become closer, so one of the forces driving the draught weakens. Wind, the positions of surrounding buildings, roof shape and opening locations also have an effect. Neighbouring homes or different storeys of the same home can therefore behave differently. Saying that all shafts stop working completely in summer would be as inaccurate as promising that a taller pipe alone will always solve the problem.
Air leaving needs air entering
It is not enough to assess a duct's draught without knowing where replacement air comes from. If all windows are airtight, inlets are closed and internal doors restrict transfer, extraction becomes limited. Aereco supply-air solutions present inlets as parts of natural, hybrid and mechanical-extract systems, not standalone whole-house ventilation.
However, a partly open window is only a change in observation conditions, not an acceptance test for the whole system. Increased movement at a grille suggests a possible influence from the air supply, but does not establish the exact flow or duct condition. Nor is it known whether enough air will reach every room at night when doors are closed again. A measurement must be accompanied by a clear use scenario.
What owners can record safely
It is useful to record the date, outdoor and room temperatures, wind and the state of open passages. Note separately whether the cooker hood, bathroom fan and externally vented tumble dryer are running. One photograph will not show these circumstances. If there are several ducts, record their data separately: different routes may have different resistance.
Do not use a candle, lighter or burning paper beside the grilles. This is not a necessary household test and creates a fire risk. Nor should you climb onto the roof to look for a blockage. If signs of smoke or combustion products entering appear, stop ordinary ventilation observation and seek help. A ventilation duct and a flue are assessed according to their respective purposes.
Illustrative example: the bathroom dries only in winter
Suppose the walls of a single-storey home's bathroom dry fairly quickly after a shower in winter, but remain damp on a July evening. The outdoor and indoor temperatures are similar. The residents turn the grille the other way, but the result does not change. In this situation, the grille's shape is not the first question: the limits of the natural system and actual moisture removal must be checked.
A specialist assesses the duct route, incoming air, transfer between rooms and the possibility of providing suitable assisted extraction. If a fan is chosen, its operation, reverse-flow restriction and compatibility with the system must be resolved together. Buying according to the highest capacity written on the box does not show how the appliance will work when connected to the particular duct. An unauthorised alteration to a shared shaft in a block of flats can also affect other flats.
When to suspect a defect rather than season
A sudden change after roof repairs, signs of a bird's nest, crumbling masonry, persistent unfamiliar smells or different performance in rooms that previously behaved alike all require inspection. Similar air temperatures do not explain every fault. It is useful to list what has changed: windows, doors, the duct cowl, kitchen equipment or an additional fan. Often, the date of the change is what helps narrow the investigation.
The PNNL air-movement explanation highlights the relationship between openings, resistance and pressure. The practical conclusion is simple: do not repair only the most visible component. A new cowl fitted at the top of a duct may change conditions without resolving a closed-door problem. Conversely, a larger gap under a door will not restore a shaft blocked by fallen material.
Why a backdraught damper is not always the answer
A backdraught damper offered in a shop may seem a simple way to stop air moving in the wrong direction. However, the component itself has resistance and particular opening conditions. It can become an additional obstruction to a weak natural draught. Moreover, a closed component does not replace the room's necessary ventilation. Suitability must be checked for the entire layout, not just according to a matching pipe diameter.
Roof-mounted draught-improving devices should be assessed similarly. The proposal should make clear what problem they address, their operating conditions and who will maintain them. If it is unknown whether the duct is clean and airtight, a new top component is a guess, not a diagnosis. A safe inspection and information about the existing system are needed first.
Pay attention to the room's purpose too. A shower room installed in a formerly dry store creates a different moisture load. The old duct may be undamaged but insufficient for the new use. This mismatch needs to be addressed as a change in demand, rather than merely by repairing sound masonry. A record of room use helps choose a justified next action.
What should the inspection deliver?
The owner needs to know more than 'it draws' or 'it does not draw'. It is worth stating the measurement conditions, identified defects, incoming-air route and intended warm-weather ventilation in the inspection conclusion. If the system depends on weather conditions, that dependence must be stated clearly, not hidden behind one successful winter measurement.
When choosing alterations, also discuss quiet night-time operation, maintenance and a power outage. Mechanical assistance can help, but does not in itself resolve a poorly chosen location or blocked airflow route. If the home will be renovated, it is worth coordinating the solution with its future airtightness. A random gap left today may disappear completely after new finishes.
General airflow-demand principles are in the airflow selection article. In your enquiry, state the season and conditions in which the problem is strongest.
Information sources
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