An air supply opening in the bedroom and extraction in the bathroom look like a complete system. Yet when internal doors are closed, the air must still have a planned route between these places. If that route is blocked, actual airflows and pressures can change. The issue often becomes apparent not during the ventilation installation itself, but when doors are fitted later, a new threshold is added or a thicker floor finish is laid. Air transfer therefore needs to be resolved before the finishes are completed.
The entire schematic must stay continuous
The DOE whole-house ventilation overview describes the principles of supply, extract and balanced systems. In practice, it is useful to mark on a plan where outdoor air enters, which areas it passes through and where it is removed. If the arrow stops at a door, the total supply and extract figures do not yet demonstrate adequate ventilation in each room.
It cannot be assumed that people will always leave doors ajar. A bedroom at night, a bathroom during use or a study may have its door closed. The actual system check must reflect these situations. If the design depends on a particular door position, this must be clearly defined, although a permanent requirement to live with doors open is often not a desirable solution.
A gap is more than height
A gap under a door is assessed by its free area and permissible resistance, not by one number of millimetres stated for everyone. The door width, floor finish, threshold and actual airflow matter. A carpet or a sealing strip added later can change the intended passage. The final condition must therefore be checked after all finishing work.
The PNNL air-movement explanation relates airflow to its route and the pressure difference. An opening that is too small can increase the pressure difference between rooms. This may show up as noise, doors that are harder to move or altered airflows. These signs are not a final diagnosis, but they are a reason to check the solution rather than automatically increase fan power.
When transfer grilles are considered
If a gap under the door is unsuitable because of airflow, privacy or other conditions, transfer grilles or another planned route can be designed. However, their free area is not the same as the outer dimensions of the frame. Pressure-loss data for the particular product and a clearly defined installation detail are needed. An acoustic version also has its own operating conditions.
The transmission of light, odours and sound must be considered. A simple open grille may be suitable in one location and unacceptable in another. The solution must reconcile the air route with the room's use, not merely allow air to pass physically. If a door or partition is subject to special fire safety requirements, it must not be cut or altered without authorisation.
Door suppliers need the brief
When ordering doors, provide the intended transfer detail in advance. A gap cut later can affect the finish, construction or properties specified by the manufacturer. This part must be coordinated between the designer, door supplier and ventilation contractor. Everyone must work to the same final floor level, not to different versions of an earlier design.
Similarly, a bathroom threshold or waterproofing detail should not be altered solely for ventilation. A coordinated solution must be found that does not impair other important functions. Air transfer does not give permission to disregard water containment, acoustics or safety. If there is a conflict, it must be resolved in the design, not by the last tradesperson working on site.
Illustrative example: the system became noisier after doors were fitted
Suppose ventilation in a new home was tested before the internal doors were installed. Doors with a small gap are fitted later, and noise develops in one room. The owner increases the supply setting, but this does not help. In this situation, it is essential to check whether the changed transfer route has created additional resistance. The earlier measurement does not cover the final condition.
A specialist compares airflows and pressure conditions with the doors closed and in their normal positions of use. If a problem is found, the required transfer detail is agreed and the system is adjusted again after the change. Simply cutting all doors to the same height is insufficient. Different rooms may have different airflows and other requirements.
Reports need the actual configuration
It is worth recording the positions or settings of doors, grilles and ventilation in the test report. If some finishing work remains incomplete, this must be identified as a limitation of the check. The Komfovent installation manual covers system installation and final checks; the documents for the particular model must be related to the whole property's layout, rather than treated as an isolated piece of paperwork.
It is useful for the owner to have a clear photograph or drawing showing where the transfer elements are provided. When later changing a carpet or adding a sealing strip, they will then understand that they are changing more than the door's appearance. This helps prevent a sound system from behaving differently after a minor repair, with the cause going unnoticed for a long time.
Test normal living conditions at acceptance
For the check, it is not enough to leave all doors open merely because this makes walking around with the measuring instrument easier. Discuss which rooms are used with their doors closed and how this will be assessed. If the owner sleeps in a closed room, that circumstance must be known rather than treated as an unexpected breach of operating conditions after installation. The same applies to a separate laundry room or bathroom.
After the final adjustment, the intended airflow route can be shown to the owner in a straightforward way, identifying the elements that must not be covered and leaving instructions. This is not merely an extra piece of paperwork: someone carrying out repairs later may not know why that particular gap was left under the door. A clear description protects against an accidental change to the system.
What not to do yourself
Do not alter the positions of adjustable supply and extract valves to compensate for a closed transfer route. This can unbalance other rooms. Do not seal grilles merely because of noise before a suitable alternative has been selected. Nor should you add another fan without checking the system. The identified cause must be addressed, rather than increasing the operating setting of every appliance.
More on airflow demand is in the selection article. In your enquiry, provide a plan, photographs of the doors and thresholds, and information on when the problem began. A clear layout of the final condition helps select an air transfer solution that will continue to work when room doors are closed.
Information sources
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