Modelling for Ventilation Design

Ventilation modelling balances two competing needs: enough fresh air to remove moisture, odours and pollutants, and little enough heat loss to keep a home efficient and comfortable. A model estimates ventilation heat loss from the air change rate, which is how often indoor air is replaced, and from the temperature difference between inside and outside. It also considers moisture production from cooking, bathing and occupants, and pollutant sources such as cooking fumes and cleaning products, to judge whether a given air change rate keeps indoor conditions acceptable.

Ventilation comes in several forms, and modelling treats them differently. Uncontrolled infiltration through gaps and draughts is variable, weather-dependent and hard to predict, while purpose-provided ventilation, through trickle vents, intermittent extract fans, continuous mechanical extract, or mechanical ventilation with heat recovery, is more controllable and modellable. A home that relies mainly on infiltration may be draughty in winter and stuffy in calm summer weather, while a home with designed ventilation can maintain steadier air quality with lower heat loss.

Heat recovery changes the balance further. Mechanical ventilation with heat recovery extracts stale air, recovers much of its heat, and supplies fresh air at a temperature closer to indoor temperature, reducing ventilation heat loss substantially compared with opening windows or using extract-only ventilation. Modelling heat recovery requires its efficiency, its bypass behaviour in summer, its frost protection and its maintenance state, because a poorly maintained or badly commissioned system recovers less heat than its rated efficiency suggests.

Moisture is the constraint that most often decides ventilation design. A home produces several litres of water vapour daily from normal living, and without adequate ventilation that moisture condenses on cold surfaces, feeds mould and damages fabric. A ventilation model should therefore show not only air change rates, but also moisture risk in vulnerable rooms and details, such as bathrooms, kitchens and cold corners, particularly after insulation or draught-proofing reduces uncontrolled air leakage.

Before accepting a ventilation design, ask for the design air change rate, the ventilation strategy by room, and the heat recovery efficiency where applicable, with its maintenance requirements. Check whether the strategy provides for boost ventilation in kitchens and bathrooms, for summer bypass or purge ventilation, and for occupant control, such as boost switches or humidity controls. A design that provides a single whole-home rate without room-level thinking may leave wet rooms under-ventilated.

Also check the interaction with airtightness work. Draught-proofing, new windows and insulation reduce infiltration, which is good for energy, but they increase the importance of purpose-provided ventilation. A retrofit package that tightens a home without a ventilation design is incomplete, and a model that shows only energy, without a ventilation note, should not be relied on for such a package.

By place, ventilation requirements and conventions differ. UK building regulations set ventilation requirements for new homes and for work on existing homes, with guidance on rates and system types, while EU member states set their own ventilation requirements through national codes under the EPBD framework. A ventilation design that meets one country's rules may not meet another's, and local regulations should be checked for the property concerned.

Occupant behaviour also affects ventilation in practice. Windows opened for purge ventilation, fans switched off because of noise, and vents blocked to reduce draughts all change real air change rates from design rates. A good design therefore considers noise, usability and maintenance, not merely modelled rates, because a system occupants disable is worse than a simpler system they use.

A common mistake is to model ventilation as a single number for the whole home, ignoring room differences and moisture sources. Another mistake is to tighten a home for energy without designing ventilation, and then to discover condensation and mould that a ventilation model would have predicted.

A further mistake is to specify heat recovery without a maintenance plan. Filters that are never changed, ducts that are never cleaned and controls that are never checked degrade performance and air quality, turning an efficient design into a neglected system.

Keep the ventilation design, the air change assumptions, the system specifications and the maintenance plan together with the energy model. That file supports commissioning, servicing and future retrofit stages.

Questions to ask include: what air change rate is designed for each room, how is moisture handled in wet rooms, and what maintenance does the system need. Those questions test a ventilation design properly.