Bigger isn’t always better for improving ventilation

One of the defaults for designing and specifying ventilation systems for buildings has often been a ‘bigger the better’ approach. But that doesn’t always mean those systems are delivering the best result, and definitely means systems can require more energy use than would be ideal from the perspective of both utility costs and NABERS ratings.

With the rising focus on reducing energy use as part of our transition to full electrification and decarbonisation of existing buildings, it’s also important to consider that simply switching out one system for another less energy-hungry one may also be suboptimal.

The lifespan of the equipment is one of the reasons careful thought needs to be given both for new design and retrofits. Heating, ventilation and air conditioning (HVAC) systems are not like lightbulbs, where we can just swap incandescent for LEDs without losing too much of the capital and resources invested in the item.

Air handling units (AHUs) have an average operational lifespan of 25 years, and if the overall system is being managed in a smart and energy efficient way, they may function effectively for 30 or 35 years. A chiller usually has an operational design life of 25 years, and with the right maintenance can generally support cooling for up to 30 years.

If we think about it - the HVAC equipment in a building is potentially going to last almost as long as the careers of the people occupying it!

That means the decisions we make as designers, specifiers and installers have a lasting impact and need to be carefully considered.

Where we need to start is with understanding how the building operates – or in the case of a new build is likely to operate. We also need to apply building physics thinking to gain an idea of how the building will function thermally in passive mode, that is, with no mechanical heating, cooling or ventilation.

Layer into this local climate conditions and the type of building it is – for example, areas where there will be heat-generating activities or equipment such as catering kitchens, laundries, sterilisation rooms or data racks, will have a higher internally-generated heat load.

A building where doors are constantly opening and closing such as a retail centre or airport may have indoor conditions that are more affected by ambient, outdoor air temperatures.

Put all of those things together and we have a design basis for thermal comfort – what the range of conditions will be through time, and therefore what degree of mechanical HVAC will be needed to keep conditions within the zone required for wellbeing.

We also need to consider how outside or fresh air comes in, and how the quality of indoor air will be optimised in both passive and mechanically-assisted modes, and again, the ideal is to use passive ventilation effectively to reduce energy waste but also while minimising leakage of cooled air in summer and warmed air in winter.

There will also be limitations and non-negotiable factors to consider such as the façade design, potential locations for plant, electrical loads, fire safety requirements for necessary penetrations, possible riser locations, floor plate configurations and so forth.

Based on our experience, there are some of these factors may need a discussion with the wider design and delivery team if we are to ensure a new building or refurbishment/retrofit delivers the best outcome for comfort and efficiency. Because a variation at design stage is always more cost-effective, time-effective and simpler than one done halfway through the build or post-construction when the building is occupied.

Variations that introduce greater flexibility are some of the most effective. For example, operable or moveable walls [internal and external] combined with a distributed and zoned smart HVAC system with multiple small AHUs rather than one large one serving the entire building enables uses of floorplates and occupancy patterns to change through time without needing to significantly alter the HVAC system. A large tenancy can become smaller ones, or vice versa.

The other advantage I’ve seen of multiple AHUs – one per floor for example – is any maintenance, upgrade or other alteration of the AHUs does not mean having to switch off ventilation for the entire building as is the case with one single, large AHU for the whole building.

Operable walls, operable windows and operable façade elements also allow for mixed-mode ventilation approaches that save on energy. For residential buildings, having cross-flow ventilation is important, so having operable doors/windows/walls on opposite sides of each room facilitates this.

For schools, having a shaded southern exposure and an operable northerly one with a façade that enables capturing winter sun internally combined with cross-flow ventilation and operable internal walls supports flexible learning spaces and the best conditions for learning.

One of the building physics approaches we often design for is the ‘stack effect’, whereby the natural tendency of warm air to rise and colder air to sink is leveraged to encourage both fresh air movement and the distribution of either passively or mechanically-cooled or heated air. But if the architectural design closes off and isolates individual floors, this does not work effectively. Designing for thermal chimneys or other structural features – atriums or connecting internal stairways – supports the stack effect but does involve a tradeoff with net lettable area.

The same is true of distributed plant and equipment – if there is mechanical equipment on every floor, NLA is reduced by a fraction. That’s where thinking about the tradeoffs between revenue from NLA and operational costs for HVAC needs to be part of the design decision-making conversation with the client.

Conversation is ultimately one of the most important aspects of HVAC design for either new or existing building projects. That includes having effective coordination between disciplines, so other vital performance aspects such as fire safety and acoustics are also factored in.