Rightsizing Systems

I call this the "Goldilocks principle" of building design: systems should be the right size for the job, neither too big nor too small. This seems obvious, but since temperature complaints are a large source of occupant discomfort and criticism in most buildings, engineers have become naturally conservative in sizing systems, making them larger than they need to be, both to avoid complaints and to provide an extra cushion for extreme events and the degradation of system performance over time. This decision costs a lot of money, since HVAC systems alone can cost 10% or more of a building's budget.

Right-sizing a system doesn't start with the HVAC system. It begins with proper building orientation; generally in the US and Canada, buildings should be oriented with the long axis in the east-west direction, allowing for less heat gain through the east and west windows, when sun angles are lower. This measure alone can save 10% or more of the energy use of a similar building oriented in the north-south direction. For better day-lighting, buildings need to be more rectangular than square, But unless daylighting can be sold as an important feature of an office building, this is not usually considered an "efficient" design from a developer's perspective, because it results in a smaller percentage of leasable area compared with a square building design, so there may be a built-in conflict here between economic efficiency and good design practice.

Then one needs to consider how to keep direct sunlight out of the building in summer and to let it in during the winter, a basic tenet of passive solar design. This can be done with selection of glazing that restricts incoming sunlight or with external shading devices that keep the high summer sun from entering the windows. Thermal-energy storage systems are used to reduce the demand for cooling during summer afternoons, often a leading cause for over sizing systems. In large projects, it may be practical to include space for installing additional cooling capacity if the type of occupancy changes, just to be on the safe side, rather than spending the extra money upfront.

Andy Frichtl, a senior engineer at a mid-sized consulting engineering firm, Interface Engineering, Inc., says that right-sizing requires engineers to go back to basics and develop system sizing from careful analysis of heating and cooling requirements rather than relying on handbooks and rules of thumb (such as 300 square feet of building area per ton of air conditioning) that already have too many safety factors hidden in them, justifying systems larger than necessary.135

Why should engineers do this? In a sustainable design project, each contributor needs to be mindful of the need to save money wherever possible to fund those green measures that are definitely going to cost more, such as photovoltaics for onsite power production or a green roof for open space, stormwater management and habitat restoration. In addition, buildings designed this way will be cheaper to operate, use less energy over time and may well be more comfortable for the occupants, leading to higher productivity and greater satisfaction.

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