Understanding heating loads in commercial buildings

We recently encountered some head-scratching results in an energy analysis of an existing office building in downtown Ottawa. This is an old building with out-of-date, metal-framed, leaky windows and we wanted to know what would happen if we replaced the old windows with well-sealed insulating windows. Expectations for energy savings for high, but when we finished the analysis the annual energy savings were only 7%. What happened?

A common trap that people fall into when thinking about energy in large commercial buildings (such as office buildings) is to think of these buildings as much larger versions of a house. Many people have an intuitive sense of how to save energy in houses. Houses in Canada typically use most of their energy for space heating, and the best way to reduce space heating energy is to add insulation, use better windows, and minimize infiltration by sealing all of the cracks, penetrations, and gaps in the envelope. If you do this very well, add a good heat recovery ventilator, and orient the building so that enough of its windows face true south you might even build a Passivhaus that can be heated with a 1000 watt hair dryer.

So what is different about energy consumption in large commercial buildings? Here are three key differences to consider:

1. They have a much larger floor area relative to the area of the walls and roofs.
2. Office buildings have a higher occupant density and require more outdoor air to ventilate the building during occupied hours.
3. Office equipment, lighting, and computers use a lot of energy and also add heat to core spaces that have no exposure to the exterior.

For example, let’s consider a ten-storey office building. In this building, the middle eight storeys will have no exposed floors or roofs, and only the spaces on the perimeter of the building will have exterior walls. A large number of the spaces will be in the core with no exterior exposure. These core spaces will be filled with people, computers, office equipment, and lights, all of which give off heat. Since the core spaces have no exterior exposure, they will need to be cooled, even in winter.¹

So how do these factors come together to explain what’s happening in our existing office building? Digging into the energy modelling outputs reveals that space heating makes up 61% of the total annual energy consumption, while lights, office equipment, and space cooling make up another 29%. Of the 61% fraction for space heating, heating outdoor air for ventilation is 63% of the space heating load and making up for heat losses through the walls, roofs, and windows is 37% of the space heating load. This means that space heating losses through the building envelope account for 22% of the total building energy consumption. It turns out that improvements to the windows cut heat losses through the envelope by over 30%. As expected, this is a dramatic reduction, but since the envelope is only responsible for 22% of the total annual energy we are left with only 7% in total energy savings. Mystery solved! In this building, it turns out that the heating load on outdoor air brought in for ventilation is a large piece of the energy pie, something that replacing the windows does not address.

Ideally, the heating system would be redesigned to use exhaust air heat recovery to preheat the incoming outdoor air, and the older, draughty windows would be replaced with well-sealed, insulating windows. Both of these strategies together would lead to 30% energy savings. Replacing the windows would also have the complementary – and more valuable – benefit of greatly improving comfort by eliminating cold draughts.