INNOVATION May-June 2012

ex ltration

f ea t u r e s

Don’t Blow it out Your Top – Cool It!

ex ltration

neutral pressure point

Jeff Besant PEng and John Lovatt PEng

in ltration

O ne of the important functions of building ventilation systems is to condition outside air. To illustrate how significant this is, consider that the mass of the air supplied by a make-up air unit on the roof of a 20-storey multi-unit residential tower in one year would exceed the mass of the entire building by a significant margin (or put another way, the volume of air processed in a year would be greater than all the air above the building from ground to outer space). Ventilation air provides health and comfort to the occupants, but excessive ventilation air can waste energy, cause problems for the building structure and the other building systems. During the winter months, it is sometimes helpful to think of a building as a giant, heavy hot air balloon. The problems come when you put holes in the top and bottom; then the balloon becomes a chimney such that the taller the balloon and the greater the indoor to outdoor temperature difference, the greater the chimney airflow. Quite literally the elevator shafts are like large chimneys that extend from the parking garage to the roof. This is called stack effect (Figure 1). In the winter, driven by stack effect, conditioned air escapes through cracks and openings in the building envelope at the top. This depressurizes the lower floors and as result cold air enters into the bottom of the building, especially through open windows and doors. Commonly, in commercial buildings, the ventilation systems have the ability to respond to variations in the pressure resulting from stack effect. However, this is typically not the case in high-rise residential buildings. Also, residential buildings are much more likely to have operable windows making it harder to control pressure differences.

The Canada Housing and Mortgage Corporation’s (CMHC) 2012 Report, “Corridor Air Ventilation System Energy Use in Multi-Unit Residential Buildings” and RDH’s Graham Finch PEng (“Energy Tune-ups for Mid- to High-Rise Residential Buildings,” 2011) have estimated that as much as two thirds of the conditioned ventilation air supplied by a make-up air unit escapes without ever reaching the suites in high-rise multi- unit residential buildings. In fact, CMHC stated that “the functionality of corridor air systems should be questioned since significant amounts of the air provided do not flow as intended.” This implies that perhaps two thirds of the energy used to condition outside air is wasted. If this is the case, in a high-rise residential building (20+ floors) that employs a roof-mounted indirect gas-fired make-up air unit with a rated input of 700,000 BTUH, 80% efficiency and a capacity of 8,500 CFM, the energy wasted in a single year may exceed 650 GJ (and at $10 per GJ, this amounts to $6,500 per year in gas costs). One leakage path for ventilation air is through the elevator doors, out the top of the elevator shafts and through openings in the walls of the elevator machine rooms. It is common for elevator machine rooms, in residential buildings, to rely on forced convection for cooling using an exhaust fan. The typical elements of a forced convection cooling system include: an intake louvre, an air filter, occasionally a motorized damper, and an exhaust fan mounted either high on the wall or on the ceiling of the elevator penthouse. The exhaust fan and motorized damper (if used) are often operated by a reverse acting thermostat that is typically set between 22ºC and 26ºC.

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