We get calls every year during periods of extreme heat from people asking why their air conditioning systems cannot keep up.  To answer this question, let us first talk about what goes into sizing the equipment and what restrictions energy codes place on this process.

Loads on a building can be broken into two categories: internal and external. Internal loads occur inside the building – people, lights and electrical. External loads come from the building envelope – windows, walls, roofs, and ventilation (including infiltration).

Internal loads can be controlled by the building’s occupants: lights can be switched off in unoccupied rooms or dimmed in occupied spaces, if daylighting allows, computers and copiers can be shut off when not in use and people can work remotely if applicable. Occupants have less control over the external load components. The ability to control these components come from drawing window coverings and minimizing ventilation air (fresh outside air) for the system.

Let’s talk specifically about minimizing ventilation air for systems as this has recently become more of an issue due to COVID. OSHA has mandated that commercial buildings with more than 10 employees must optimize the amount of outside air circulated through its existing HVAC systems to the extent the system(s) can do so. So many companies have been increasing the amount of outside air introduced into their systems to the highest extent possible. This poses an issue because increasing the percentage of outside air increases the mixed air (mixture of return air and outside air) temperature of the system. This increase in mixed air temperature results in a 24% load increase over design for a 5-ton system, which means the system is no longer able to cool to design indoor temperatures.

HVAC systems are sized with very specific parameters as dictated by Code and ASHRAE. Systems across the Portland metro area are sized to cool to an indoor temperature of 75 deg F (dictated by Oregon Energy Code) on a 90 Deg F day (dictated by ASHRAE) with minimum ventilation air (dictated by Oregon Mechanical Code) being introduced into the system.

As you can see though, we are in unprecedented times with outdoor temperatures far exceeding design and increased outside air loads due to COVID measures. So please understand that your systems will probably not maintain 75 deg F during these times as they were not designed to do so. And please don’t turn your thermostats down below 75 deg F, this will not help. Employ as many of the control measures as possible – close windows coverings, dim lights and turn off electronic devices when possible. These measures will help your systems overcome the overwhelming temperature burdens they will be facing.