Every building across Canada we need some form of HVAC system to keep the occupants comfortable and healthy.

HVAC is an acronym for: Heating, Ventilation and Air Conditioning and in this blog we will focus on the Ventilation component.

What does ventilation mean?

Ventilation is the means to provide fresh outdoor air (make-up air) and remove contaminated air such that the occupants are comfortable and healthy.  The code related to complying with this is ASHRAE Standard 62.1.

In this particular installation we will explain how the ventilation system will fail and be unable to provide fresh make-up air to the occupants.  Being that this is a doctor’s office makes it even more imperative to provide proper ventilation.  

First, we need to explain the basics of what a heat pump does. A heat pump, heats or cools the air to keep the occupants thermally comfortable as per ASHRAE Standard 55.

The drawing below is of the base building HVAC system.

  • RED is the make-up air ducted from the outside to the heat pump.

  • BLUE is the return air that pulls air from the space into the heat pump to be heated or cooled and is mixed with the RED make-up air.

  • GREEN is the supply air that has been treated and is pumped back into the room.


Like water; air follows the path of least resistance.

Since the make-up air is a longer and smaller duct compared to the return air, air will not flow through the make-up air but 100% through the return air.  As a result no fresh outdoor air will be provided to the occupancy.

The solution is either to add booster fans in the make-up air duct or to add balancing dampers in the return air duct thus forcing more make-up air to be pulled into the heat pump.

In the photo below the base building mechanical engineer provided the 2nd option: dampers in the return and make-up air ducts denoted by the ‘L’ shape object.

To get the desired make-up air, the return air damper will be closed off by a certain percentage such that make-up air will be pulled into the heat pump(s).



The problem is when Evolve Mechanical Solutions visited the site no balancing dampers were installed in the return or make-up ducts.

Luckily we caught this early on because if we had not the project would have been delayed at final occupancy.  This would have caused the doctors not to move in and be able to treat their patients. 



As always please hire experienced trades and engineers to design and inspect your projects to avoid these costly issues. 

Feel free to comment and let me us your thoughts.

James Hicks P.Eng.

Evolve Mechanical Solutions


#Fail #HVAC