Summer HVAC Check-up: Are You REALLY Getting Enough Fresh Air?

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Submitted by MIAQC Member Kris Anderson.  Kris is a professional engineer and president of K.G. Anderson, an engineering and consulting firm from Bath, Maine specializing in hands-on whole building and building-systems problem-solving.  

Summer is finally here and with the seasonal changes comes a variety of IAQ issues. One issue I see regularly, which creates air quality problems for commercial buildings, is the lack of outside ‘FRESH’ ventilation air being introduced into the occupied space. Now is the time to have your heating, ventilation and air-conditioning (HVAC) system checked.

Way back when the building’s HVAC was first designed, or redesigned for the current occupant, the outdoor ventilation rate, expressed in cubic feet per minute (CFM), was established. It is typically based on the number of occupants in the space, and many other factors which may come into play. Outdoor air ventilation is important to dilute interior pollutants such as food odors, people odors, scents, CO2 (which is exhaled by people), and VOC’s off-gassing from building materials and equipment such as printers. It also helps to pressurize the interior space to prevent unfiltered and unconditioned air from leaking into the interior space creating uncomfortable perimeter zones. It also prevents dust, pollen, and odors like vehicle exhaust from entering the occupied space. Pretty important.

You might be saying, ‘I understand these issues, but if it was all set up and working originally, what would change?’

Well, a lot can change. One major thing that I find repeatedly in buildings is that during the fall or winter the outside air damper has been closed for some reason and never reset to operate at its minimum ‘designed’ position. This seems to occur most frequently on single or 2-stage gas-fired roof top units in an attempt to solve the complaint that there’s a cold draft when the gas heat is OFF. The simple fix is to just close the outside air damper fully and not worry about the air quality.

You can do a simple self-check to see if your roof top unit (RTU) is drawing in outside air by placing a piece of paper over the intake to see if it holds the paper firmly to the intake screen. If the paper doesn’t hold, contact your HVAC service company and ask them to investigate.

The outside dampers can be reset to some minimum open position to introduce some fresh air by removing a panel on the RTU and finding the economizer damper controller. On this controller there should be a small adjustment screw marked ‘MIN POS’, or something similar. With the unit running adjust the screw slowly in the direction indicated and give the controller a chance to modulate the damper open. If you find that the damper doesn’t move and the screw can be adjusted fully both ways, then the economizer controller needs to be replaced. The amount of outdoor air introduced will still be a guess at this point until it can be measured or calculated. However, just opening the damper about 25% will go a long way toward helping your IAQ.

You may also be surprised to find that your RTU has no outside air being introduced at all like this unit I found recently. Not a good situation.

Remember that HVAC systems differ and yours may look different than these pictures, but they all basically operate similarly. Have your HVAC technician show you your system and how the outside air damper works.

Have a healthy and happy Summer.

Kris Anderson
KG Anderson, Bath, Maine

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