When Fresh Air Isn’t Fresh

Posted · Add Comment

Maine has an environment that is somewhat difficult in which to design an HVAC system and building envelope. Temperatures can swing wildly within 24 hours causing the HVAC system to cycle from heating at night, to economizer cooling (using outside air to cool), to mechanical cooling. So the interior environment can see a variable amount of outside “fresh air.” That’s a good thing, right? Well, it depends on what is being drawn in with the outside air.

Outside air is loaded with moisture, biological and non-biological dust, dirt, mold, bacteria, insects, pollution, odors, etc. Don’t get me wrong; fresh air is good for you and helps to create a healthy inside environment. But, if the fresh air is not “fresh,” it can also be a contributor to poor indoor air quality. In this column we’ll look at the outside air intake environment.

First, consider the path of the outside air entering the building. You want all outside air to enter through the HVAC system so it can be filtered and conditioned before entering the occupied space. This means the building should be at a neutral to a slightly positive pressure relative to the outside (+.05 WG maximum).

Next, survey the outside air intake location(s). Hopefully you have some, but I have found that is not always the case.

No Outside Air Intake

Outside Air Intake Hood Installed

These may be hooded louvers on the side of rooftop or ground level air handlers, or fixed louvers in the exterior wall of the building, above or below ground level. It could be the outside air is coming through an air-to-air energy recovery unit, which would also have an outside air intake. Make sure the intake locations are drawing air from a clean well-ventilated environment. I investigated a YMCA once where the outside air intake for the gym was located in a pigeon infested ally completely closed in by buildings. Dead bodies, nesting material, and feces were everywhere, and feathers completely plugged the intake louver.

Raised plumbing vents to prevent sewer gas entrainment

On roofs around the intakes look for ponding and stagnant water, bird or insect nests, outlets of exhaust fans, other AHU exhaust air discharges, boiler or heater exhaust stacks, and plumbing vents. Determine if an exhaust nearby is particularly bad, such as a dental vacuum discharge, boiler or generator exhaust, or a fume hood with high VOC or biohazardous effluent.

Ponding around OA intake

Boiler stack up wind of OA intake in prevailing winds

For wall louver intakes, look at parking and loading dock locations, designated smoking areas, exhaust fan discharges, and dumpster locations, etc. All of these can result in re-entrainment of exhaust gasses or undesirable odors and contaminants.  Sides of buildings are susceptible to exhaust contaminants, even those from roof discharges, when the exhaust is trapped up against the walls due to wind vortex currents.

Alley intake subject to car/truck exhaust fumes

Below grade, wet, trashy intake area

For ground level AHU’s consider all the sources mentioned, but also look for landscaping mulch (which is made-up of decomposing moldy wood fibers), mowing activity exhaust gases, and cut grass blown into the intake screen.

All outside air intakes will have some sort of screen protection to prevent birds and insects from entering. These screens can become blocked by debris preventing proper outside air flow, and the debris can become a site for mold growth. If there are trees nearby consider leaf and pollen blockage. If there are outside lights nearby, the intake can become clogged with insects. Laundry discharges can produce lint, and loading docks can produce paper and plastic debris, which can cause blockages in the screens. This high level of debris in the outside air can lead to varying levels of air filter loading. If filters are not changed on the basis of filter condition, but by a set schedule, the results can be collapsed filters and dirt and pollen coated coils and ductwork. Not a good scene.

Outside Air Intake Hood

Plugged intake mesh screen, and wet debris pond

Clogged Intake screen

Moist collapsed air filter with mold growth

Inside some duct systems there may be other locations that, if not inspected and maintained regularly, can contribute to poor air quality by blocking outside air flow or becoming a breeding ground for mold. Devices like turning vanes, flow straighteners, dampers, and flow measuring stations can collect debris in the unfiltered air stream.

Honeycomb air flow straightener plugged

In the winter, make sure the intakes are not susceptible to being covered with deep or drifting snow. I’ll be discussing designing outside air intakes for snowy regions in another article.

Be sure the inspection of the outside air intake and its environment are part of your preventive maintenance program. Every building is different, so survey yours and learn how your building lives and breathes to ensure a healthful indoor environment.

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.  


Join Our Mailing List

Join Our Mailing List

Join our mailing list to receive the latest news and updates from our team.

You have Successfully Subscribed!

Sign Up for The Informer

Sign Up for The Informer

Receive the quarterly newsletter of the Maine Indoor Air Quality Council

You have Successfully Subscribed!