Negative Truth About Outdoor Allergies, Air Filters and Room Air Purifiers

Air filter for air conditioner

December 1, 2015

Bought one of those TV sold submarine tested super air purifiers, MERV 12 pleated allergies filters or filter less electrostatic precipitators and still suffer from allergies? The problem isn’t with the filter or the system; it’s probably with your home and its energy efficient heating and air conditioning system. Hospitals, most commercial buildings and the military use over pressure, or positive air pressure to keep unwanted particles and gases out of their air..

Your humble dwelling leaks air. Most homes are under negative air pressure, which draws in pollen, dust and other bad stuff from the outside under doors and cracks around windows. The average home relies on re-circulating conditioned air from inside the home to save energy. Unless your HVAC system brings in air from a controlled outside source known as supply or make up air your home acts like a big vacuum. This brings in all the bad particles (pollen, dust and bacteria) you are trying to eliminate. Most modern commercial office buildings and all hospitals are under positive air pressure and most homes are not. Simply put, air must be forced into a building or room to create positive pressure.

You can easily test your home for positive air, turn on the fan in your system and slightly crack the front door. Place a very small piece of tissue paper near the crack. You can do the same thing with smoke from incense. If it blows back into the house your home is sucking air. When the surgical suite at the local hospital goes under negative air pressure alarms go off and the operating room is closed until the problem is fixed. Patents typically have suppressed immunities and normally harmless bacteria can be life threatening. Ideally, if you suffer from outside allergies you want your dwelling to leak outwardly and push the bad actors out not bring them in.

Now that you know the problem, survey your home. See where the supply air intakes are located (many times this is where the filters are located) as well as the registers that distribute conditioned air. If your systems supply air is coming from your bedroom, it almost certainly under negative air pressure. Use a tissue or incense to test the bedrooms. Are they positive to the rest of the home or under negative pressure?

Your HVAC contractor can install a supply air intake along with an auxiliary filter box for your system. Ideally this will be located in the attic or under the roofline to reduce moisture. Please note this may require more capacity or tonnage to keep your home comfortable and it will require more energy to run. A MERV 8 or better air filter will capture most pollen on the first pass and under positive pressure particle counts of allergens and dust will stay low as long as you continue to run the fan.

Here are other options you may want to consider. (1) If your bedroom has no supply air intake and you have purchased a room air purifier with inflow ducting you can bring in supply air from the next room or from outside. Kits are available from the manufacture or distributor if the system is designed for use to overpressure a room. This makes this room positive to the rest of the house and maybe you can finally get some relief and rest at night. (2) If your bedroom has a supply air intake you may consider having your contractor moving it into another room or changing bedrooms during allergy season. (3) Purchase inflow duct supply air purifiers for other rooms you spend time in. Please note that you must draw supply air from another room or from outside. If you bring in supply air from outside it will not be conditioned but it will be filtered. If you bring air from the next room that room will be under negative air pressure (4) Use ventilation fans in bathrooms and kitchens as well as chimney flues sparingly because they create negative pressure.

Good filtration is still better than no filtration; it just works better with overpressure.

Dean Philpot is a Certified Air Filtration Specialist and has provided indoor air solutions to a number of hospitals and commercial buildings. He works for Filtera Commercial Air Filtration. The company manufactures and distributes air filters and air purifiers from its website He recommends the IQAir HEPA System along with a inflow ducting kit to positive pressure and filter outdoor allergens.

Source: IAQ Times – Archived Air Quality News May 05, 2004

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