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Outdoor Air Ventilation

What happens if you pass blower door testing?  You’re right to worry about making the house “too tight”.  Here’s the bottom line: Seal up the house tighter than a submarine and add mechanical outdoor air ventilation.

Combution, Makeup, & Ventilation Air Needed. Proper ventilation is needed to control indoor air pollutants and provide sufficient combustion air for fireplaces and gas appliances.  Besides people polluting the inside of their house through routine living activities, just about every manufactured item in the house outgases.  It’s easy to back draft fossil fuel and wood burning appliances in a tight house.

Do Not Use Air Infiltration. It’s irresponsible to use air infiltration as the means for providing ventilation, combustion, and makeup air.  Besides needlessly wasting energy, the air sources are uncontrolled, pulling in hot, humid, and dirty air in the summer and cold dirty air in the winter.  Air infiltration is also largely responsible for problems with dry skin and static electricity.

Outdoor Air Ventilation Methods. There are a variety of methods for providing outdoor air ventilation to a house.  They include Air Cyclers, enthalpy recovery ventilators (ERVs), heat recovery ventilators (HRVs), and ventilating dehumidifiers.  The method chosen depends on personal preferences and house location & construction.  Home InSight can help with selecting appropriate mechanical outdoor air ventilation, but an HVAC contractor is usually required to install it.  That’s what the “V” in the HVAC industry is for: ventilation!

Computing Outdoor Air Ventilation Needs. To figure out how much mechanical outdoor air ventilation to provide, the formula is simple.  This is the amount of air that needs to be deliberately brought into the house all day long, every day while the house is occupied.  The amount of air increases proportionally to the fraction of an hour the ventilation system runs to achieve the same amount of air exchange.

Ventilation CFM = (Floor Area / 100) + (7.5 * (# Bedrooms +1))

For example, a 2000 sqft house with 3 bedrooms needs 50 cfm of ventilation air every minute.  If a typical Air Cycler is installed, the air handler unit runs about 20 minutes an hour, so 150 cfm of ventilation air every minute is needed.

Add Additional Combustion & Makeup Air. Combustion air and makeup air needs are in addition to outdoor air ventilation needs.

Get Reliable Ventilation Information. The whole subject of how and how much ventilation air is hotly debated by various interest groups.  Don’t be surprised at the amount of confusing information out there.  The approved standard to reference is ASHRAE 62.2, Ventilation and Acceptable Indoor Air Quality in Low-Rise Residential Buildings.  Follow it for the latest advice on how much to provide. 

In this area, Home InSight’s preference is for ventilating dehumidifiers, especially when there's a basement or crawlspace.  Otherwise, Air Cyclers are typically chosen in Georgia.

Bottom line: Seal up the house tighter than a submarine and add mechanical outdoor air ventilation.


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