To many people, the idea of tightening up a house to prevent air leakage and then ventilating seems like a contradiction. Of course, the issue is control. Controlled ventilation brings a specific amount of outside air and distributes it through the house. Random leakage isn't effective ventilation, because it's not reliable, regulated or distributed.

Nevertheless, pulling outside air into a space and then heating it up consumes energy and costs money. This is especially true of "exhaust-only" ventilation systems that have no heat recovery. So how much does exhaust-only ventilation cost? Not much, as it turns out.

To illustrate this point, Ted Haskell, Oregon State University Extension Agent, put together the following calculation. It's not a rigorous engineering equation, but it will give you a good idea of the heating load associated with exhaust-only ventilation. The procedure appears on this page with an example filled in.

Remember this is just an estimate. Many factors can affect the outcome from the actual air leakage of the house to the climate. Choices for several key factors, including building air leakage, heating degree days and heating system efficiency, appear in the box at the bottom of this page.

Air Tightness

One way to express the air tightness of a house is air changes per hour, which is the number of times the amount of air inside the house changes in one hour. The air leakage in a house is usually measured with a blower door. In many areas, you can find a specialist to conduct the test for you. Check with electric utilities, heating contractors, insulation contractors and home inspectors.

Adding Leakage and Ventilation

To calculate the total air coming into the house you don't simply add natural leakage to ventilation, according to Larry Palmiter of Ecotope, Inc. Exhaust-only ventilation systems place the entire house under a slight suction (called negative pressure). The fan pulls air through the same openings that also provide natural air leakage, so you don't want to count the same air flow twice. Based on Palmiter's field research on air leakage, he developed the method for calculating total air flow presented in Step 4. If the fan's air flow is less than two times the natural leakage, you add natural leakage to one-half the fan air flow. This condition occurs in homes with typical levels of natural air leakage, such as the one used in the example. In especially tight homes, where the fan air flow is equal to or greater than two times the natural leakage, all the air flow is driven by the fan. In this case, you would simply use the fan air flow.

This illustration doesn't apply to air-to-air heat exchangers, which are "balanced" ventilation systems that blow the same amount of air in as they pull out. In balanced systems, ventilation air is added directly to leakage to arrive at total air flow. The other factor not accounted for in this equation is heat recovery.

Worst Case

This estimate makes several "worst case" assumptions:

• The fan operates 24 hours a day.
• The fan actually delivers the air flow listed on the nameplate.
• There is no temperature setback on the thermostat.
• The heating system operates whenever the outdoor temperature falls below 65°F, which is the basis for heating degree days.

Even though this calculation leads to a ballpark estimate, it gives you a general idea that the investment the homeowner makes is a relatively small investment in exhaust-only ventilation.