When it comes to understanding heating systems, most of us are comfortable with the basics. To warm up your house on a cold day, you need a source of heat in your living room — say, a wood stove or a radiator. To keep the heat in your house, you need insulation.
That’s the way most builders understood heating from 1935 to 1980. Somewhere around 1980, however, building scientists began to realize that the old picture was imperfect.
We now know what was missing from the old picture: an understanding of air pressure. A house is a dynamic system, and that system is greatly affected by air pressures inside and outside the home. Forces affecting the performance of the home include wind, the stack effect, and many different types of fans. These forces operate in buildings, and all buildings include leaks in their thermal envelopes. Moreover, most homes have ducts with leaky seams. Finally, many of these ducts are partially indoors and partially outdoors.
If you put all these factors together, you realize that you can’t understand home performance without understanding the effects of air pressure.
What happens when the furnace comes on?
Building scientists use a variety of techniques to measure air leakage through a home’s thermal envelope. Air leaks faster on cold days than mild days (because the stack effect is stronger when the outside temperature is cold), and it also leaks faster on windy days than on still days. No surprises there.
Let’s say that technicians are studying a typical American home. They are measuring the air leakage rate (the infiltration and exfiltration rate through the home’s thermal envelope) while the furnace is off. They get a reading. Suddenly, the furnace turns on, and the technicians notice the air leakage rate has suddenly increased dramatically — for example,…