The Chimney Affect

Chimney affect is the technical phrase used to describe how buildings will act like a chimney when its colder outside than it is inside. If you clicked here from Atmospheric Combustion Appliance, you already understand how chimneys work; the same physics are taking place in the whole house, with one caveat.

The difference is this: there will be a negative pressure at the bottom of the house, namely the basement. But on the top of the house, that is the ceiling of the top floor, there will be a positive pressure. This pressure difference can be measured with very sensitive diagnostic equipment. Some smart guy long ago came up with some cool math: with only called 45 F difference between indoors and outdoors, there is 1 Pascal per meter of building height. This isnt a lot of pressure, but any pressure is enough to cause heat to leak out at the top and cold to be sucked in at the bottom.

Chimney affect is more so a problem in taller buildings. A one-story house had minimal pressure differential (engineering folks might us the phrase Delta P or ΔP). A three story building has much more. Engineers have a really hard time with sky-scrapers (this is where the top engineering firms in the world apply their brain power, can you imagine a 1000 foot building and keeping it from drafting?).

We once installed a new heating system in an older 3-story house. The homeowner when the weather got cold and mentioned the first floor wasnt heating up as well as the second and third floors. This was a concern for our company; obviously we strive to get the heating system sized properly the first time. As it turned out, there was too much chimney affect taking place. The old house was not tight on the bottom (the basement air was being pulled right into the living space), and was not tight on the top (all the positive pressure was driving into un-insulated areas, like old closets that were anything but air tight).

One simple way to check how your house is doing when it comes to chimney affect is open your front door ever so slightly on a very cold day. If you can feel a breeze going buy you, your house is starving for air; you probably have a inefficient cap or air barrier on the top of your home. Please call us or any other building professional for an in-depth analysis.