All buildings continually trade air with the outdoors. The rate of that exchange, called infiltration and exfiltration, varies with many parameters, such as wind speeds, fireplace and furnace operation, etc. But it came as a surprise to find a special situation where infiltration increased noticeably during the operation of one home's warm-air heating system, quite separate from any combustion process.
This unexpected phenomenon first came to my attention in a home whose fireplace draw was marginal. It was a bit difficult to start a fire when the heating system was not running, but very difficult when it was. Opening a window slightly helped get the fire started; and one could feel the increase in incoming air whenever the furnace blower ran. Since the house is electrically heated, the increase could not be blamed on a furnace's appetite for combustion air.
Some exploration revealed that a large percentage of the home's supply ductwork ran through attics and crawlspaces, all of them ventilated.The return ductwork, however, ran largely through the interior of the house itself, as is true in most modern homes with two or three return grilles in centrel hallways.
One assumption (that ductwork is rather leaky), and one basic principle (that the amount of air leaving the blower must return to the blower) - and the logic of the situation became inescapable. Air was escaping from the leaky supply ducts to the attics and crawlspaces, and thence to the outdoors. An equal amount of air and had enter the return ductwork to be reduced to the blower, and the house, therefore, had to suck in this make-up air from the outside.
How valid is this assumption? Unfortunately quite valid; ductwork is often leaky. Although such leakiness is always wasteful, it only affects the room-to-room distributiotion when all the ductwork is within the confines of the home's conditioned space, and is not very important unless the leaks are quite gross. But when much of the supply ducting is, in effect, out of doors, remedial work becomes worth while.
The leaks are least difficult to pinpoint in cold weather when the system is running. Expect them mostly in joints, bends, ets., and under the duct insulation, of course. Duct tape will close most of the smaller cracks, some sheet metal work may be needed for any large gaps.
Think about this problem when you design and build your next home.
William Papian is a registered professional engineer with the home-inspection firm of Claxton Walker & Associates.