So, How DOES a Chimney Work?

When people report problems with the performance of their wood burning (or fireplace) systems, the cause is often traced to the chimney. If you are building a new home or remodeling an existing one…Selecting the correct chimney and designing the layout of the installation is critical to solid performance. And of course, knowing how chimneys work will be very useful in the day-to-day operation of the appliance.

Chimneys operate on the principle that hot air rises because it is less dense than cold air. When a chimney is filled with hot gas, that gas tends to rise because it is less dense than the air outside the house. The rising hot gas creates a pressure difference called draft which draws combustion air into the appliance and expels the exhaust gas outside.

Two factors affect the amount of draft produced by a chimney.

1. Heat: the hotter the gases in the chimney compared to the air outside, the stronger the draft.

2. Height: the taller the chimney, the more draft it will produce at a given temperature difference.

The table below shows how heat in the chimney and chimney height work together to produce draft.

Pressure differences resulting from various temperature differences and various stack heights, in pascals

  1. The red figures in the body of the table are the number of pascals (Pa) of pressure difference that result from the intersecting temperature difference and stack heights. One Pa is equal to 0.004 inches of water column (“wc).
  2. The lower two rows of the table body can be used to roughly estimate total stack effect (- & + values) in houses during cold weather.
  3. A single point flue gas temperature measurement, usually at the flue pipe, will give a higher temperature than the average for the total system because of heat loss through the chimney, and therefore will inflate predicted draft levels unless a correction is applied.
  4. Combustion and venting system height measurements should be taken from the base of the firebox; i.e. from where it gets hot.

The chimney’s function is to produce the draft that draws combustion air into the appliance/fireplace and safely exhausts the combustion gases to the outside. But a good, proper chimney must do more than that. It really should:

  • protect the house structure from the hot gases passing through it;
  • tolerate the high gas temperatures that can result from chimney/fireplace fires;
  • conserve flue gas heat to produce strong draft;
  • resist corrosion on the inside and weather effects on the outside;
  • be sealed to prevent leakage (on the sides as well as on the top, with a chimney cap).

It’s essential to realize just how important proper chimney and fireplace installation and operation truly are to the well being of your home and family… it can make a BIG difference! For any help on detecting if YOUR chimney or fireplace is in solid, working condition, simply gives us a quick call today – and we will be glad to assist you: 817-791-4606