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How does chimney draft work?

How does chimney draft work?

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Manage My Life
Draft

is the flow of air and exhaust gasses up a chimney. Chimneys may used natural draft or forced draft. The ability of chimneys to carry exhaust gases away through

natural draft

(or

stack effect

) is a function of several things, including - 1. the chimney height 2. the flue size 3. any offset from vertical in the chimney 4. the appliance size 5. the number of appliances using the chimney 6. the temperature difference between indoors and out 7. the direction of the prevailing wind 8. tall structures near the chimney 9. whether the chimney is interior (running up the center of the building), or exterior (enclosed in, attached to, or adjacent to an exterior wall). 10. the smoothness or roughness of the flue passage.

Generally speaking, the warmer the chimney, the easier it is to maintain good draft. A chimney going up through the center of the house will be warmest. A chimney on the exterior wall, but with three sides interior and only one exterior side (flush with the exterior wall), is the second warmest. The next coldest is a chimney built on an exterior wall, but projecting out from the wall with three exterior sides. The coldest is a freestanding chimney outside the house.

While it's not common with masonry chimneys, metal chimneys are sometimes separate from the house wall but supported by it. The entire outside diameter of these chimneys is exposed to the cold over most of the length of the chimney.

Taller chimneys usually draw better than short chimneys, at least within the height ranges we usually see on houses.

Homes that are partly one-story and part two-story may have draft problems. A chimney on the one-story section often drafts poorly because of the wind turbulence and downdraft created by the two-story section.

It takes a while to establish positive draft when the appliance first comes on. There is often a column of cold air in the chimney and the weight of this cold air has to be overcome by the buoyancy of the warm air (combustion products). It's not unusual for appliances to spill or backdraft during startup.

The type of appliance also has some effect on draft.

Natural draft

appliances such as fireplaces and conventional furnaces and boilers rely simply on the buoyancy of warm air rising to create chimney draft. These appliances also need

dilution air

or

draft air

which helps keep things moving upward.

Forced-draft

appliances push fresh air into the combustion chamber and may help push the exhaust gases up the chimney. While it's possible to have a cold column of air overcome this, a forced-draft appliance is less likely to suffer draft problems than a natural draft appliance.

An induced-draft appliance is similar to a forced-draft appliance, except that the fresh air is pulled through the combustion chamber from the exhaust side, rather than pushing it through from the intake side. Induced draft fans may also push the products of combustion up the chimney and may be less likely to suffer draft problems than natural draft appliances.

Note: Many induced draft fans are designed

not

to pressurize the chimney. The discharge is designed to be at or near atmospheric pressure.

In some cases, fans are provided in chimneys to overcome poor draft. These

draftassisting

or

draft-inducer fans

are expensive and are exposed to a severe environment. They do not have a long life span. These should not be confused with

power venters

which are typically used

instead

of chimneys on sidewall vented appliances.

There are several solutions to draft problems. The fans that we talked about are one example. Rain caps at the top of chimneys may also help by deflecting the naturally occurring downdrafts.

In some cases, metal extensions are put on chimneys with a 90 bend and a weather vane on top. The vane or fin on the top is moved by the wind so that the discharge point of the elbow will be facing downwind. These swivel-type extensions may resolve draft problems, but like any mechanical device far from convenient service points, they may not work over the long term.

Other strategies to improve draft include extending chimneys, changing flue sizes, changing appliance sizes, etc.

We mentioned that the size of the flue has a bearing on the quality of the draft. The flue size is determined by a number of factors, including - 1. the fuel that is used 2. the appliance size 3. the number of appliances 4. the chimney height General Rules include - 1. wood needs a larger flue than oil or gas 2. a larger flue is needed for larger appliances 3. a smaller flue is needed for taller chimneys, all other things being equal

Flues that are undersized or oversized will tend to backdraft and spill combustion products into the house. Condensation is more likely if the flue size is wrong.
by Manage My Life
April 26th, 2007
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