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Downdrafts & Wind Loading

Smoke flowing back into living areas rather than up and out the chimney can occur for a number of reasons. The backpuffing smoke signals that one or more factors may be affecting performance.

Wind induced downdraft is one cause of smoke. Wind induced downdraft is the downward flow of air into a chimney. It can be caused by the deflection of wind by obstacles higher than the chimney such as neighboring tall trees, hills and buildings

wind induced downdraft

wind related downdraft from hill

wind induced downdraft from tall building

Wind induced downdraft
from nearby tall trees

Wind induced downdraft
from nearby hills

Wind induced downdraft
from nearby tall buildings


Dynamic wind loading may occur in buildings when the wind causes negative air pressure. Negative air pressure and consequent tell-tale backpuffing may be particularly noticeable when a door or window is opened on a side of the building other than the windward side. To restore normal pressure in the building, make up air is apt to enter via the chimney and down into the living area. If the appliance is operating, smoke is then carried into the room.

This can be remedied by cracking open a window on the windward side of the house and closing windows on the other sides. The windward side is the side the wind strikes. A Vacu-Stack is recommended as a partial solution to this problem as well.

dynamic wind loading smoke puffback

dynamic wind loading puff back solution


Wind Loading on a multi-story building is another common cause of smoke and backpuffing. As the wind flows against the second story vertical wall, a high pressure zone develops at the chimney top. Although a Vacu-Stack is not guaranteed as solution to this problem, field reports show satisfactory results in more than 50 percent of the installations.

The occurrence of smoke and backpuffing when air is calm may be caused by a number of structural factors that relate to the heating system. The chimney may be too short or the flue opening may be too large or small for the heating appliances. Smoke may also occur if the chimney is blocked with soot, creosote or debris.

In a relatively air-tight house, negative pressure problems can be caused by appliances that exhaust the inside air, such as cooking ranges, exhaust fans, a clothes dryer vent, furnaces or other devices. Your individual chimney problem may be the result of a variety of factors. 

multi story building wind loading high pressure puff back


How Important Is The Connection Between Stove And Chimney?
The dimension and the seal of the stove pipe to the chimney also influences performance. Specifically, the pipe size, number of turns and length of the pipe can affect the draft.

Other Factors
Affecting Performance
The location of the home with respect to hills, high trees or adjacent buildings can cause problems with downdrafts. Sometimes starting a fire or keeping one going is difficult in an airtight house.


Fireplace Insert Installation

Top-vented Installation


Fireplace Adaptor Installation

An airtight seal is essential to prevent the formation of creosote which could cause chimney fires.

Pictured here are three   installations with airtight seals.


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