Fireplaces basics

While fireplaces are no longer used as the primary source for heating homes, there are few things nicer than a cheery fire – especially on a cold winter night. By the same token, there are few things more distressing than a fireplace which doesn’t draw smoky air up into the chimney:  belching smoke into the home, chasing people out, setting off smoke detectors, and covering your house in dirt.

So why do some fireplaces draw perfectly, and others so poorly? Usually it comes down to the design of the fireplace.  Some of the factors which affect fireplace performance include:

1) Ratio of the Fireplace Opening to Chimney Flue Size

The area of the flue should be roughly 1/10th the size of the opening area. If the flue is too small, the fireplace will smoke.

2) Chimney Height

In this case, the taller the chimney the better – chimneys that are too short will cause draw issues.  A good guideline to follow is to have your chimney at least 1000mm above the roof and 600mm higher than anything within 3m of it.

3) Damper Size and Location

It’s important to ensure that your damper isn’t too small, too low or too far back. It should be the full width of the firebox and at least 150mm above the top of the opening. The damper is usually closer to the front of the fireplace than the back.

4) Smoke Chamber Slope and Smoothness

The smoke chamber above the damper should be as smooth as possible, and should slope no more than 45 degrees as it funnels the smoke from the damper opening into the chimney.

While most fireplaces break at least some of the rules of good design, many tend to work well despite this. Fireplace design is more of an art than a science. As there are so many factors which affect the draw, it is impossible to know how “perfect” the unit has to be in order to work properly.

If you’re having issues with your fireplace not drawing well, consider some of the following solutions:

Reduce the Opening Size – This can be achieved by laying an additional row of firebrick on the floor of the firebox. Before you start laying brick, you can test the solution by holding a piece of metal over part of the opening and watching to see if the draft improves.

Extend the Chimney – This is expensive but often successful. Less expensive alternatives include a rain cap or a metal draft hood which rotates with the wind so that smoke is always released downwind.

Verify the Chimney is Unobstructed – Sometimes unwanted guests can make their home in your chimney. Verify that the flue is unobstructed all the way up. Occasionally, this can be done from below with a mirror when the damper is open. Otherwise, a top down look may be required. (A specialist may be required for this review).

Move the Fire Back – Oftentimes, the fire is simply too close to the front of the firebox and needs to be moved back. Unfortunately, if the fireplace is too shallow to permit this, the fireplace may have to be rebuilt.

Add Air – A fireplace which is starved for air won’t work properly. Sometimes opening a window in the room with the fireplace will supply enough air. Fireplace draw is more difficult to achieve if the house is under negative pressure. Negative pressure occurs when the air pressure in the house is less than the air pressure outside the house.  Don’t have exhaust fans on or your clothes dryer running while trying to start a fire. This is particularly true for newer, tighter houses.  You will also find it’s easier to start a fire when the furnace is in an off cycle. In addition, glass doors help to protect the fireplace from negative pressure effects in the house, especially if combustion air can be brought in from outside.

Warm the Flue – This is a trick most people know about. Pushing a burning piece of rolled-up newspaper up past the damper will help overcome the column of cold air in the chimney and allow a good draft to be established quickly.

Damper or smoke chamber modifications are possible but should be considered last resorts, especially as they are expensive. If you’ve worked through this list of solutions and still find yourself with fireplace issues, it’s time to call a specialist as more extensive work may be required (image courtesy of thompsonshearth.com).