Smoky fireplaces, part 3: constant fireplace smoking

It is one of the most valued features in our homes. And all too often, the fireplace is labeled "non-working" and sits unused, because the homeowner doesn't know what to do about a smoking problem. Here I will show how most smoking problems can be handled relatively easily, with little or no expense. We'll cover:

Here's a trick for monitoring smoky fireplaces: If you can't see the smoke spilling from the fireplace, shine a flashlight across the fireplace opening. The light reflecting off the smoke particles will make it easy to see. 

Constant fireplace smoking

If the fireplace always smokes, the problem might be:

Fireplace blockage

Sometimes, a smoking problem is caused by a partial or complete blockage of the flue.

Depressurized House

Although this problem sometimes only shows up when the fire is burning low, for some super-tight houses it is a constant problem. This is the first possibility to explore if you have a tightly- insulated house, an air exchange system, or exhaust fans in operation. If there is not enough air available in the house to allow the fireplace to draw, it will smoke.

Fireplace Smoke Chamber is too small

The triangular cavity (above the damper) that funnels the smoke up into the flue liner is called the smoke chamber. While an oversized smoke chamber can lead to difficulty in getting the draft going, if the smoke chamber is too small, smoke sometimes curls out at the top of the fireplace opening and spills into the room.

When the fire is out, open the damper and shine a flashlight up into the smoke chamber. 

Safety Alert:  Wear gloves and eye and ear protection any time you work on your fireplace. Damper edges are sharp, and will cut you. Debris in the flue could get into your eyes and ears, causing injury.

The side walls of the smoke chamber shouldn't be more than 45 degrees from vertical. Usually, the smoke chamber is about three feet high. If yours is closer to 18 inches, or shorter, it could be the culprit. A smoke guard or fireplace doors might solve the problem. 

Fireplace smoke guard

A smoke guard is a piece of non-combustible material (usually steel) placed over the top of the fireplace opening, to lower the opening. 

Before you buy one, do this: Take a piece of aluminum foil a little wider than the fireplace opening, and fold it in half width-wise, to make it sturdier. When the fire is going, tape it over the top of the opening, covering a few inches of the opening. (Go easy on the tape! You will want to adjust it).  Monitor the fireplace as the fire burns. If it spills smoke, lower the foil; if it doesn't, raise it. Adjust it until you find out (a) if it works at all, and (b) how much of the opening you need to cover in order to solve the problem. 

If your foil smoke-guard works, go out and buy a smoke guard at least as wide as you need. 

Fireplace doors

Fireplace doors are generally constructed of tempered glass panels in a metal frame. The frame usually covers a couple of inches of the fireplace opening, so fireplace doors act something like a smoke guard, lowering the effective opening of the fireplace. 

And, if the fireplace starts to smoke, you can close the doors. Fireplace doors also save heat, since you can close them before you go to bed, preventing heat loss up the flue. (Remember, you can't close the damper until the fire is completely out, often a day or more later.) 

Fireplace flue is too small

If the flue is too small, it might not be able to handle the volume of airflow needed to draw the smoke up and out. 

For the facts & figures buffs

The rule of thumb for flue liner sizing is for the cross- sectional area of the flue to be about one- tenth to one-twelfth the area of the fireplace opening. But other factors, such as overall height, location of the chimney, and chimney construction, also play a role.

Most masonry fireplace flues are lined with 8" x 12" terra-cotta tiles. But this is a nominal dimension, and the actual inside dimension is usually closer to 6.5" x 10.5". 

Calculating the correct flue liner size is pretty complicated. But as a rough-and-ready guess, if you answer yes to all of these questions, an undersized flue might be the problem.

  • Is the fireplace opening pretty close to 36" wide x 28" tall (or larger)?
  • Is the flue liner smaller than an 8" x 12" nominal liner?
  • Is the house fairly open, or drafty, rather than being tightly insulated? (If it is very tight, consider Depressurized House first.)
  • Is the chimney built up the inside of the house? (If it is on the outside, see “Cold Flue by Design” above).

If so, the next step is to hire a chimney professional to examine the entire system. If your chimney professional determines that the flue really is too small, the solution is a smoke guard or fireplace doors. These two devices will reduce the opening size of your fireplace, changing the ratio of the opening size to the flue size, improving the fireplace's ability to draw all the smoke up and out. 

Fireplace damper is too low

Similar to the small smoke chamber problem is the misplaced damper. Normally, there is some room above the top of the fireplace opening for smoke to gather before it moves through the damper and into the smoke chamber. But if the damper is installed right at the top of the fireplace opening, smoke sometimes spills into the room. 

This is especially problematic if the damper opening is particularly narrow, or if the damper is installed at a steep angle, acting like a baffle. 

A "baffle" is anything that slows or re-directs the flow of smoke and gasses. 

The solution to a misplaced damper is either (a) to remove and replace the damper, often a costly and involved proposition, or (b) to install a smoke guard or fireplace doors. Smoke guards and fireplace doors work by lowering the top of the fireplace opening, adding extra space for the smoke as it collects prior to passing through the damper. 


Smoke Signals: Have a question about your chimney?