Smoky fireplaces part 2: smoky fireplace endings

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. 

Smoky fireplace endings

Some fireplaces work just fine as long as you keep a rip-roaring fire going. But let it burn low, and it starts to smoke. (You might find that this type of fireplace is also smoky at startup, as discussed elsewhere.) The solution is to keep it burning high until spring, when you can open the windows... Just kidding. But seriously now, read on if your fireplace is okay during high-fire, but smokes as it burns low. 

Sometimes this is due to a design problem that can be easily solved. In other cases, it is more serious, and difficult to remedy.  There are several possible design problems that could cause smoky endings. Some of these are easy to check. Others require the skills of a chimney professional. Here are some to look for: 

Cold fireplace flue by design

If a masonry fireplace chimney is built on the outside of the house, and not up the middle of the house, the chimney structure, and therefore the flue, will sometimes cool quickly as the fire burns low. Especially on windy or rainy days, when weather conditions draw heat from the masonry structure more rapidly, this cooling can create a reverse draft problem: Cold air sinks, and the smoke sinks with it right into the living room. 

It is easier for masons to build chimneys on the outside of the house, after the house structure is mostly completed. But venting specialists recommend building masonry chimneys up the inside of the house structure, to avoid cold flue problems. 

Sometimes you can alleviate this type of problem with a good set of glass fireplace doors. As the fire burns low, close the doors to keep the smoke from entering the house. This is the easiest and cheapest possible solution and the first option before you spend money on structural changes.

A note about fireplace glass doors

Glass doors are useful, even for non- problematic fireplaces, since you can close the doors before you go to bed and prevent heat loss up the flue. Remember, you can't close the damper until the fire is totally out, usually a day or more later. And an open damper is like an open window.

Inadequate  fireplace chimney height

This one is fairly easy to check. First, check to see if the chimney meets the basic chimney height rule:  The chimney should be at least 3 feet higher than the highest point where it passes through the roofline, and at least 2 feet higher than any part of the house within ten feet of it.

In other words, all chimneys must extend at least three feet above the roof. And to find out if yours needs to be taller than three feet, measure horizontally from the side of the chimney, ten feet out on all sides. If you hit the roof, or an addition, or any other structure, then the chimney must be at least two feet higher than your measuring tape. If the chimney is any shorter, you are likely to experience draft problems, especially (but not only) when it is windy. 

There are some reasons for building a chimney higher than the minimum according to the rule. For example, if the chimney is built down near the gutter line, and the house itself is much higher than the top of the chimney, you might need a higher chimney. This is due to the stack effect of the house.  (Stack Effect: movement of air in the house created by the difference in pressure between air in the house and air outside).

Here is how it works

If lots of heated air is exiting the house at the top (from a leaky or poorly insulated roof, open windows upstairs, etc.) then the house acts like a chimney, or stack, drawing air up from downstairs to replace the heated air that's exiting. If the stack effect of the house is stronger than the chimney draft, you have a problem, since the house will draw air from the chimney, too, causing smoke and exhaust spillage. 

Measuring this gets pretty complicated, so if you suspect a stack effect problem, read through the other possibilities first (especially Depressurized House, below), then consult your chimney professional for advice if you still think stack effect's the culprit. 

Over-sized fireplace flue

Your fireplace flue liner might be too large for the fireplace. Some contractors seem to think "bigger is better."  But if you have too large a flue, then the amount of heat produced by a low fire might not be enough to "drive the draft" – that is, to keep that big flue warm enough. If cold air starts to rush down... you get the picture. 

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". If you have a larger-than-normal fireplace, you might need a larger flue. 

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 oversized flue might be the problem.

  • Is the fireplace opening pretty close to 36" wide x 28" tall (or smaller)?
  • Is the flue larger 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", below, 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 large, the solution is to re-line the flue. 

Depressurized House

Especially in newer, tighter homes, or homes with mechanical air exchange systems, depressurization of the inside of the home can be a problem. A fireplace chimney draws a huge volume of air out of the house. If either (a) something else is also drawing air out of the house, like an open window upstairs, an exhaust fan, or an air exchange system, or (b) the house is so tightly insulated that not enough air is leaking back inside to make up for the air exiting through the fireplace flue, then there is the possibility that the house will draw air from the chimney, and smoke along with it. This is even more likely as the fire burns down, emitting less heat to keep the flue warm. 

Test for depressurization in well-insulated homes

First, make sure there are no exhaust fans, air exchange systems, etc. in operation, and close any open windows upstairs. Crack open a window, in the same room with the fireplace. (NOT in another room, and definitely not upstairs!) This will allow outside air to enter the room, equalizing the pressure. If you find that the smoking problem stops, then depressurization is the culprit.

Note: This test is best done on a calm day, since wind pressure against the house adds another factor. Depending on wind speed and direction, air might actually be drawn out of the cracked window, giving an incorrect result.

Since an open window isn't particularly desirable, consider an outside air duct. An outside air duct, as the name implies, is an air duct (usually a metal pipe) that allows outside air to enter the home, relieving the depressurization problem. Talk to a chimney professional about installing one. 

Improper air space around the fireplace liner

This is a tough one. If you have a brick or stone chimney, chances are you have a terra-cotta (clay) liner inside the masonry work. There is supposed to be an air space between the liner and the brick/stone work. Sometimes, this air space is too large, especially in large, decorative chimneys that hold only one or two flues. A small air space is a terrific insulator. But a large air space can actually cause heat to be drawn away from the liner, causing lower flue temperatures, and consequently, draft problems. 

If your chimney is quite large, and especially if it is built on the outside of the house rather than up the middle of the house, this could be a factor in a smoking problem. But consult a chimney professional on this one, as some remedies (such as a new, insulated liner) could prove costly, and less drastic remedies might suffice. 

Note: In some regions, the space between the liner and the casing might be filled deliberately. An example is in California, where chimneys are reinforced with steel bars, and the space is required to be filled, in an effort to help the chimney withstand earthquakes. The technical ramifications of this relatively new construction practice are beyond the scope of this work. And in most regions, the air space is considered an integral part of good chimney design, provided it does not exceed the recommended size.

 

 

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