Chimney Airflow Problems

Understanding chimney draft problems is not necessary for most people. Usually, if you notice smoke not rising from your chimney, you can call on a professional to fix the issues.

This information is for those who really like to understand; it may be too much information for many people. I’ll do my best to keep it as interesting as it can be. If you’re a do-it-yourselfer or the owner of an old home, you’ll probably get a lot of out of it.

Understanding Chimney Draft Issues

To understand the problems, you need to understand what draft is. Draft is what we name the effect of how the air flows up the chimney. It’s measured in “inches of water column.” Draft then is the combination of volume, speed, and pressure of the flue gasses. And temperature of the gasses comes into play here as well.
For matters of this discussion, chimney draft is usually thought of as the speed at which the vented gasses travel up the stack, or pressure of the gasses. This can also be referred to as the stack effect. A common question might be “how strong is the stack effect?” Good draft conditions mean that the vented gasses are traveling up the chimney quickly rather than slowly or not at all.

How Does Chimney Draft Work?

The reason smoke (or other flue gas) goes up the chimney at all is because of the vacuum in the chimney. The question you should ask now is “a vacuum relative to what?” The general answer is that it’s relative to the air in the house. Don’t read too much into that because it gets tricky (for example, how does replacement air get into the house?- because the house environment is a relative vacuum to the outside. Yet the inside of the house is not a vacuum compared to the chimney.) Let’s keep this simple and just talk about the chimney. The pressure in the chimney is typically less than that inside the house. Thus, the draft effect is caused by air inside the chimney being pushed up the chimney by the house air.

And why is there a difference in pressure in and out of the house, or in and out of the chimney? There can be a few reasons, but the biggest and most important reason is the temperature difference from one place to another. Remember that when air is heated it expands? The same amount of air occupies a larger space, or you could say the same amount of space has less air (fewer molecules of air.)

The air outside the house in the winter is colder and heavier than the warmer air in the house. It pushes its way into the house (or is it pulled, depending on how confused you want to be.) The air in the chimney just came from a fire so it’s really hot and expanded and being pushed up the chimney to the cooler air outside where warm air rises, right? That’s buoyancy. Problems occur when these processes don’t happen correctly.

Diagnosing Chimney Draft Problems

Draft is measured with a pressure meter that has a probe which goes into the smokepipe. The meter should register a negative number, and generally speaking for residential heating appliances that number would range between -0.02 to -0.04. Zero or a positive number means the gasses are not going up the chimney. And too large a negative number can have its own set of consequences; but that isn’t usually the problem. Mostly “a draft problem” means the gases are not going up the flue, this is merely a minor chimney repair.

Causes of Draft Problems

Now there are other reasons for draft problems. One is called Dynamic Wind Loading. or “DWL.” DWL is caused when the wind blows on one side of the house and causes a positive pressure, and creates a corresponding negative pressure on the other side of the house.

If the windward side of the house is tight and the lee side (negative pressure side) isn’t, the vacuum resulting from the wind can suck air out of the house. And the most likely source of that air is the chimney; it’ll pull down on the chimney, smoke and all and keep it from exiting your house! Or if a gas furnace is being vented you won’t see smoke but you still get the carbon monoxide.

The way to deal with that is to tighten up the lee side of the house and then put in an outside-air source. There are kits for that or you can just crack a window on the windward side of the house.

Chimney Draft Issues Caused by Fans

The other large reason for bad draft is when chimneys have to overcome fans in the house. Kitchen fans, bathroom fans, radon fans. It doesn’t take much of a fan to overcome a natural draft appliance (such as a fireplace or woodstove) Again, the best answer is to allow “make up air” into the house.

The problem with that of course is that you don’t want a draft across the floor and you hate to purposefully introduce freezing cold air into the very house you’re trying to heat. It’s a Catch 22, but I can tell you CO poisoning is a bad thing, and smoke in the house is a bad thing. You just may have to make some choices.

Air Flowing Down Your Chimney

Finally, sometimes air actually blows down the chimney, but less frequently than you’d guess- it’s usually something else. But maybe your chimney is short and next to a larger part of the house or a bigger building. The same problem occurs if your house is located at the base of a mountain. If you have this problem, a Vacustack is a good solution if you can’t raise the chimney to the proper height.

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