Fireplace Smoke

A Liberty Foundry Co. smoke guard installed on a fireplace with a fire burning in the firebox

Everything You Need to Know about Fireplace Smoke Guards

There’s nothing quite like the comfy, cozy, warm feeling that a fireplace offers, especially on a dreary fall or winter day. That feeling of content can fade quickly, though, if your fireplace starts belching smoke into your home.

And the blaring smoke alarms will be the least of your worries. Smoke buildup in the home can have ill health effects, leading to coughing, stinging eyes, or even the buildup of dangerous carbon monoxide.

So, how do you avoid smoke backdrafts from your fireplace?

One easy, effective, and inexpensive way is to add a smoke guard to the top of the firebox.

But what are these guards? How do they work? And should you add one to your fireplace? As a smoke guard manufacturer, we know plenty about how smoke guards help to mitigate smoke buildup in your home. And we want to pass what we know along to you.

By the time you’re finished here, you’ll understand what a smoke guard is, how it works, and whether or not you should add one to your fireplace.

Why Does a Fireplace Smoke Back into a House?

It can be difficult to pin a smoking fireplace back to one particular factor, because there are several variables that may cause the problem. But we’ll outline a few of the major causes here to give you a better understanding of what may be going on.

Outside and Inside Temperature Differences

Most homeowners light fireplace fires in the fall or winter. And that’s good, because the bigger the difference between the outside temperature and the temperature in your home, the less likely you are to have a backdraft problem.

Warm air rises and displaces cold air along the way. Let’s say it’s 70°F in your home, and 30°F outside. The warm air inside the house travels into the fireplace, and as it rises through your flue, it comes into contact with (and pushes through) colder air from the outside.

This is called a convection current. In the hearth world, we also refer to this process as a draft — the act of a flue drawing air from the firebox and out through the chimney.

If it’s a bit warmer outside (say, 50°F), the draft won’t be quite as strong (i.e., the air won’t circulate through the flue quite as quickly). This smaller temperature differential may be one cause of smoke backing up into your home.

Chimney Height

This factor is one that you can’t necessarily control, but it’s still important: the taller your chimney, the better draft it will create.

Air pressure accounts for this fact. A taller chimney has less air pressure inside because, as air rises, it both speeds up (causing lower pressure) and disperses (causing lower pressure). A shorter chimney with higher air pressure could cause a weaker draft, resulting in smoke backup.

Extending the length of your chimney is (unsurprisingly) an expensive process. But, in some cases, it’s the only way to stop a backdraft from happening.

A Dirty Chimney

A single-flue chimney flue clogged with creosote overlooking a suburban yard littered with leaves

If you don’t follow proper burn practices and use wet, moisture-filled firewood in your fireplace, creosote can build up in the flue pipe. Creosote is a kind of wood tar that results from burning organic compounds. If you burn enough moist wood, it will accumulate in your chimney.

It should come as no surprise that this buildup of creosote can clog your chimney and restrict airflow. The more creosote that collects in your flue, the more likely it is that you’ll experience smoke in your house.

To prevent this problem, hire a chimney sweep, or use a chimney cleaning kit to clean your flue.

Negative Pressure Caused by Other Vents

This is rare, but it’s not unheard of, either, so it’s worth mentioning.

Your house utilizes several vents to push different kinds of exhaust out. Bathroom ceiling vents, dryer vents, foundation vents, soffit vents — circulation in a home is key, and all of these different vents work to push bad air out and keep good air in.

Moving air creates low pressure, though, and if too many of these vents are working too hard all at once, believe it or not, that can have a negative effect on your chimney’s draft. Again, it’s rare, but at least be mindful of how many vents your home has and whether or not they’re all active at once.

The Ratio of Your Firebox Opening to Your Flue Pipe

Picture this: your sink has a clog in it. The water still drains, but not very quickly, and as you wash your hands, the water in the sink starts to pool faster than it drains. When you finally turn the sink off, the water takes 20 or 30 seconds to drain fully.

This is exactly what can happen in a chimney, except the “clog” is the size of your flue, the “sink” is your firebox, and the “water” is the smoke.

When the smoke isn’t allowed to “drain” out of your flue pipe quickly enough, it will accumulate in your firebox (like the water in the sink) and “spill” back out into your house.

This is one of the most common causes of a smoke backdraft, and it’s also one of the few that you can control.

How a Smoke Guard Helps Alleviate Smoke Buildup

A side-by-side comparison of the same fireplace, one without a smoke guard and one with a smoke guard, with blue arrows indicating that more air flows into the fireplace without a smoke guard and that less air flows into the fireplace with a smoke guard

When it comes to your fireplace and your chimney, the size of your flue pipe is pretty much set in stone (or concrete, more literally). As a result, without completely gutting your chimney, you can’t expand the size of your flue.

Among the other factors we covered (like temperature differences, chimney height, etc.), one of the few that you can control is the amount of air flowing into your chimney from inside your home. Too much air entering the chimney too quickly (like water filling the sink) can cause smoke to pour out of the firebox.

Adding a smoke guard to your fireplace reduces the surface area of your firebox opening, mitigating just how much air is flowing into the fireplace. Less air relieves the pressure on the entire system, allowing the flue to draft properly.

Think back to our sink analogy — adding a smoke guard is just like turning the knob on the sink to reduce the flow of water. Once you do, you can continue to wash your hands, and even though you’ll wind up with a bit less water pressure, the water will at least drain properly and won’t pool in the sink.

Should You Install a Smoke Guard on Your Fireplace?

Hopefully you remember at least a little bit of your high school physics, because as you can tell, there are a lot of complicated factors that go into a smoking chimney. Air temperature, air pressure, chimney height, firebox surface area — it can all be a lot to keep straight.

So, to simplify it all: should you add a smoke guard to your fireplace?

Truthfully, that depends on a lot of other factors.

If your flue is too small while your firebox opening is too wide, there may simply be a fundamental flaw with the design of your entire fireplace that can only be fixed by extensive repairs.

If your flue is dirty after heavy use, a good cleaning may solve your backdraft issues without the use of a smoke guard.

But if you’re experiencing smoke issues from your fireplace and you’ve tried and failed to mitigate the problem, a smoke guard could be just what you need. If you’re cautious, it’s also a good preventative measure, designed to keep irritating, dangerous smoke out of your home.

They’re relatively inexpensive, easy to install, and could be just what you need to change an overwhelming, frustrating problem back into a warm, comfortable evening by the fireplace.

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