How To Prevent Backdrafts In Your Woodstove

If you heat your cottage with a wood stove you may notice smoke entering the camp via the stove. In other words, instead of going up the chimney, the smoke is coming back out into the camp. Of course, this can be dangerous. Deadly dangerous.

Why Is Smoke Coming Back Into The Camp

Back drafts, smoking back and down drafts can be caused by something blocking the chimney which stops the smoke from escaping upwards and it will eventually come back down.

That’s why periodic checks on your chimney are a good idea as well as cleaning or having your stove pipe and chimney cleaned.

That is woodstove 101 keep your chimney clean and clear of obstructions and of course keep your chimney and stove pipe free of creosote build-up to prevent chimney fires.

Another reason can be a lazy, smoky fire from burning wet or “unseasoned wood” that doesn’t burn very hot and smolders instead of burning bright, hot and quickly. Sometimes the smoke from these types of fires is slow to rise because the fire is not hot enough to force itself up the chimney.

And another reason can involve the seal around the door of your stove or wood burning appliance. If the seal is worn, ripped or coming apart, the door may leak dangerous smoke and carbon monoxide into the room. Always inspect your door seals and your chimney stove pipe.

And yet one more reason can involve neighboring roof lines, nearby trees or other structures affecting the airflow around the top of your chimney. In that case, sometimes adding a special chimney cap like the one shown can help lessen the problem.

This type of problem is sometimes referred to as a “down draft” as opposed to a back draft, but essentially the result is the same, smoke not wanting to go up your chimney or being forced back down your chimney.

The Stack Effect

But sometimes backdrafts are caused by something not quite as straightforward as a family of skunks living in the chimney or a leaky seal around the stove door.

Sometimes it has to do with differences between the indoor and outdoor air pressures and the difference between the air pressure on the floor where the stove is located and the air pressure above it, where the smoke is supposed to exit. In other words, there is a pressure difference between the outside air and the air inside the cottage which is caused by the difference in temperature between the outside and inside air.

Hot air and smoke rise, but there are down drafts, or back drafts that are caused by negative air pressure inside buildings owing to a difference between the inside and outside temperature.

Warm air rises while colder air doesn’t, or not as much. Where the warm air meets the cold air you have a somewhat invisible blockage, usually somewhere up inside your chimney, that stops the warmer air from rising up and exiting the top of your chimney. This is called the stack effect.

The stack effect in chimneys: the gauges represent absolute air pressure and the airflow is indicated with light grey arrows. The gauge dials move clockwise with increasing pressure.
The stack effect in chimneys: the gauges represent absolute air pressure and the airflow is indicated with light grey arrows. The gauge dials move clockwise with increasing pressure.

Because of the warmer air rising upwards, there are pressure differences in your cottage and inside your chimney.

Typically the pressure is lower in the bottom of your wood stove pipe than at ceiling height. At the midway point, there will be a mid-zone or neutral zone where the pressure is neither higher or lower. This is what can cause the stack effect or back draft. Actually, it is the stack affect that is causing the back draft.

It is often worse in two story buildings or camps with high roof lines that require tall chimneys. And it is often worse in cottages where the wood stove is located in the basement where the pressure is the lowest.

If the woodstove is above the neutral pressure level spot in the building, the problem is lessened and maybe even non-existent. That is why woodstoves located on a floor above the basement often do not have an issue with pressure. However, not all of us have camps with basements.

I am reminded of a friend of my Dad who had a woodstove in his house that he could not use because of smoke coming back. It was a two story building and the pipe ran up the outside wall. It suffered from stack effect problems from day one. It ended up not being used because he could not remedy the problem. In retrospect, the well insulated house was on a hill, the pipe was on the outside and quite long. I can imagine that there would have been significant differences in the air pressure inside and outside.

Camps and cottages, especially older ones, may not be constructed as ‘air-tight’ as newer buildings, which can actually mean less problem with the stack effect in the chimney. Air tight building construction often can cause stack effect in your chimney because the outdoor air pressure has been effectively shut off from getting into the building and vice versa.

If your camp has an old foundation or basement that is less than air tight, the neutral pressure point or pressure zone, will be lower in the building as the neutral pressure level usually is located close to the area of the building that has the most air flow.

The same happens in reverse if your upstairs is less air tight than your basement.

How To Eliminate Chimney Back Drafts and Stack Effect

Ok…all that is good to know, but what do you do to prevent or lessen backdraft in your cottage chimney?

One simple fix is to open a window on the bottom floor of your cottage. That will help to bring the neutral pressure zone down to the lower level, in other words, out of your chimney, and should eliminate the negative pressure the chimney smoke has to get through to escape through the top.

You can check for the stack effect by putting your hand inside your wood stove (when the fire is out) and the damper is in the open position. If you feel cool air coming down the chimney you know you have a pressure problem above the stove pushing the air and subsequently the smoke back down. You can do the same thing with a piece of paper lit on fire, even a match or lit cigarette. If the match or cigarette smoke come back towards the door of the stove rather than rise up, you know the pressure point is above the stove. Now try it with a window or door open. If the smoke from the lit cigarette or piece of burning paper goes up the chimney, you have reduced or lowered the pressure point.

Now you can light the fire, leaving the door or window open until the chimney is drawing good and the back draft is no longer a problem.

Another fix for persistent back drafting is to add a length of stovepipe to your chimney especially if you think nearby structures or trees are affecting air flow around your chimney.

There are also some chimney caps that will help prevent the back draft affect on your chimney smoke, particularly if it is caused by nearby structures.

The Danger of Back Drafts

OK, so you figured out the above information and now you have a nice fire going in your cottage woodstove.

The fire is burning well, there is no back draft, everything is good. Except you have to go to bed.

That means that eventually, while you sleep, the fire is going to burn down and the chimney in turn will cool down and as that happens your air pressure problem can return and along with it your back draft problem.

Only now, it is even worse because if enough of the smoke and gases from the fire enters the camp, you could die from carbon monoxide poisoning in your sleep.

This is particularly likely to happen in winter when the outside air is very cold and the chimney cools down quickly. Add that to your newly insulated cabin that is very air tight and you can have yourself a problem.

Two things you can do. 1) Open a window and leave it open all night which will help with the pressure situation and provide a source of fresh air into the camp. And or;

2) add a direct source of outside air into your wood stove.

This is usually in the form of metal duct pipe that brings air from the outside directly into your stove. Many new stoves, particularly those designed for smaller buildings are supplied with one, and others have the connection built in so that you can add one.

rais fireplaceOur wood stove has one although we do not have the duct installed, and most if not all pellet stoves come with them.

Check with your wood stove supplier to see if one if available for your particular stove.

So…long story short. Check out your stove pipe. Make sure it has no obstructions, make sure your door seals tightly on your wood stove.

Test for negative pressure as described above, open a door or window on the same floor and test again. I personally suggest you always leave a window open an inch or two to keep the air pressure consistent throughout the day and night.

There ya go, some advice of sorts on how to prevent back drafts in your wood stove at the cottage. It can be problematic, but it is not too difficult to fix. Try the open window approach first, you will probably be amazed at how easily it fixes the problem. Probably could have just posted that and we’d all be happy. I should have just said at the beginning….open a window.

How about you? Have you had trouble at your camp with back draft or stack effect on the stove?

Stack Effect Diagram: Wilkipedia Creative Commons






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