Downdrafts & Smoke Backup Causes

Fireplace/chimney systems are pretty basic until something goes wrong. When you’re experiencing smoke backing up into your home, you may not have a clue why it’s happening. Fortunately, there are only a few possible causes for backdrafts or downdrafts, and most are fairly easy to fix.

Drafting basics

In order for smoke to flow up the chimney, five things are required:

1. Sufficient positive air pressure in the room where the wood-burning fireplace is located
2. Air within the chimney that isn’t excessively cold
3. A flue that’s free of obstructions
4. The “normal” amount of smoke from the fire
5. A fully open damper

Common causes of fireplace smoke backups

Negative air pressure

Air-tight homes are notorious for insufficient air pressure inside them to push smoke properly up the chimney. Air-pressure issues also are common when the fireplace (or stove) is located in a basement or a lower floor of the home.

This problem usually can be solved by cracking a window in the same room as the appliance. You may need to do this with several windows – try different approaches and see which works best.

Cold flue air

Cold air is heavier than warm air. If the air in your flue is particularly cold when you start a fire, it may prevent warmer air from rising and exiting the chimney. This is a problem seen in regions with very cold winter temperatures.

To warm up your flue air before getting a fire going, try lighting a rolled-up newspaper and sticking it up past the damper for a minute or so. Once the air begins to warm up, you can start your fire.

Obstructions in the flue

Excess creosote and soot, which are byproducts of smoke condensation in the flue, as well as tree leaves and twigs and nesting materials of small animals all can narrow the flue and cause smoke to back up into your home.

Having your chimney cleaned by a CSIA-certified chimney sweep will remove all types of drafting obstructions. Creosote should be cleaned out once a year to reduce the chance of a chimney fire. As to foreign debris, installing a custom, full-width chimney cap will solve the problem.

Excess smoke from wet wood

Wood that hasn’t had time to cure (dry) is too moist to be burned in a fireplace. It will produce large amounts of smoke, often too large for your chimney to effectively draft.

A rule of thumb is to let wood logs dry for about six months, and keep them free of rain and snow while doing so. Test logs by banging two together: a dull thud means there’s too much moisture; a cracking or popping sound means the logs have dried out.

Damper is closed

This is the easiest backdraft/smoke backup problem to solve, and it’s a problem you’ll know about immediately. The damper must be fully open to properly vent your fireplace. If it’s rusted or damaged, it may only partially open. Have your damper inspected, if you suspect this is the reason for smoke backing up into your home.

Black Moose Chimney & Stove of Antrim, NH, provides CSIA-certified chimney sweeping, chimney inspections, and all manner of chimney repair, modification, and rebuilding work. When you have excess smoke – or any other problem – we have the solution. Call us today at (603) 525-7905.