This can be one of the most frustrating issues for a wood burner. There are several causes, but it can usually be solved if you are methodical. Chimneys work on pressure differential, so you need to think about pressure issues. In general, air pressure is higher the lower you are, so the bottom of a chimney is higher pressure than the top of a chimney, so the air gets forced up. This is the same thing that makes straws work.
Chimney size - if your chimney is smaller or larger than the flue collar on your stove, it can cause downddraft. A chimney that is too small increases the flow resistance and increases the pressure in your stove. This causes smoke spillage. A chimney that is too big will cause a pressure drop in you chimney and can change the pressure dymanics again.
Competing draft - dryers, bathroom fans, range hoods, and attic fans can decrease the pressure in the house and pull air/smoke down the chimney to make-up for the exiting air. If you have an intermittent problem, check these things.
Tight houses - today's houses are very air tight to reduce energy loss. Stoves need a constant supply of make up air to "make up" for the air going into the stove/chimney. Try cracking a door or window open to see if the problem goes away. If it does, a make up air kit can be installed.
Maintenance - soot, broken tiles, air leaks in connecting pipes and other maintenance related issues can be a leading cause of down-draft.
Hills & Dales - If you live in a valley or around a lot of trees, air can get trapped in the depression and cause a locally higher pressure. This is a hard one to solve, but sometimes a power-vent can be used.
Weather & Inversions - If you have problems in fall & spring or when it is raining it could be weather patterns or a local inversion. These can usually be overcome by pre-heating the chimney or adding height. The rising heat & elevation differential should overcome the inversion.
Other - low end stoves, living in the 4th dimension, bad luck, and not going to church can sometimes cause down-draft. This can be the hardest one to solve because it usually doesn't make sense.
Call Black Duck Chimney if you are having down-draft problems, maybe we can help.
I decided to start a blog to share some of the things I learn as I travel around and work on chimneys. The heating season is off to a roaring start. The combination of high energy prices, poor Michigan economy, and available tax credits have made many people take a second look at wood heat.
A word about safety:
- Do not cut corners with wood heat! I have seen too many people try to do a wood stove installation for a minimal cost. Many are finding old used stoves and putting them in their living room for heat. BE CAREFUL. The mechanical codes exist for a reason. We have heard of several house fires in West Michigan this fall, one resulting in the death of 3 people.
- Fire can be your best friend or your worst enemy. It all depends on the installation. Modern wood stoves can be very safe and efficient if installed properly.
- Do not vent a wood stove into an old un-lined chimney. You don't know what is going on with mortar joints and old bricks/blocks. At least have it inspected by us to make sure it is safe. This service costs $55. Money well spent for peace of mind.
- Be careful with old wood stoves. Wood burning technology has come a long way, and modern wood stoves burn very clean and efficiently. (70-80%) Exactly the opposite of stoves made before 1990 which burn dirty and in-efficiently. (40-50%)
- Clean your chimney every year. I often take bushels of creosote out of chimneys that have not been serviced in years. If that fuel ignites, it burns at 2000 F and there is no good way to put it out. Minimum damage - cracked tiles. Maximum damage - house fire.
Matt Waite - Chimney Guy Owner Black Duck Chimney MI Mechanical Contractor - Specialty Wood & Gas