1. Problem: Grate is too large
Solution: You need a smaller grate
2. Problem: Grate isn't high enough
Solution: Elevate grate
In every case, the grate should be all the way back against the fire wall.
1.Damper should always be all the way open when you're burning a fire in a regular fireplace
Did you remember to open the damper? As offensive as this question sounds, this happens all the time.
1. The flue is cold when you start the fire so you may have a little turbulence in the chimney. (i.e. cold air pushes down while warm air rises, so as the two are passing by each other, the cold air will bring some smoke into the room until the flue warms up.) To warm up your flue, roll up some newspaper, light it, and hold it up by the damper so that the warm air will have it easier pushing out the cold air. Or if you have a log lighter, run it for a few minutes before you start the fire.
Over the last ten years the building codes have gotten very strict on making sure a house is built air tight. Because of this reason, the fireplace will smoke. In short, what happens is while the fire is burning it is consuming a lot of oxygen from inside the house. So the house tries to replace that oxygen as soon as possible. After the fire dies down a little, the house will start winning the battle for oxygen and start pulling air right down the chimney.And with the air will come smoke. Unfortunately the only cure for this is to open a window or door near the fireplace while you're burning a fire. Also, if your fireplace is equipped with an outside air kit, make sure it's open while burning a fire.
A vaulted ceiling can actually fight with the fireplace to be the chimney.So if you have the grate sitting towards the front of the fireplace, you'll want to move it back so that the chimney will win the battle. If the grate is sitting forward because of the log lighter, have the log lighter moved further back also.
If you have glass doors installed on your fireplace, believe it or not, they may be the cause of your problem. While you're burning a fire, the doors should either be all the way open or all the way closed. If you have them only half way open, this will cause smoke and flame to be drawn into the house.
If you have central heating, this may be your problem all by itself or it may be contributing to your problem. The heating system moves a lot of air, and it is forced air so it will win the battle for oxygen over your fireplace. Ironically, the intake for the heaters is usually somewhere close to the fireplace. So bottom line... It's a good idea to not run the heaters while your burning a fire.
Wind at the right angle will cause smoke to come into the house. Sometimes if you have a chimney cap on your fireplace, the wind will deflect off of the rain cap and force air & smoke down the chimney. A Wind Directional Cap is a good idea (only available for masonry fireplaces).
Look around you, is your house a one story surrounded by a bunch of two story houses? Or are there big hills all around you? Do you live at the bottom of a hill? All these situations, unfortunately, work against you. The only hope in a situation like this is a flue extension. Sometimes it works, sometimes it doesn't.
A general rule of thumb is...The height of the opening of the fireplace should be no more than 2/3 the width. If it exceeds that, the fireplace can smoke (i.e. If the width of the opening is 36", then the height should not exceed 24"). In many cases, the height exceeds that. A good idea for fixing this problem is to install a smoke guard.
Sometimes the way the air flows over your house will cause a restricting of your draught. A good idea, would be a flue extension. By elevating the flue above that air channel, it will no longer effect the draught. This has been a cure in a lot of cases.
Another rule of thumb: The chimney should be built or installed at a minimum of 2" higher than anything; 10" away with a minimum of 3" of extension above the roof. If this isn't the case with your fireplace, then a flue extension may help.
Finally, there is a little bit of science that's involved in building a masonry fireplace. And believe it or not, it is possible that the person who built yours didn't do his math properly. There are so many possibilities, the only way we would be able to tell if a mathematical problem is the case, is to actually see the fireplace. All that can be done with these fireplaces is a combination of corrections in hopes that they will fix the problem.
And there is always the question of when the last time the chimney was cleaned. You can get enough build up in fireplace to actually work against the draught. Sometimes even the spark arrestor can be so full of creosote that you don't have enough air flow. Regular maintenance and a cleaning of the chimney at least once every two years is recommended.
There are a few other small possibilities for why a chimney could smoke. What we have done here, is given you the main reasons behind a smoking fireplace. Hopefully one of these solutions will be able to help.