How do I keep a constant temperature of 225 degrees when smoking for several hours?
Keep the lid closed as much as possible (see Cold Weather BBQing for more information). Regardless of the type of smoker you have, raising the lid will cause you to lose heat. Although it must be done, avoid the temptation of peeking too often.
Offset smokers work very well, because you are able to add wood or charcoal without disturbing the lid covering the meat. The old barrel type (like I use a lot) requires you to open it up to add fuel. To make things worse, the grate must be moved too. My best advise is to hurry.
Have a plan, and then don't waste time getting it done. Sometimes an extra person can help, too. Usually you must open the lid, move the meat, raise the grill, add the wood or charcoal, replace the grill, replace the meat, and close the lid. It's a real pain in the grill.
Be sure to also use a thermometer to monitor the internal heat of the smoker. Most grills come with one built in now days.
Don't complicate things by trying to adjust both the air intake vent and the exhaust vent. Only concern yourself with the intake. Open the exhaust vent all the way and leave it alone.
Once the fire is going, adjust the intake to allow more air flow, if you need a hotter fire. Cut down the air flow by closing the vent slightly to decrease the heat.
It takes some practice to maintain a steady temperature while smoking. It will still never be as constant as using an electric oven... but then, you give up that luxury when you prefer to smoke.
I guess that's why it's considered of the art of smoking. Not everyone can do it.
Keep those questions coming. Simply Contact Me with your questions and comments.