I own a PM Safe and have tried many ways to keep the temp below 250. The pits are well made, extremely insulated and they love to cook hot if you don't add water. The only way I have found to keep temps @ 225-245 is to use Kingsford Blue and water in the pan. You don't need much fuel. They are very efficient, especially once they reach your desired cooking temp. For instance, I cooked (2) 13lbs briskets @ 235-245 last weekend (11 hr cook) and used a total of 9-10lbs of briquettes. The key to low and slow on these units is how you start your fire. It is critical that you catch your temp on the way up. Once the pit passes your desired temp, the only way to bring it down is to open the cook door to cool off the chamber and remove the burning charcoal from the unit...No bueno. Below is the method I use for a 235-245 deg fire. I do not use a forced air draft system (Stoker, Guru, etc). I've found these pits work better w/ out forced air systems.
1. Open your exhaust 100% and open both air intakes 100% as well. My unit is one of the original models and has 2 sliding air intakes. The newer units only have 1 sliding air intake.
2. Fill your water pan w/ water (I like use boiling water if possible).
3. Add 10-15 briquettes to the back left corner of the ash pan and start w/ a weed burner. If you do not have a weed burner, start the charcoal in a charcoal chimney and when they start to turn grey add them to the charcoal tray as noted. Leave the charcoal door open 2-3 inches to allow a good flow of air into the coals.
4. Next take the weed burner and heat the cooking chamber for 2-3 minutes. Pass the wand over each cooking grate. This will help to clean off the left over grease and fat and get your temps going. Again if you do not have a weed burner, you will need to increase the amount of briquettes to approx 20-25, leave your charcoal door open 5+ minutes and allow more time for the pit to get hot.
5. Close your cook chamber and charcoal chamber door.
6. By now you should see your temps close to 150-160 degrees on the door thermometer.
7. When your thermometer reaches 200, close your exhaust to 1/2 open and close one of your air intake sliding doors completely and the other close to 1/2 open. If your pit only has the 1 sliding air intake, close it to 1/2 open. Once your temp hits 225 close your exhaust to 1/3 open and close your air intake slide to 1/2" (thickness of a book of matches) and your on the right track. At this point all 10-15 charcoals should be glowing and ashed over.
8. Next, add a single row of charcoals along the inside edge of your charcoal tray making sure they touch each other and that 1 briquette comes in contact w/ the lit charcoals. It should form a U shape. This will start the "domino" affect. You can add small chunks of your favorite wood every 5 coals. This will keep you in the 235-245 area for about 3-4 hours. Once you see the temp drop to 225, open your charcoal door, move all the remaining lit charcoals to the left back corner and make another single row of briquettes creating a U shape. At this point you will need to add water to the pan again. You will see some 10 degree spikes during the cooking process, but I've been able to hold consistant temps using this method. PM me if you need any additional info, always willing to help if I can. Good luck. They do cook some good grub once you figure them out.