In some cases, you may have vents, pipes and spigots from furnaces, dryers and water heaters in the way of your ledger board attachment. You have a few options to avoid these obstacles.
The easiest method is to install the ledger board around the vents by breaking the ledger board into smaller sections. You may need to adjust your joist spacing so that a joist doesn't fall on a vent. The ledger board doesn't have to be one continuous board. It can be a series of broken sections if it is solidly attached. This will require extra bolts and installing a 2x nailer across the top of the vent to secure the decking along the house wall.
The second method is to cut holes in the ledger board around the vents with a jig saw. This is more difficult than simply breaking the ledger board.
If the joists will fall on the vents, you will need to box frame around the vents. To do this, you will need to run a double joist on both sides of the vent and run a double header in between to pick up the intermediate joists. Double joist hangers should be used to secure the H-frame around a vent.