Steam and condensate flow in the same direction, and the pitch is less than one-inch in 20 feet.
There should be no areas where condensate can gather when the system shuts down. If you let condensate lay between cycles, steam will pick it up on the following cycle and create water hammer.
Check the mains with a line level. Don't trust your eyes.
Steam and condensate flow in opposite directions, and the pitch is less than one-inch in ten feet.
Don't let condensate hang around during firing cycles. Make sure it can drain quickly back to the boiler or condensate receiver.
Notice how the pitch of counterflow pipes is twice what it is on parallel flow pipes. This is a very important consideration in steam heating. The condensate and the steam have to be able to get out of each other's way.
Don't trust your eyes; use a line level.
Horizontal runouts to risers feed a second (or higher) floor, and the pitch isn't right.
In a one-pipe steam system, the condensate and the steam have to pass each other in that horizontal runout to the riser. If the riser feeds a radiator on the first floor, the pitch is not so critical because there's usually not that much condensate flowing back. However, when the riser feeds an upper floor (and you can't drip it) you have the additional burden of the piping to consider. A lot of condensate is going to be fighting that steam on its way to the radiator. To avoid water hammer and uneven heating, you should give that runout to the riser a pitch of at least one-inch per foot. Use a line level to check it.
The risers aren't dripped.
If you have a one-pipe steam riser that feeds up more than one floor you should, ideally, drip it into a wet return or into a dry return through a loop seal or a steam trap.
Dripping two-pipe steam risers isn't as critical because the condensate returns through a separate line. Just keep in mind that any steam pipe will be more efficient if you drip it. To avoid water hammer, always keep the steam and the condensate as far away from each other as you can.
The mains aren't properly dripped.
Even if the pitch is good, you can still get water hammer if you don't drip those mains. In mains where the steam and condensate travel in the same direction, there should be a drip line every 150 feet. If the steam and the condensate flow in opposite directions, you need a drip line every 50 feet.
If you drip into a gravity wet return, you don't need steam traps. If you drip into a dry return, or a return that ends in a condensate-or boiler-feed pump, use steam traps.
Want more troubleshooting tips? Check out A Pocketful of Steam Problems (With Solutions!).