Pipe the vent about a foot inches back from the end of the main, and six-to-ten inches up on a nipple to keep it away from any end-of-main water hammer. If you can't get these exact dimensions, do the best you can. And remember, the bigger the hole, the faster the venting, so it pays to install a tee with a three-quarter-inch tapping for the vent near the end of the main. Don't try to get by with a tiny hole drilled into the main. Tiny holes don't vent quickly.

Install a "Y" strainer before the main vent. An old steam system can be plenty dirty! And since steam is moving at high velocity (typically, about 25 mph in a one-pipe system), it picks up particles of rust and sediment. Eventually, this stuff winds up inside the main vent. Before long, your main vents clog and can't shut. They spit water and lets steam pass to the atmosphere. This creates water-level problems at the boiler. A "Y" strainer, installed vertically before the main vent can protect the vent from system debris and increase its life. Use the strainer as part of your six-to-ten-inch elevation for the main vent and you'll improve the system's performance dramatically. Then make sure you clean the strainer from time to time!

Vent the radiators based on their size. If your goal is to get all the radiators hot at about the same time on the coldest day of the year, you'll have to handle the air in a special way. First, as I said before, vent the mains quickly. That's important. Then vent the radiators in relation to their size, not necessarily their location throughout the building. The main vents will help get the steam to each radiator at about the same time. Since big radiators contain more air than small radiators, big radiators should have larger air vents than small radiators. That makes sense, doesn't it? Nevertheless, it's a fine point in one-pipe steam that's often misunderstood.