Balancing One-Pipe Steam Systems
It can be done, though. You just have to follow a few rules that the Deadmen followed when they first installed these old beauties. Do these things and you’ll have happier clients.
First, vent the mains quickly. This will help the steam get to all the radiators at about the same time. Steam is a gas and it will always look for a way out of the system. When it leaves the boiler, it heads toward the air vents. The bigger the air vent, the more inclined steam will be to head that way. If the system heats unevenly, install a good, high-capacity main vent near the end of the each main and watch the difference it makes. 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 allow air to 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 (up to 60 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 won't shut. They’ll spit water and let steam pass to the atmosphere. This creates water-level problems back 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 someone cleans the strainer from time to time.
Vent the radiators based on their size, not on their location within the building. 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 so important. Then vent the radiators in relation to their size, not 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.
Use two vents on oversized radiators. Oversized radiators are always a challenge. No matter what size vent you use, that vent will close once the steam reaches it – even if much of the air remains in that huge radiator. The Deadmen often faced this challenge by drilling and tapping those oversized radiators for a second air vent. They positioned the second vent a few inches lower than the first. The two vents then worked together to let the air out. When steam reached the first vent (the higher of the two), that vent closed. But the second vent (at the lower level) continued to vent air from the radiator. As a result, the radiator heated more completely, and brought the Deadmen a step closer to system balance. This trick can work just as well for you.
Insulate all the steam lines. When steam condenses and turns into water it stops moving. That's why the Deadmen spent so much time insulating their steam mains. They wanted the steam to condense in the radiators, not in the basement piping. If the asbestos is gone, replace it with fiberglass. This will help you balance the system because the steam won’t condense as quickly in the basement pipes. Uninsulated steam pipes have about five times the heat loss of insulated steam pipes so wrap them well and give the steam a chance to get where you want it to go.
Clean the system (and a dozen times if necessary). One-pipe steam systems are old and open to the atmosphere and constantly corroding. All that corrosion winds up in the boiler water, and if the boiler water is dirty, the steam will carry water with it when it heads off into the piping. This leads to water level problems at the boiler, of course, but it also creates balancing problems throughout the system. The steam gives up its latent-heat energy to the mist of water that's traveling with it. That stops the steam dead in its tracks. The radiators furthest from the boiler remain cold while the radiators near the boiler room get warm. The burner often short-cycles when the steam quality is poor. This, too, leads to balancing problems. Check the boiler manufacturer's cleaning instructions. It can take days to get a boiler's water back in "clean-steam" shape, but this is often the only solution to those balancing problems.
Lower the steam pressure. Steam heating systems ride a wave of pressure from the "cut-in" to the "cut-out" setting of the pressuretrol or the vaporstat. The system must cycle up and down on that wave because that's how the air vents work. Steam pushes the air from the vents; the vents then shut on temperature. When the steam condenses, the vents are supposed to open to allow venting to continue. But if the system pressure is too high, the air vents might stay closed. Since air can't escape from a closed air vent, the radiators stay cool and the system goes out of balance. The air vents and the pressuretrol or vaporstat work together to move the air from the system. If you set the "cut-in" setting at one-half psi on a pressuretrol, or at about four ounces on a vaporstat, you'll never lock the air vents closed. The "cut-out" pressure should be as low as possible. There is no reason to raise the steam pressure any higher than it has to be. High-pressure steam actually moves slower than low-pressure steam. So when you're trying to balance that one-pipe system, lower, don’t raise, the pressure.
Check the near-boiler piping against the boiler manufacturer's specs. If you want a modern steam boiler to produce good-quality, dry steam, you’ll have to pipe it the way the boiler manufacturer recommends. Replacement boilers are much smaller than the boilers of the steam era. They use their near-boiler piping to separate the water from the steam. If you want the steam to reach those faraway radiators, it has to be dry. Proper near-boiler piping plays a huge role in the one-pipe-steam balancing act.