The steam traps have failed in the closed position.
If a thermostatic radiator trap fails in the closed position, no air will pass through it, so little or no steam will arrive at the radiator.
Open the top of the trap and examine the bellows. If it's cool and fully distended, the trap has probably failed in the closed position. If the system is running when you remove the top of the trap, notice whether air rushes from the radiator. This is another good indication that the trap has failed in the closed position.
Repair or replace the trap.
The steam traps have failed in the open position.
A two-pipe steam system is like a ladder. One side of the ladder is the supply line; the other side is the return line. The rungs of the "ladder" are the radiators, and at the end of each rung there is a steam trap.
The trap's job is to pass air into the return side (the no-pressure) of the ladder, to close when steam arrives, and to reopen when condensate forms. If a trap fails in the open position, steam will pass into the return side of the "ladder," causing water hammer as it meets the condensate.
Repair or replace the steam traps.
There's a water leg before the condensate- or boiler-feed pump receiver.
In two-pipe systems, you have steam traps on the radiators instead of air vents. The steam pushes the air through the traps, into the dry returns and toward the condensate- or boiler-feed pump's receiver. Since the receiver is vented to the atmosphere, that's where the air is heading. If your return line drops below the inlet to the receiver, however, you'll have problems. That line isn't under pressure because it's downstream of the radiator and end-of-main F&T traps. As condensate drains from the radiators and pipes, it will pool in that water leg and form a seal. Air can't vent through that water seal, so the building will take forever to heat.
If the traps are defective, they'll mask this problem because there will be enough pressure to force the condensate out of the water leg. A lot of water hammer usually accompanies this evacuation of the water. That encourages the building owner to have his traps fixed. When you fix the traps he'll have no heat because the air can't get out. At this point, you have two choices. Raise the return line to eliminate the water seal, or install main vents at the outlet side of the end-of-main F&T traps.
The Dead Men piped the system for a coal burner.
In the days of coal-fired steam boilers the coal pile would burn all day, nudging the steam gently through the pipes and toward the radiators. Knowing that the firing cycle would last at least eight hours and never cycle on and off, the Dead Men often used long perimeter mains to carry the steam from the boiler to the radiators. The main typically followed the foundation wall and dropped below the boiler water line only when it was done wrapping its way around to the basement. From the main, they ran relatively short takeoffs to the radiators.
As oil- and gas-fired systems grew in popularity, the Dead Men faced a problem. Unlike the coal-fired boilers, these new burners cycled on and off. The thermostat often shut off the burner before the steam reached the end of that long main.
To solve their problem, the Dead Men began to pipe their jobs differently. Rather than use a single long main with short takeoffs to the radiators, they used several mains, each heading in a different direction. With these shorter mains, the runs to the radiators usually had to be longer. But that didn't present much of a problem as long as the Dead Men dripped and pitched their lines properly.
If you find yourself facing one of those old, coal-fired piping arrangements, put as many air vents as you can near the end of the main. Your goal is to get the air out of the main quickly so the steam favors that route. If you can fill the main with steam as though it were a long heating trough, you'll do a much better job of balancing the heat throughout the building.
The venting strategy is wrong.
Keep in mind your goal is to heat the building evenly. That means that on the coldest day of the year you want all the radiators to heat all the way across simultaneously. This is a challenge because some radiators are larger than others. Big radiators contain more air than small radiators. If you want a big and a small radiator to heat evenly, you'll have to vent the big radiator more quickly. This is why manufacturers make adjustable air vents, and air vents with various sizes of fixed vent holes.
But some installers get mixed up. They install the quick air vents far from the boiler, and the slow air vents close to the boiler. They don't pay attention to the size of the radiator when they're doing this. You see, if you use main vents, the steam will favor the main and arrive at most of the radiator supply valves simultaneously. That's why you should vent one-pipe radiators in relation to their size, not their location in the building. This is the key to balancing one-pipe steam heat.
The device that controls the firing cycle is defective or in the wrong place.
Larger steam-heated buildings have heat-timing devices. These devices will fill the piping and radiation with steam on a call for heat. Then they'll run the boiler for a certain time based on the outdoor temperature.
Some control manufacturers use a pressuretrol to figure out when steam fills the piping and radiation. It's easy to trick a pressuretrol - all it takes is a dirt in the pressuretrol or pigtail. If the heat in the building is uneven, check that pressuretrol.
Other heat-timing devices use a thermistor to sense temperature rather than pressure. You'd usually place the thermistor at the end of the longest steam main, but there are no fixed rules. It varies from building to building. However, if the thermistor is on a main that has a clogged air vent, the burner will run all the time. That's because the trapped air will keep the steam from reaching the thermistor.
Check, too, for thermistors that wind up on cold water lines, drain lines and, yes, even gas lines. It happens.
If you have a gravity-return system, make sure the thermistor is high enough on the main. It needs to be below the "A" or "B" Dimension so the rising condensate doesn't cover and cool it. (See The Lost Art of Steam Heating for a complete discussion of "A" and "B" Dimensions.)
It's a gravity-return system and the ends of the main tie together above the boiler water line.
If steam leaves the boiler and travels in two or more directions through separate mains, those mains will join again at some place in the system. Often they meet on the opposite side of the basement where they drop into a wet return and return to the boiler as a single line.
Steam fills each main because the air escapes through the main vents. Some mains are shorter than others, however, and the steam will usually travel through these more quickly than it will through longer mains. That's why it's important in a gravity-return system that the ends of all the mains join below the boiler water line. If they join above the water line, the steam will zip through the shorter main. It will shut the longer main's air vent before the steam in that pipe can reach the vent. That leads to a very uneven heating system.
The Dead Men used boilers that had higher water lines than you'll find in modern, low-water-content boilers. Their end-of-main piping connections may have come together below the water line of their old boiler, but it might not be the same with your new boiler.
Check your water line against those connections. If you find they're coming together above the water line, drop the returns to the floor and connect them there.
The boiler or the burner is undersized for the radiation.
If the burner is too small for the system, it will run 24 hours a day and not heat the building. It's like putting a pot on simmer. You're putting in enough heat to make the water boil mildly, but not enough to deliver steam to the ends of the mains. Remember, the boiler's ability to produce steam has to match the system's ability to condense steam.
If you suspect the boiler or the burner is too small, measure the radiators in the building, add a suitable pick-up factor for the piping and check it against what's there. The burner must fire to this load as well.
The radiators aren't the right size for the space they serve.
If the radiator is undersized for the space it serves, the room will be cold and the heat will be uneven. Also, look for excessive infiltration. A drafty window can cause many comfort-related complaints that have little to do with the steam system.
The pipes aren't pitched properly.
The pitch of pipes in a one-pipe steam system is crucial. The supply main is also the return main. If you don't quickly get rid of the condensate quickly, it will condense the steam as the steam tries to travel further down the line. That leads to uneven heating, and very often, water hammer. If the steam travels in the same direction as the condensate, the pitch must be at least one inch in 20 feet. If the steam and condensate flow in opposite directions, you need a minimum pitch of one inch in ten feet. Ideally, you should drip the horizontal runouts to risers. If you can't drip them, pitch them at least one inch per foot back toward the main.
The take-offs from the main leave at a 90-degree angle instead of a 45-degree angle.
Unless you're dripping the riser, the condensate that returns from the radiators has to flow back into the main. It's important to make this connection to the main at a 45-degree angle so the condensate can hug the side of the horizontal main and flow immediately to the bottom of the main. If the horizontal runout to the riser leaves the main at a 90-degree angle, the condensate will splash into the flow of steam and keep it from reaching the rest of the radiators. Either get rid of the 90-degree connection, or drip the base of the riser so the condensate can't return to the main.
There are no drips on the risers.
The condensate from the radiators falls back into the main. This causes the steam to condensate before it can fill the rest of the radiators. Add drips to the risers.
If it's a gravity-return system, drip into a wet return or into a loop seal and then to a dry return.
If the job has a condensate- or boiler-feed pump, use a steam trap at the base of the riser drip and flow by gravity from the trap to the pump's receiver.
If you run the system at very low pressure, you can use the loop seals with a dry return instead of the riser traps. But if you do this, keep in mind that if someone raises the system pressure, you'll have water hammer problems and steam at your condensate- or boiler-feed pumps.
The steam quality is bad.
The quality of the steam greatly affects steam distribution. When steam condenses, it stops traveling. Dirty water or water with a too-high pH creates wet steam. Look closely at the boiler's gauge glass. If the steam is dry, the part of the gauge glass above the water line should be dry as well.
Try raising the water line to within an inch of the top of the gauge glass. If the water in the boiler is clean it will not surge over the top of the gauge glass.
Check the pH of the water with pH paper. A good pH for a steam system is between seven and nine. If the pH gets to 11, the water will begin to prime and foam and carry over into the system, causing water hammer. Dead Men often added vinegar to steam heating systems to lower the pH and improve the steam quality.
If the building heats unevenly, make sure the water is clean.
Someone added something to the water.
What sort of chemicals are they, and how much is in there? Too much of the wrong type of chemical can cause the water's pH to rise, and that will make the water foam. Check the pH, and lower it if necessary.
Did anyone add pipe dope to cure a leak? If so, the boiler is probably producing wet steam. Wet steam doesn't travel far. If the steam can't make it to the far radiators, the building will heat unevenly.
The thermostat is not working, it's in the wrong place, or it's the wrong type of thermostat.
A smaller steam system will usually run off a space thermostat. Typically, the thermostat will be somewhere in the center of the building, but it might also be in the coldest room. If it is in the coldest room, the other rooms might overheat. If the thermostat's in a central location, some rooms might not heat evenly.
Check to make sure you have the thermostat properly calibrated. And use an ammeter when you're checking; don't guess at that anticipator setting. If the thermostat has a mercury switch, make sure it's hanging level on the wall. See if the thermostat is subject to cold drafts or if it's hanging on a poorly insulated outside wall.
The near-boiler piping doesn't meet the manufacturer's specs.
Nowadays, boiler manufacturers consider the near-boiler piping to be a part of the boiler. They use it to help dry the steam before it heads out toward the system. If the near-boiler piping doesn't meet the manufacturer's specs, you could be throwing water up into the piping. This will cause very uneven heating as the steam condenses in the carried-over water.
Get the boiler manufacturer's installation-and-operating manual and check the piping on the job against the drawings in their booklet.
There are no end-of-main air vents.
The key to balancing one-pipe steam systems is to treat the air in the mains differently than you would the air in the radiators. You should vent the mains quickly, and the radiators in proportion to their size.
There should be a large main vent near the end of each main, but not right at the end in a tee. If the main vent is at the end of the main, water hammer might damage it. Place the vent at least 15 inches back from the end of the main, and up on a six-inch nipple. This gets it out of the way of any water hammer damage.
If you vent the mains properly, the steam will travel more evenly through the piping system, and many of your uneven-heating problems will disappear. Missing main vents can also make the burner short-cycle, and this can lead to uneven heat throughout the building.
The end-of-main air vents are clogged with rust and sediment.
When the steam comes up, the air goes out. As these two gasses rush toward the main vents, they carry pieces of rust and sediment with them. Over time, those tiny particles can build up inside the vent and clog it. Remove the vent and see if you can blow through it. If you can't, try boiling it in vinegar for an hour. If that doesn't clear the vent, replace it.
The pipes aren't insulated at all.
A bare steam pipe will put out more than five times the heat of an insulated pipe. When you remove the insulation, you increase the heat in that room. This often happens in basements. Someone removes the asbestos and the basement overheats. And since there's only so much heat available from the boiler, most of that heat winds up in the basement. As a result, the building may heat unevenly.
Insulate all steam pipes.
The steam pipes are partially insulated.
You're supposed to insulate the supply pipes in a steam system so the steam doesn't condense on its way to the radiators. If some insulation is missing, the steam may be condensing quickly in some areas and slowly in others. This can create pockets of vacuum that cause the steam to flow unevenly throughout the system.
Insulate all the steam pipes.
The main vents close because water backs into the steam mains.
If your one-pipe steam system has a gravity-return, you have to have at least 28 inches of vertical height between the boiler water line and the bottom of the lowest steam main. We call this the "A" Dimension, and it plays a big part in putting water back into the boiler. If the system doesn't have enough "A" Dimension, returning condensate will back into the main, shut off the main air vents, and create water hammer. If the air can't escape through the main vents, the building will heat unevenly.
There are bullheaded tees on the boiler header or at the ends of the main.
When a steam line enters the bull of a tee, we say that line is bullheaded. Steam and bullheaded tees don't get along well because carried-over condensate usually bounces off the back of the tee and winds up in the distribution piping or the radiator.
Get rid of any bullheaded tee you find in a steam line.
Want more troubleshooting tips? Check out A Pocketful of Steam Problems (With Solutions!).