The air vents aren't working.
And if they're not, the system will trap air and drive the fuel bills up. Steam and air are both gases, but steam is lighter than air so the two won't mix. When the steam heads down a pipe, it pushes air ahead of itself. If the air can't escape from a point near the end of the pipe (through a vent), the steam will just compress it. The pressure will build and the burner will shut off on the pressuretrol. The building will stay cold because the radiators and mains contain air instead of steam. Someone will probably show up and raise the pressure, and that's when you'll start to burn more fuel. The higher pressure compresses the air a bit more, but usually not enough to heat the building. The higher you raise the pressure, the bigger the fuel bills will be.
If you want to save fuel, fix the air vents. Take them off and try to blow through them. If they're clogged, boil them in vinegar for an hour. If they still don't work, replace them.
The burner is short-cycling.
It could be because the air vents aren't working. A short-cycling burner runs very inefficiently, and that leads to greater fuel usage. Check the air vents, and change them if necessary.
Check the firing rate of the burner as well. You should be firing to the boiler's connected load (the piping and radiation) and no more. If your burner is too large it will raise the system pressure very quickly, and then shut off on the pressuretrol.
If you combine an oversized burner with clogged air vents you'll magnify the problem - and you'll increase the fuel bills. If the burner is oversized it will also throw water up into the system. That leads to wet steam, and wet steam can also make the burner short-cycle. The pressure builds quickly as the steam leaves the boiler and confronts the air. But because there's so much water in the steam, the steam quickly condenses, and that makes the burner come on again.
Short-cycling wastes fuel. Find the cause and eliminate it.
The steam traps aren't working.
If the steam traps on the radiators or at the ends of the main fail in the open position, steam will enter the return lines. Once there, it will equalize the pressures on the supply and return sides of the system. When that happens, the flow of steam stops. The people in the building have no heat. Someone raises the pressuretrol settings, and adjusts the thermostat to make the burner run longer. The higher pressure, and the longer steaming cycle, will overheat the radiators that were already getting steam. Those people will probably open their windows. The radiators that weren't heating before will get a bit hotter, but at what cost?
When steam traps are not working, fuel bills soar. There is no substitute for steam trap maintenance in a steam heated building. Find the defective traps and repair or replace them.
You can check thermostatic radiator traps with a thermometer. There should be at least a ten-degree temperature drop from one side to the other. Float & thermostatic and bucket traps have no temperature drop across them. Check these by opening the line downstream of the trap. You should see mostly condensate with some flash steam come from the trap. If you see live steam, repair or replace the trap. Before you replace a bucket trap, make sure it's primed with water. If a bucket trap loses its prime, it will blow live steam.
The burner is undersized.
If the burner is too small, 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 gently, 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.
Think for a minute about a high/low-fire burner. You begin with the big flame because both the insulated pipes and the radiators are cold. The big flame lets' you drive steam out to the ends of the mains. Since the piping represents at least one-third of the total load, you need that big flame on start-up. But once you've made the insulated piping hot, the burner can drop to the low-fire setting because the insulated piping's ability to condense steam has lessened.
Now imagine you start with the little flame instead of the big flame. Can you imagine how difficult it will be to push steam to the furthest radiator? This is why you should never undersize burners. If the burner is too small, the fuel bills will be very high. Always fire to the connected load.
There's a hole in the boiler.
Now, this isn't a hole you can see. It's an internal hole caused by oxygen corrosion. It happens to a cast-iron boiler that takes on a lot of fresh feed water. The oxygen boils out of the water and eats a hole through the metal at the boiler's water line. The water steams off and goes up the flue. You can't see it unless you're looking at the chimney. It looks like white smoke, but it's not smoke; it's water vapor. The fuel bills increase because the burner never shuts off on high pressure.
And since a lot of steam goes up the chimney, you wind up with the same effect you'd have if the burner was undersized. It runs on and on, but
the furthest radiators never get hot.
To check for a hole, flood the boiler up into the header piping. You'll know you have a problem if you see water pouring out of the boiler's jacket. By the way, if you have an automatic water feeder, it pays to install an inexpensive water meter on the feed line. Keep a log of the meter reading and you'll have an early warning of a system leak or a hole in the boiler.
The device that controls the firing cycle is either 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 these pressuretrols. All it takes is a bit of dirt either in the pressuretrol or the pigtail. If the fuel bills are high, check that pressuretrol and pigtail.
Other heat-timing devices use thermistors to sense temperature rather than pressure. Usually, you 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!
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.)
The thermostat is either not working or it's in the wrong place.
A smaller steam system will run off a space thermostat. The thermostat probably 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, and that will make people open their windows. An open window will always mean a higher fuel bill.
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 the thermostat hangs level on the wall. See if cold drafts hit the thermostat, or if it's hanging on a poorly insulated, outside wall. All these things lead to higher-than-normal fuel bills.
The boiler is dirty.
If the boiler is sooty, it's going to burn lots of fuel. Check it out, and give it a good vacuuming if it needs it. If the boiler has a chamber, make sure that chamber hasn't collapsed, either fully or partially. If you're burning oil, check to see if you have the correct nozzle. Check both the firing rate and the flame pattern. If the burner runs on gas, check the gas pressure and clock the meter.
A great investment you can make in any steam system is to install a stack thermometer in the breeching. Keep a log showing the stack temperature. Start the log right after you've cleaned the boiler thoroughly. When you see the temperature rise, you know it's time to clean the boiler again.
Want more troubleshooting tips? Check out A Pocketful of Steam Problems (With Solutions!).