Steam Main Vents

One Pipe Steam Heating Systems- Venting Mains Steam heat distributes itself by differential pressure. As the vapor is formed it expands to 1700 times the volume of liquid water. The expansion pressurizes the boiler and then the race is on as the system tries to equilibrate with the outside world- high pressure seeking low pressure. The goal, the bait, the place where equilibrium can be reached is at the designed holes in the system, in the vents. Steam heads for those holes as soon as it is formed, and it is this ability to move without mechanical help that made steam the central heating choice in the era before electricity. (No electricity, no pumps.) A steam system at rest is full of air, right down to the water line of the partially full boiler. Once the boiler fires and heats the water, steam begins moving up through the boiler, pipes, and radiators, pushing air in front of it. You may hear a hissing sound from the vents as air is forced out of the radiators. When the steam reaches a functioning vent, the heat in the vapor will cause the vent to shut, often with an audible click. The trapped steam then condenses in the cold radiator, giving up its heat and transforming back to liquid, which returns to the boiler for another cycle. A steam vent which is stuck shut doesn’t allow air to pass through it; steam can’t get into the radiator and no heating can take place. A vent stuck open will allow steam to blow right past; the system will lose water like a teakettle left on the stove too long, not to mention removing the wall paper and setting off the smoke alarms. The locations and sizes of the holes in the steam system’s vents control how fast the rooms heat and therefore how soon the thermostat is satisfied and the burner shuts off. Quick distribution equates directly with the size of the fuel bill. The sooner the thermostat is satisfied, the sooner the burner shuts off, the smaller the fuel bill. There are vents on each radiator, but also on the mains, the big pipes which distribute steam from the boiler and gather the condensate to return it to the boiler. These main vents are very important because they allow the air to be removed from the big piping quickly, (having bigger holes and being located at the level of the mains). If the main vents are missing or broken, the only place air can leave the system (and let the steam come after it) is at the radiators. When this happens, all the air from the large main pipes has to be pushed out through the radiator piping and small radiator vent holes. The entire system takes longer to heat- the burner is running while the steam is held up in traffic, backed up waiting for the air to leave, and the last radiators in line are still waiting for their steam while the first are baking. Those last-in-line radiators may never even get their steam, if the thermostat is located near the first-in-line radiators. If you have cold rooms, or rooms that only heat on a very cold day, this may be the cause. Ideally, the main vents on your system should be located on the main piping right after the take-off to the last radiator. The vent should be as high above the main as possible, but often the vent or vents will be in a tee fitting at the end of the return mains where the pipe drops down to rejoin the boiler. Installers did this because it was convenient, but it is a dangerous place for the vents as each cycle of steam, moving at up to 20 mph, roars down the pipe and crashes into the fitting at the end, squirting condensate and pipe debris up into the base of the vent and hastening its demise. When we find main vents which are no longer working, we recommend moving them back at least 12” from the end of any pipe run and setting them as high as possible. The cost of repiping will quickly be repaid in shortened run times for the burner, and better balance as rooms heat at the same time.