1. Be a Float and Thermostatic style vent, which is normally open so the steam entering the system pushes the air ahead of it out of the radiators.
2. Close shut in the presence of steam. There is a diaphragm bellows at the base of the vent where the float and pin sit. As steam approaches the vent, the surrounding temperature increases, causing the alcohol and water mixture in the diaphragm to flash into vapor. The corresponding increase in mixture volume causes the diaphragm to expand, driving the pin into the port opening at the top of the vent. The vent closes, preventing steam from leaking.
3. Open to break any vacuum that forms when the steam in the radiator condenses, as well as vent any air that is still in the radiator. For this to happen, the system pressure must be very low. Each steam vent has a pressure rating known as “drop-away” pressure. This “drop-away” pressure is the maximum pressure the float inside the vent can drop down against to reopen the vent.
4. Close in the presence of water. If some water made its way into the radiator or if someone left the make-up water valve open by accident, the float inside the vent would “float” the pin into the port opening, closing the vent. These float vents have a syphon tongue which is hinged at the bottom of the vent in the 1/8” nipple where the air, water and steam pass each other. The tongue ensures any water entering the vent shell drains out of the body and doesn’t hold the vent closed.
Probably the most important detail and least understood subject that governs the success of one-pipe steam systems is velocity. The main in a one-pipe system supplies steam to up-feed risers, which in turn feed the radiators on each floor. The piping system that feeds these up-feed risers is referred to as the horizontal runouts. What occurs inside this piping is critical to the operation of the system. Steam heads towards the radiator while the condensate gravity drains back into the main. As long as the velocity of the steam doesn’t exceed a critical point, everything coexists perfectly. But if the steam’s velocity crosses that critical point, the velocity is so high the condensate can’t drain back. The steam carries this condensate back up the risers, causing gurgling and sloshing noises in the piping and the condensate eventually spits out the radiator’s vent valve!

Old pipe sizing handbooks featured charts showing the required pipe sizes based upon the connected load and the designed pressure drop. In these pipe charts, there was a column dedicated to the size of the horizontal runouts and up-feed risers. The pipes selected for these columns were solely based upon this critical velocity limitation.
Whenever you are faced with solving the problem of a spitting radiator vent, think about how the system was supposed to work and ask yourself what’s changed. Here is a list of some of the more common causes of spitting radiator vent valves.

Check the size of the horizontal runout, vertical up-feed riser and the supply radiator valve vs. the connected radiation load and make sure they are adequate.
Make sure the runout pitch meets the recommended minimum(1/2” per foot, and if the runout exceeds 8’, use the next larger pipe size).
Make sure the radiator valve is completely open. If it’s partially closed, or if the disc falls off the seat, the diameter of the opening is reduced and the velocity increases dramatically!

Check to see if the insulation has been removed from the piping. Cold pipes create a lot more condensate.
Make sure the radiator is pitched slightly towards the supply valve. The condensate should drain when the system shuts down.
Replace the vent if dirt, scale and rust prevent the vent from seating tightly.
If someone relocates the radiator, make sure the new piping adheres to the same rules as when the system was first installed.
A large radiator with a large capacity vent may cause the radiator vent to spit. A fast vent can allow too much steam into the radiator. This creates too much condensate, which is trying to drain out the supply valve while more steam is trying to enter. Instead of one quick vent, try two smaller capacity vents, installed one above the other.

If you have questions or problems with any steam related systems, call your McDonnell & Miller or Hoffman Steam Team representative. They have the solutions to your problems.