Steam Radiator Valves
A radiator air valve, also commonly called a radiator air vent or steam vent, is a small but essential component on steam heating systems. Its primary purpose is to allow air to escape from a radiator so that steam can fill the unit and provide heat.
How a Radiator Air Valve Works
The operation of a radiator air valve is based on a simple, yet effective, thermostatic principle:
- Air Pushes Out: When the boiler begins a heating cycle, it produces steam. Steam is much lighter than air, so as it enters the radiator, it pushes the cooler, heavier air out through the open radiator air valve. This is the "hissing" sound you often hear from old steam radiators.
- Valve Closes: Inside the air valve is a heat-sensitive element (often a sealed metal capsule containing an alcohol-based liquid or a bi-metal strip). As the hot steam reaches the valve, it heats this element. The element expands and causes a small pin or plunger to rise and seal the valve, trapping the steam inside the radiator.
- Steam Heats the Radiator: With the steam now contained, the radiator can efficiently radiate heat into the room.
- Valve Reopens: As the steam cools and condenses back into water, the radiator's temperature drops. The internal element in the air valve cools down and contracts, which opens the valve again. This allows air to re-enter the radiator, which can cause the radiator to become cool again until the next heating cycle.
Types and Variations
One-Pipe vs. Two-Pipe Systems: The most common application for these valves is in one-pipe steam systems, where a single pipe carries steam to the radiator and returns condensed water back to the boiler. Two-pipe systems have a separate return pipe, and the valves may work slightly differently, but the principle of venting air remains the same.
Adjustable vs. Fixed-Rate Vents:
- Fixed-Rate Vents: These are the most basic type. They have a pre-set orifice size that determines how quickly air can escape. Different sizes (often labeled with numbers like #4, #5, #6, etc.) are used to "balance" a heating system. Radiators far from the boiler may use a larger-sized vent to heat up faster, while those close to the boiler may use a smaller one to prevent them from heating up too quickly and leaving the far radiators cold.
- Adjustable Vents: Some modern or specialty vents have an adjustable dial that allows you to change the venting speed. This gives the user more control over how quickly a specific radiator heats up, which can help in balancing the heat distribution in a building.
It's important to note that a radiator air valve should not be confused with the main control valve, which is typically located at the bottom of the radiator where the supply pipe connects. The control valve is for manually turning the heat on or off, while the air valve works automatically to vent air.