3/4" x 1/2" Steam Air Valve, Female x Male

Bell & Gossett 401434 Series 75 Main Steam Air Vent – Thermostatic Float-Type, 1/2" x 3/4" NPT

The Bell & Gossett 401434, also known as the Hoffman Specialty Series 75, is a float-type main air vent designed for low-pressure steam heating systems. This thermostatic valve is engineered for use on medium and large residential or light commercial steam mains where rapid and efficient air removal is required during system startup.

Equipped with a single non-adjustable orifice, the Series 75 maintains consistent venting performance without field calibration. Its internal float mechanism provides automatic shutoff when steam contacts the thermostatic element. The valve body features a 1/2 inch FNPT inlet and a 3/4 inch MNPT outlet connection, enabling straightforward installation at main line vent points.

Product Features

• Vent Type: Float-actuated thermostatic air vent for steam mains
• Configuration: Straight pattern body with FNPT x MNPT connections
• Inlet Size: 1/2 inch FNPT
• Outlet Size: 3/4 inch MNPT
• Orifice: Fixed (non-adjustable) venting port
• Design Pressure: 3 psi operating, 15 psig maximum
• Application: Main venting for low-pressure steam distribution systems

Technical Specifications

• Model Number: 401434
• Series: 75
• System Type: Steam (non-vacuum)
• Maximum Pressure: 15 psig (1.0 bar)
• Operating Pressure: 3 psig (0.2 bar)
• Connection Type: 1/2 in. FNPT inlet x 3/4 in. MNPT outlet

Installation Requirements

The Series 75 should be installed in an upright vertical orientation at the end of the steam main, 10–15 inches above the return and a minimum of 18 inches above the boiler waterline. Use compatible steam-rated thread sealant. Proper vent sizing in relation to pipe length and system volume is critical for effective air removal. This valve is not vacuum-rated and should not be used in vacuum-return systems.

Use Cases

• Low-pressure steam main venting in multi-radiator systems
• Suitable for both one-pipe and small two-pipe steam layouts
• Applications requiring reliable non-adjustable venting in medium-capacity systems