Hoffman Air Valves

Hoffman air valves are thermostatic float-type and disc-type venting devices designed for use in steam heating systems, available in radiator, convector, main vent, and water main configurations to serve every venting point in a one-pipe or two-pipe steam system. A Hoffman air valve opens to expel air from a radiator, convector, or steam main as steam advances, and closes automatically when steam or heat reaches the valve, preventing steam from escaping into the room. Without properly functioning air valves, air trapped in the system blocks steam from reaching radiators, creates uneven heat distribution, and causes banging and hammering as steam meets pockets of cold air. The Hoffman air valve lineup at SupplyHouse spans the Hoffman 1A adjustable angle radiator valve, the Hoffman 40 non-adjustable angle radiator valve, the Hoffman 41 and 508 straight convector valves, the Hoffman 4A and 75 steam main vents, the Hoffman 75H high-pressure main vent, and the Hoffman 78 water main air valve. Hoffman air valves are the most widely specified brand in single-pipe steam heating service and replacement work.

KEY SUBCATEGORIES / PRODUCT TYPES

Hoffman 1A  Adjustable Angle Steam Radiator Air Valve

The Hoffman 1A (SKU 401422) is a 1/8" NPT angle connection, float-type, adjustable steam radiator air valve with six port settings ranging from slow (1) to fast (6) for true proportional venting. The 1A is the correct choice when different radiators in a system need different venting rates, typically because radiators farther from the boiler need to vent faster to heat at the same rate as closer ones. The maximum operating pressure (drop away pressure, the maximum pressure against which the vent can open) is 1.5 psig, with a maximum pressure of 10 psig. The 1A is non-vacuum and float-type, using alcohol-filled float technology that closes the valve when steam heat vaporizes the alcohol, preventing steam from escaping. Manufacturer recommendation is to replace these air vents after 10 years of use.

Hoffman 40  Non-Adjustable Angle Steam Radiator Air Valve

The Hoffman 40 (SKU 401440) is a 1/8" NPT angle connection, float-type, non-adjustable steam radiator air valve designed for ordinary one-pipe steam systems that don't require proportional venting. The 40 has a single non-adjustable port and is best suited for radiators close to the boiler where fast, unrestricted venting isn't needed and where all radiators in the zone heat at a reasonably consistent rate. The 40 is non-vacuum. It's the simpler, lower-cost option for steam radiator service when adjustability isn't a system requirement.

Hoffman 41 Straight Steam Convector Air Valve

The Hoffman 41 (SKU 401455) is a 1/8" NPT straight shank, float-type, non-adjustable steam convector air valve designed for small steam systems using convectors rather than cast iron radiators. The 41 features a telescopic siphon tube with an angle cut that assures drainage back into the convector, preventing water accumulation at the valve. Maximum operating pressure is 6 psig (drop away), with a maximum pressure of 10 psig. The straight shank connection suits the port geometry common on convector heating elements, making the 41 the correct choice on a steam convector where an angle-connection radiator valve won't install properly.

Hoffman 508  Disc-Type Straight Steam Convector Air Valve

The Hoffman 508 (SKU 401475) is a 1/8" NPT straight shank, disc-type (not float-type) steam convector and radiator air valve with a built-in check valve, a maximum steam pressure of 15 psig, and a design that allows the valve to be disassembled and cleaned without removing it from the line. The 508 is the replacement for the obsolete Model 500. Unlike the float-type 41, the 508 uses disc-type thermodynamic operation to open and close, can be installed vertically or horizontally, and can also serve hydronic systems when the cap is set to the hydronic position. The built-in check valve and serviceable design make the 508 well-suited for convector and baseboard applications where the valve may require periodic cleaning.

Hoffman 4A  Straight Steam Main Air Valve

The Hoffman 4A (SKU 401413) is a 1/2" NPT female and 3/4" NPT male straight shank, float-type, non-adjustable steam main air valve for residential or small one-pipe or two-pipe systems. Maximum operating pressure (drop away) is 2 psig, with a maximum pressure of 10 psig. The 4A installs 6 to 10 inches above the horizontal return and 18 inches above the boiler water line. It's designed to expel air from the steam main at startup so steam can fill the system quickly, and closes when system pressure reaches 2 psig. For residential systems operating at typical pressures below 2 psig, the 4A is the standard main vent specification.

Hoffman 75  Straight Steam Main Air Valve for Medium and Large Systems

The Hoffman 75 (SKU 401434) is a 1/2" NPT female and 3/4" NPT male straight shank, float-type, non-adjustable steam main air valve built for medium and large steam systems. Maximum operating pressure (drop away) is 3 psig, with a maximum pressure of 15 psig. The higher drop away pressure of 3 psig compared to the 4A's 2 psig makes the 75 suited for systems with higher operating pressures or longer main runs where the 4A would close prematurely before all the air has been expelled. The 75 features solid copper construction, contributing to its reputation for durability in larger steam systems.

Hoffman 75H  High-Pressure Straight Steam Main Air Valve

The Hoffman 75H (SKU 401437) is a 1/2" NPT female and 3/4" NPT male straight shank steam main air valve with a maximum operating pressure (drop away) of 10 psig. The 75H serves steam systems operating at higher pressures where both the 4A (2 psig) and the 75 (3 psig) would close before the system has fully purged air. For commercial steam systems or any installation where boiler operating pressure exceeds the drop away ratings of the standard main vents, the 75H is the correct specification.

Hoffman 78  Water Main Air Valve

The Hoffman 78 (SKU 401485) is a 3/4" NPT straight shank water main air valve with a cast brass body, a built-in check valve, a top-tapped 1/8" NPT safety drain connection for discharging moisture, and a body that unscrews for easy cleaning. Maximum operating pressure (drop away) is 150 psig, maximum hydrostatic pressure is 450 psig, and maximum temperature is 250°F. The 78 serves high-pressure hot or cold water, glycol mains, and process applications with a specific gravity greater than 0.7, making it the correct valve for hydronic heating mains, chilled water mains, and process piping rather than steam applications.

BUYING GUIDE / HOW TO CHOOSE

Selecting the right Hoffman air valve starts with identifying the venting location: radiator, convector, or steam main. Radiator valves (Hoffman 1A and 40) use 1/8" NPT angle connections that thread into the side tapping of a cast iron steam radiator. Convector valves (Hoffman 41 and 508) use 1/8" NPT straight shank connections that thread into convector heating elements. Main vents (Hoffman 4A, 75, and 75H) use 1/2" female and 3/4" male connections that install at the end of steam mains and on the return piping.

For radiator valves, the next decision is adjustable versus non-adjustable. The Hoffman 1A's six-position adjustment allows a technician to balance heat delivery across a system by slowing the vent rate on close radiators and opening it up on distant ones, so all radiators heat at a consistent pace. The Hoffman 40 is non-adjustable and works well for close radiators or systems where all radiators are at roughly equal distances from the boiler. The general principle: radiators close to the boiler vent slower; radiators far from the boiler vent faster.

For main vents, the critical spec is the drop away pressure: the maximum pressure against which the vent can open. The 4A drops away at 2 psig, the 75 at 3 psig, and the 75H at 10 psig. The correct main vent matches or slightly exceeds the system's normal operating pressure. If the system runs at 1.5 to 2 psig, the 4A is correct. If it runs higher, the 75 or 75H is required. Using a main vent with a drop away pressure lower than the system operating pressure means the vent closes before all the air is expelled, leaving air in the mains and slowing heat distribution to the radiators.

For convectors, the choice between the 41 and 508 depends on installation geometry and whether serviceable, in-line cleaning is a priority. The 41's telescopic siphon tube assures drainage back into the convector. The 508's disc-type design allows disassembly and cleaning without removal from the line, and can also serve hydronic applications.

FAQs

What does a Hoffman air valve do in a steam heating system?

A Hoffman air valve is a thermostatic venting device that opens to allow air to escape from a steam radiator, convector, or main line as steam advances through the system on startup, then closes automatically when steam or heat reaches the valve, preventing steam from escaping into the room. Without a properly functioning air valve, air trapped in the system blocks steam from entering the radiator, resulting in a radiator that's slow to heat, partially cold, or completely unresponsive. Hoffman air valves are float-type or disc-type: float-type valves use an alcohol-filled float that closes when alcohol vaporizes from steam heat, and disc-type valves use thermodynamic disc operation to open and close.

What is the difference between the Hoffman 1A and Hoffman 40 steam radiator air valves?

The difference between the Hoffman 1A and Hoffman 40 is adjustability. The Hoffman 1A (SKU 401422) is an adjustable float-type angle valve with six port settings from slow (1) to fast (6), allowing a technician to control how quickly each radiator vents air and receives steam. The Hoffman 40 (SKU 401440) is a non-adjustable float-type angle valve with a single fixed port, suited for ordinary one-pipe systems where proportional venting isn't needed. Both use 1/8" NPT angle connections. The 1A carries a maximum operating pressure (drop away) of 1.5 psig and a maximum pressure of 10 psig. Proportional venting with the 1A is the recommended approach for systems where radiators are at significantly different distances from the boiler.

What is the difference between the Hoffman 4A, 75, and 75H steam main air valves?

The difference between the Hoffman 4A, 75, and 75H is the drop away pressure, which is the maximum pressure against which the vent can open and remain open. The Hoffman 4A (SKU 401413) has a 2 psig drop away pressure, suited for residential or small one-pipe and two-pipe systems. The Hoffman 75 (SKU 401434) has a 3 psig drop away pressure, suited for medium and large systems with higher operating pressures or longer main runs. The Hoffman 75H (SKU 401437) has a 10 psig drop away pressure, suited for commercial systems operating at higher pressures where the 4A or 75 would close prematurely. Selecting a main vent with a drop away pressure at or above the system's normal operating pressure is required for the vent to fully expel air before closing.

What is the Hoffman 78 water main air valve used for, and how does it differ from steam air valves?

The Hoffman 78 (SKU 401485) is a water main air valve designed for high-pressure hot or cold water, glycol mains, and process applications, not steam systems. The 78 carries a maximum operating pressure (drop away) of 150 psig, a maximum hydrostatic pressure of 450 psig, and a maximum temperature of 250°F, which are specifications in a different class from the steam valves in the lineup. The 78 features a cast brass body with a built-in check valve, a top-tapped 1/8" NPT safety drain connection, and a body that unscrews for cleaning. Steam radiator and main air valves in the Hoffman lineup operate at pressures below 15 psig. The 78 is the correct specification for hydronic heating mains, chilled water distribution, and process piping.

When should a Hoffman 41 convector air valve be used instead of a standard radiator air valve?

A Hoffman 41 convector air valve (SKU 401455) should be used in place of a standard radiator air valve when the heating element is a convector rather than a cast iron radiator, and when the connection point requires a 1/8" NPT straight shank rather than an angle connection. The 41's telescopic siphon tube with an angle-cut bottom assures drainage back into the convector, preventing water accumulation at the valve body that could cause spitting or dripping. Maximum operating pressure on the 41 is 6 psig drop away with a 10 psig maximum, giving it a higher pressure tolerance than the Hoffman 1A or 40 radiator valves. For convector installations where in-line serviceability matters, the Hoffman 508 disc-type valve (SKU 401475) is the alternative, as it can be disassembled and cleaned without removal from the line.