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Boiler Relief Valves

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Boiler Relief Valves

What Is a Boiler Relief Valve?

A boiler relief valve is a safety device designed to protect a boiler from over-pressure (and, in some cases, over-temperature).
If pressure inside the boiler gets too high, the relief valve automatically opens and releases water or steam.

Without a relief valve, a boiler could rupture or explode, causing serious injury and equipment damage.
Relief valves are required by code and are not optional.  

 Boiler Relief Valves

Types of Boiler Relief Valves

There are two main types used in HVAC heating systems:

1. Hot Water Boiler Relief Valves

Used on hydronic (hot water) heating boilers.
Typical characteristics:

  • Pressure relief only (no temperature element)
  • Common set pressure: 30 psig, verify on boiler name plate
  • Discharges hot water when pressure is exceeded
Where are they used:

  • Commercial and residential hot water heating boilers

2. Steam Boiler Relief Valves

   Used on steam heating boilers.

Typical characteristics:

  • Designed specifically for steam 
  • Higher pressure ratings than hot water systems
  • Discharges steam (very dangerous if exposed)
Where are they used:

  • Low-pressure or high-pressure steam heating systems
  • Older buildings and industrial facilities
Key Differences: Hot Water vs Steam Relief Valves

Feature


Medium

Typical Set Pressure

Discharge

Temperature Rating

Valve Design

Code Requirements

Hot Water Boiler


Water

30 psig

Hot water

Lower

Liquid service

ASME Section IV

Steam Boiler


Steam

15 psig (low-pressure) higher

Steam

Much higher

Vapor service

ASME Section I or IV

*Never substitute a hot water relief valve on a steam boiler.

What’s Needed to Select a Boiler Relief Valve

When selecting a relief valve, you must know the following:

1. Boiler Type

  • Hot water or steam
  • Heating boiler (not domestic water heater)
2. Maximum Allowable Working Pressure (MAWP)

  • Found on the boiler nameplate
  • Relief valve set pressure cannot exceed this value
3. Valve Capacity (BTU/hr)

  • The relief valve must be able to relieve all heat the boiler can produce
  • Valve capacity must be equal to or greater than boiler input
  • Capacity is stamped on the valve
4. Temperature Rating (Hot Water Systems)

  • Common rating: 210°F
  • Must match or exceed boiler design temperature
5. Connection Size

  • Valve inlet must match the boiler tapping
  • Outlet piping must NOT be reduced
  • Discharge piping must be: Full size, Directed to a safe location, Not capped or Valved
6. Code Compliance

  • ASME certified
  • Labeled for the correct service (water or steam)
  • Installed per local code and manufacturer instructions

Simple Rule to Remember

The relief valve protects the boiler, not the system.
Always size and select it based on boiler ratings, not piping or pumps.

New to Boilers? Read on!


Why Pressure Builds in a BoilerPressure can rise due to:

  • Water expanding as it heats
  • Failed expansion tank
  • Faulty pressure-reducing valve
  • Closed isolation valves
  • Control or sensor failures
  • Firing without proper circulation

Maintenance

  • Verify controller settings at every PM
  • Document all setpoints
  • Confirm direct-acting vs reverse-acting device orientation

The relief valve is the last line of defense.

Common Failure & Maintenance Points

  • Valve weeping or dripping (often caused by system issues)
  • Corrosion or mineral buildup
  • Valve stuck closed due to lack of operation
  • Improper replacement (wrong pressure or type)

Relief valves should never be adjusted or repaired in the field. If faulty, replace them.

Installation Basics

  • Install directly on the boiler
  • No valves between the boiler and relief valve
  • Discharge piping: Cannot be plugged or restricted, Must terminate safely
  • Relief valve discharge should be visible (to detect failures)


Boiler Relief Valve

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