Safety valve

The safety valve is the component that opens automatically when the pressure in a circuit exceeds a set threshold (setting) to release the fluid and protect the system (boiler, water-to-water heat pump, domestic hot water tank, heating/cooling network) against overpressure. It acts as the last line of defense, after the expansion vessel, and prevents material damage, deformation, and, in the worst-case scenario, ruptures. In commercial boiler rooms, 3-bar safety valves are commonly found on the heating side, while in domestic hot water systems, the equivalent function is performed by the safety group (often 7 bar).

Safety valve: operational definition, selection and implementation

A valve consists of a calibrated spring, a seat, and a lifting device (test lever or sealable cap). When the upstream pressure exceeds the set pressure, the obturator lifts, and the fluid is discharged into a dedicated drain pipe; when the pressure drops, the valve closes.

Key points for selection and installation:

- Setting & compatibility : choose a setting < test pressure and ≤ allowable pressure of the equipment; in heating, typically 3 bar.

- Discharge capacity : size according to the flow rate to be evacuated (kW generator, volume, fluid water/glycol, manufacturer's coefficients) to guarantee pressure limitation.

- Materials : brass/bronze/stainless steel body, EPDM/FKM seals depending on temperature and fluid (glycol, DHW).

- Installation : directly connected to the equipment to be protected (without an isolation valve between the two), vertical connection if possible, short section and accessibility for testing.

- Evacuation : dedicated discharge pipe, visible or with indication in technical room, gravity slope towards sewer (visible siphon/interruption), non-reducing diameter, anti-scalding if hot fluid.

- Identification : legible plate (tarification, DN, Kv/coefficients), seal and mention in the DOE.

- Test : periodic lifting (lever) if the manufacturer allows it; otherwise, programmed pressure check.

Advantages, limitations and points to consider regarding a reliable valve

Interests

• Safety: limits overpressure during temperature increases, regulation failure, closed valve, vessel fault.
• Compliance & assurance: presence and traceability required, sealing and identification.
• Simplicity: passive component, instantaneous action without power supply.

Boundaries

• Reopening if cause not treated (damaged vessel, false P0, missing insulation) → repeated discharges.
• Aging: dirty seat/seals (sludge, limescale), worn spring = calibration drift.
• Poorly executed evacuation: backflow onto occupants/equipment, soil corrosion.

Points to consider

• Upstream diagnosis: a valve that "leaks" is not the culprit but a messenger: check expansion vessel (sizing, P0), temperatures, untimely valve closures.
• Maintenance: visual inspection of emissions, annual test (according to instructions), periodic replacement in aggressive environments (glycol, high temperatures).
• Glycol networks: increased expansion, increased viscosity → discharge capacity to be recalculated; compatible materials and seals.
• ECS: does not replace the sanitary safety group (integrated expansion/non-return valve + valve).
• Documentation: up-to-date single-line diagram, intact seal, identified discharge line, test log.

Anecdote — “The leak that spoke to Lille”

In Lille, a gas boiler room was experiencing a problem with its 3-bar pressure relief valve releasing a few deciliters of fluid each morning. Initially, the valve was suspected… before a pressure reading (P0) of 0.6 bar was measured for a static head of 12 m (approximately 1.2 bar required). P0 was readjusted, the temperature difference (ΔT) and insulation were checked, and then the valve was preventively replaced (the seat was marked by the micro-lifts). The result: zero leaks, stable pressure within the specified range, and a condensing boiler that condenses more frequently. The moral of the story: if the pressure relief valve is malfunctioning, the problem lies upstream.