Check valve

A check valve (or non-return valve) is a unidirectional device that allows fluid (hot water, chilled water, domestic hot water, condensate, etc.) to flow in only one direction and closes automatically if the flow attempts to reverse. It prevents backflow, thermosiphoning (unwanted circulation when pumps are off), protects pumps/heat exchangers, and stabilizes HVAC/DHW systems. Its closure is ensured by gravity, a spring, or upstream pressure, and its sensitivity depends on the opening pressure setting.

Non-return valve: definition and uses

Main types:

- Hinged (swing): hinged shutter; low pressure losses, suitable for high flow rates and moderate speeds.

- Spring-loaded (lift/spring): axial shutter with spring; faster closing, good against sudden return, more sensitive to fouling.

- Double wafer flap: compact between flanges, appreciated in small technical rooms.

- Ball type (rare in HVAC): tolerates particles, suitable for use with dirty water.

Where to use them:

- After pump (on discharge) to prevent return to standstill.

- On DHW loops to avoid backflow between columns / cold water rising.

- On heating/cooling networks to cut off thermosiphon via generators/exchangers.

- In condensates (PAC/CTA/boiler) to avoid reflux towards the tanks.

- In aerodynamics, the equivalent is the backdraft damper on ducts.

Installation points:

- Direction of arrow is imperative; respect the orientation (some horizontal models only).

- Minimum distance after elbows/tees to limit turbulence (noise/wear).

- Accessibility for maintenance (spring/seal), drain pocket possible.

- Choose appropriate diameter/Kv: too large a valve = flapping/noise, too small = losses.

Advantages, limitations and points to consider regarding the non-return valve

Interests

• Protection of pumps/exchangers against backflow and water hammer (if closing quickly).
• Hydraulic stability: cuts off parasitic circulations (thermosiphon), maintains ΔT.
• Sanitary quality in DHW: avoids mixing and backflow from one column to another.

Boundaries

• Added pressure losses (depending on type/opening).
• Noise/vibrations if there is a beat (flow rate too low, turbulence).
• Possible fouling (sludge/scale) → valve that remains open or no longer opens.
• Anti-pollution ≠ valve: for public network health protection, a dedicated disconnector is used.

Points to consider

• Selection: type (swing/spring/wafer), tare, materials (brass/stainless steel/cast iron) compatible fluid/temp.
• Clean hydraulics: sludge removal, upstream screen filters, continuous thermal insulation.
• Anti-water hammer: if necessary, combine an anti-water hammer device (vase/attenuator) on large networks.
• Maintenance: periodic inspection, cleaning of flap/seat, checking of spring and seals.
• GTB coordination: consistency with 2/3-way valves, pump sequencing, possible bypasses.

Anecdote — “The thermosiphon calmed in Rouen”

In Rouen, a boiler room was experiencing surprisingly hot return water at night, despite the pumps being switched off. The cause: the absence of check valves on nearby connections and turbulent bends. The upgrade included installing wafer spring check valves after the discharge, replacing straight sections of pipe before the check valves, and adding a screen upstream. The result: the thermosiphon was eliminated, the temperature difference (ΔT) was maintained, and the condensing boilers received cooler return water (increased efficiency). The facility manager explained: "We installed one-way valves... and the water stopped flowing back."