Do combi boilers need check valves?

Yes — Fluid Cat 3 protection on the cold inlet is mandatory. Most combis include internal double check.

UK trade guideControls & Valves 4 min readUpdated 9 July 2026

How to Replace a Check Valve (UK)

Fit single or double check valves on WRAS-compliant Fluid Cat 2 and 3 installations. Spring vs swing.

RelianceAltecnicPegler

Fluid Cat 2 (mixed hot/cold) — single check. Cat 3 (chemicals, garden hose) — double check or RPZ. All under Water Regs 1999.

15 mm compression. Orientation arrow — flow direction printed on body. Never fit backwards.

Do combi boilers need check valves?: Yes — Fluid Cat 3 protection on the cold inlet is mandatory. Most combis include internal double check.

Shop the spares

Browse controls & valves — 2,870 live parts.