W4 Non-Return Check Valve 12mmW4 Push Fit 12mm Non Return Check Valve This is a check valve, also known as a one way valve or non return valve. These are used with most water heating systems to prevent water exiting the heaters input line. This fitting is not suitable for flexible hose. Single Check Valve ensures protection against reversal of flow. The low headloss design and fast installation time make the valve the ideal selection. The valve is designed for use with liquids