Double check valve or isolating valve? Read the job sheet before ordering

Both types can appear on the same materials list, and both can be sold as 15 mm fittings. The key question is what the installer needs the fitting to do. A double check valve is used to restrict reverse flow. An isolating valve provides a deliberate shutoff for servicing.

Read the task as well as the size

If the list says isolate for maintenance, ask which shutoff is specified. If it says prevent reverse flow, confirm the protection the installer has selected. A description such as ‘15 mm valve under the sink’ leaves the job and connection open.

Use the actual pipe and system identification

Record the pipe size, material and manufacturer markings. The John Guest 15DCV example is a 15 mm Speedfit push-fit double check valve for hot and cold water. John Guest excludes central heating for this model. Ask the installer to confirm suitability for the intended water service and pipe system.

Check what will be accessible later

A service point may need space to reach a shutoff or service the arrangement. Photograph the existing pipe layout from far enough away to show both sides, and provide the installer’s fitting specification. Leave layout, orientation and selection of backflow protection to the installer.

Make the order line specific

A useful line reads ‘15 mm JG Speedfit double check valve, 15DCV, for the specified hot or cold water service’. List any separate isolating valve and pipe inserts on their own lines once the installer has confirmed them.

Model reference and selection details. John Guest 15DCV product page.