Isolation Valves and Service Valves: Types, Sizes, and Where to Fit Them in UK Plumbing

Isolation Valves and Service Valves: Types, Sizes, and Where to Fit Them in UK Plumbing

Isolation valves (also called service valves or quarter-turn valves) are fitted throughout domestic plumbing systems to allow individual appliances, fixtures, and sections of pipework to be isolated without draining the entire system. They are a mandatory requirement in many situations under Water Regulations 1999 and are considered best practice in all domestic plumbing work. This guide covers the types available, correct sizing, and where isolation is required.

Why Isolation Valves Matter

A system without isolation valves means that servicing a dripping tap, replacing a failed toilet fill valve, or changing a kitchen mixer requires isolating the entire cold water supply or draining a large section of the hot water circuit. In a multi-occupancy property or commercial premises, this causes significant disruption. Isolation valves at each appliance allow:

• Replacing a tap, valve, or fill valve without affecting other fixtures
• Isolating a leak at source without shutting down the whole building
• Routine maintenance without draining the system
• Compliance with Water Regulations Regulation 14 (fittings must be accessible for maintenance and renewal)

Types of Isolation Valve

Flat-head (Screwdriver Slot) Quarter-Turn Service Valve

The most common domestic isolation valve. A flat-head or cross-head screw slot in the valve body rotates 90° between open (slot parallel to pipe) and closed (slot perpendicular to pipe). No handle protrudes — the valve presents a low profile and can be concealed behind panels. Available in 15mm (for most domestic appliances) and 22mm (for larger supplies).

Used for: isolating individual appliances (washing machines, dishwashers, toilet cisterns, WC fill valves, under-sink water filters, refrigerator ice-makers).

Lever-Handle Quarter-Turn Ball Valve

A quarter-turn ball valve with a lever handle that moves from parallel (open) to perpendicular (closed). The open/closed position is immediately visible — the lever position shows the valve state at a glance. Larger profile than flat-head valves — not suitable for concealment behind panels without a service access point.

Used for: accessible isolation points on mains supply, boiler service valves, irrigation supplies, outside taps, and anywhere where the valve will be operated regularly and visibility of state is important.

Full-Bore vs Reduced-Bore Ball Valves

Ball valves are available in full-bore (the ball aperture matches the pipe bore — minimal flow restriction) or reduced-bore (the ball aperture is smaller than the pipe bore). For isolation valves that should not restrict flow, specify full-bore. Reduced-bore is acceptable for infrequently operated isolation points where flow rate is not critical.

Inline Isolation Valves (with compression ends)

Fitted inline in a pipe run using compression fittings — the valve body has a compression olive-and-nut connection at each end. Ideal for installing in existing pipe runs without soldering. Available in 15mm and 22mm. Most domestic screwdriver-slot service valves are this type.

Angled Service Valves

An angled (90°) isolation valve turns the supply through 90° and connects directly to the appliance inlet — common under wash basins, sinks, and WC cisterns where the pipe comes up from the floor and the appliance connection is horizontal. The angled body avoids a separate elbow and compression coupling. Available with 15mm compression inlet and ½" BSP (12mm) male outlet for direct connection to flexible tap connectors.

Where Isolation Valves Are Required

Water Regulations 1999 Schedule 2 and the WRAS Water Fittings and Materials Directory set out requirements. Key mandated and best-practice locations:

• At every tap: An isolation valve on both the hot and cold supply to every tap — most importantly on the kitchen cold supply (drinking water) and at bathroom taps. Angled service valves under basins and sinks are standard practice.
• At every WC cistern fill valve: A service valve on the cold supply to each WC. Where a WC fill valve fails (common), the service valve allows replacement without isolating the whole cold system.
• At every washing machine and dishwasher connection: A service valve on both the hot and cold (or cold-only for modern machines with a single fill) machine connection. These are specified by Water Regulations Reg. 14.
• At outside taps: An isolating valve and non-return valve on the inside supply to each outside tap — required by Water Regulations to prevent back-siphonage.
• At boiler connections: Service valves on the flow and return connections to a boiler allow the boiler to be removed without draining the full heating circuit.
• At heating zone valves and pump: Service valves either side of each zone valve and the circulating pump allow replacement without a full drain-down.
• At the cold water storage cistern outlet: A full-bore ball valve on the outlet from the cistern allows the cistern to remain full while isolating the distribution below — useful for flat-roof cisterns that are awkward to access.

Gate Valves: Still in Use, but Not Best Practice

Gate valves (with a rising spindle and wheel or T-bar handle) are still found in existing systems, typically on the cold distribution from older cisterns. They are not quarter-turn — they require multiple full turns to open or close. Gate valves cannot be repacked under pressure, are prone to the gate disc detaching from the spindle over time, and should not be used for regular isolation. When encountered in a system under repair, replace with a quarter-turn ball valve for reliability.

Sizes and Standards

Most domestic isolation valves are 15mm (½" bore, compression olive-and-nut to 15mm copper or plastic pipe). Key size guide:

• 15mm inline: All domestic taps, WC cisterns, washing machine connections, dishwasher connections, 15mm pipe runs
• 22mm inline: Kitchen main cold supply isolator, mains entry valve, incoming supply before the cold water cistern, shower supply valves, 22mm pipe runs
• 28mm: Sub-main supply isolation, large boiler flow/return connections
• ½" BSP angled: Under-basin and under-sink angled service valves (½" BSP × 15mm compression)

Specify DZR (Dezincification-resistant) brass body valves for systems in hard water areas (London and South East) or where copper pipe with high chloride content water is used. Standard brass alloys can dezincify (lose zinc selectively) over time in aggressive water, causing valve failure. DZR brass is marked with a CR symbol.

Isolation Valves at APM Plumbing & Electrical

APM stocks a full range of isolation and service valves: 15mm and 22mm inline quarter-turn flat-head service valves, angled ½" BSP service valves for under-basin and sink connections, and full-bore lever ball valves in 15mm, 22mm, and 28mm compression ends. DZR brass body options available for hard water area installations. All valves comply with Water Regulations 1999 and BS 1010.

Browse our isolation valves and service valves or visit the Acton trade counter. Trade orders placed before 2pm ship same day across London and the South East.

Products Available from APM Plumbing & Electrical

Key isolation valves and service valves available for trade collection or delivery:

Get It from APM Electricals

APM Electricals, 24 Western Avenue, Acton, London W3 7TZ. Call 020 8702 8080 or visit www.apmi.uk for same-day trade counter collection and next-day delivery across London and the UK.