Isolation Valves and Service Valves: Which to Use and Where

An isolation valve is one of the most-fitted items in any plumber's bag — and one of the most frequently confused. Flat-faced, angle, full bore, service valve, gate valve: the names blur together on a counter, but the differences matter when you're under a sink specifying a valve that needs to hold for 20 years. This guide covers every type in the Embrass Peerless range at APM, when to use each, and the two things that most commonly cause callbacks.

What an Isolation Valve Actually Does

An isolation valve allows a single appliance or branch of pipework to be shut off without turning off the water to the whole building. Fitted on the cold feed to a basin tap, toilet cistern, washing machine, or boiler, it means a tap washer change or appliance swap doesn't require a trip to the stopcock.

In practice, every tap, toilet, dishwasher, washing machine, and any other appliance connected to water should have an isolating valve on its supply. Water Regulations require that any appliance requiring maintenance must be isolatable without disrupting the rest of the system.

Types of Isolation Valve: Flat-Faced vs Angle vs Full Bore

Flat-Faced Isolating Valve

The most common type. Fits inline on a straight run of 15mm copper or compression-jointed pipework. A flat-faced slot on the body takes a standard flat-blade screwdriver to operate — quarter turn to close. The slot indicator shows open (inline with flow) or closed (across flow).

• Embrass Peerless Flat-Faced Isolating Valve 15mm × ½" — £1.64 — the standard under-tap isolation for 15mm copper to ½" BSP tap tail. The everyday fitted item.
• Embrass Peerless Flat-Faced Isolating Valve 15mm × 3/8" — £2.64 — for fittings with a 3/8" BSP connection (common on older lever taps and some continental fittings).
• Embrass Peerless Chrome Isolating Valve 15mm — £1.49 — inline 15mm compression-to-compression for mid-run isolation where no BSP connection is needed.
• Embrass Peerless Chrome Isolating Valve 22mm — £3.38 — 22mm version for larger supplies: washing machine inlets, dishwasher inlets, or isolating a full branch.

Angle Isolating Valve

The angle valve turns the flow 90° as well as isolating it. Used where the pipe runs along the wall or floor and the tap tail comes straight down — the angle valve connects the two directions in one fitting, saving a separate elbow. Common under kitchen sinks and bathroom vanity units with bottom-entry taps.

• Embrass Peerless Angle Isolation Valve CP 15mm — £3.68 — 15mm compression inlet, ½" BSP outlet. The angle valve that goes under the vast majority of kitchen and bathroom taps.
• Embrass Peerless Angled Service Valve 15mm × ½" — £1.37 — a simpler service valve (no screwdriver slot, uses a flat-blade to operate the internal spindle) for situations where the valve will rarely be touched. Lighter duty and cheaper — used on cistern feeds, appliance inlets.

Full Bore Isolating Valve

Standard isolating valves have a reduced bore internally — the waterway through the valve is smaller than the pipe bore. This creates a small pressure drop. For high-flow applications (main supply, shower feeds, combi boiler cold mains inlet) a full bore valve maintains flow rate through the fitting.

• Embrass Peerless Full Bore Isolating Valve 15mm — £2.75 — full bore design for applications where pressure drop matters. Use on shower supplies, high-pressure combination boiler inlets, and anywhere you're already dealing with marginal pressure.

Tee Isolating Valve

The tee iso valve is a three-way fitting: it splits the supply into two outlets, with an integrated isolating valve on one branch. Common for fitting a washing machine or dishwasher cold supply off an existing 15mm run without adding a separate tee and valve. One fitting does the job of two.

• Primaflow 15mm Tee Isolating Valve CxC — £5.50

Gate Valves: When They're Still Used

Gate valves are the full-bore isolation valves with a rising spindle and large oval handle — you've seen them on the cold feed from the storage tank in airing cupboards and at the boiler. They are no longer fitted on new installs (ball valves replaced them) but remain common in maintenance work on systems installed before 2000.

Key point: a gate valve does not give a reliable shut-off if the gate disc has corroded or the rubber seat has deteriorated. On an old system, when you need a reliable shut-off, fit a new ball valve in its place rather than depending on an old gate valve.

• Embrass Peerless 15mm Gate Valve, BS 5154, WRAS — £3.99
• Embrass Peerless 22mm Gate Valve, BS 5154, WRAS — £6.41

Double Check Valves: When You Need One

A double check valve (DCV) prevents backflow — contaminated water siphoning back into the mains supply. Under the Water Supply (Water Fittings) Regulations, a DCV is mandatory on certain connections: garden hoses, bidet showers (submerged outlet), washing machines, dishwashers, and any fitting where backflow risk is classified as Fluid Category 3.

• Embrass Peerless 15mm Double Check Valve DZR — £3.09 — DZR (dezincification resistant) brass, essential on water fittings connected to the mains. Fits inline on 15mm copper pipework.

If you're fitting a garden tap, always fit a DCV on the cold supply — it's a Water Regulations requirement. The same applies to any hose union bib tap.

The Two Most Common Isolation Valve Callbacks

1. Seized valve after years of non-use

Isolation valves that are never operated will seize. The internal ceramic or rubber disc bonds to the body. When someone eventually needs to shut the supply, the screwdriver slot shears before the valve turns. Prevention: operate every isolation valve once a year — a quarter turn open and back. On a gas service, this is standard practice; on water it's rarely done but makes a real difference.

When a valve seizes and won't turn: don't force it past the point of resistance. At that point the supply is probably not stopping and the valve body may crack. Shut down at the next isolation point up the system, cut out the seized valve, and fit new.

2. Wrong valve for the pressure

Standard flat-faced isolating valves are not rated for sustained high-pressure applications. On a direct mains-pressure system (combi boiler, unvented cylinder) always use a full bore valve rated for mains pressure. On gravity-fed systems with a cold tank in the loft, a standard valve is fine — the pressure is typically under 1 bar at the outlet.

Summary: Which Valve for Which Application

All isolation valves and service valves above are in stock for same-day collection from our Acton trade counter. Buy in quantity — if you're doing a full bathroom or kitchen fit, isolation valves are small money against the cost of a callback.

APM Plumbing & Electrical | 24 Western Avenue, Acton, London W3 7TZ | 020 8702 8080 | apmi.uk