Double Check Valves: Backflow Prevention for Outside Taps and Appliances

Most homeowners fitting an outside tap have never heard of a double check valve. Their plumber either fits one without explaining it, or (less helpfully) leaves it out entirely. The Water Supply (Water Fittings) Regulations 1999 make a double check valve mandatory on every outside tap in England and Wales. The reason matters: without one, contaminated water from a garden hose can be drawn back into your drinking water supply under certain pressure conditions. The risk is real, the fitting is inexpensive, and fitting it correctly takes about ten minutes.

What backflow is and why it matters

Backflow is the reversal of normal water flow in a plumbing system. It happens in two ways.

Back-pressure is when pressure in the downstream system (your appliance or hose) exceeds the supply pressure. This can push water backwards from the appliance into the supply pipe.

Back-siphonage is more common. It happens when supply pressure drops suddenly (a main bursts nearby, the water company flushes a hydrant, or a pump starts on a neighbouring connection), creating a partial vacuum in the supply pipe. That vacuum draws liquid from whatever the pipe is connected to, back into the supply.

Picture a garden hose submerged in a bucket of water mixed with fertiliser, weedkiller, or dirty pond water. If back-siphonage occurs while that hose is submerged, the contaminated water is drawn up the hose, into the outside tap, and into your drinking water supply. The same principle applies to a hose left coiled in a bucket, a pressure washer draw-off connected to an outside tap, or any flexible connection that might be dipped in something other than clean water.

The public health consequence is serious. Waterborne contamination events from backflow are documented in UK water industry records. The Water Fittings Regulations exist specifically to prevent this.

The Water Fittings Regulations: what they require

The Water Supply (Water Fittings) Regulations 1999 (SI 1999/1148) govern all plumbing in England and Wales. Scotland has equivalent regulations. The regulations classify water supplies into five fluid categories based on their contamination risk.

Category 1 is wholesome water direct from the supply: safe to drink, no treatment needed.

Category 2 is water that has slightly deteriorated (through temperature change or ageing) but poses minimal health risk.

Category 3 is water that represents a slight health hazard: weak chemical or taste contamination (a washing machine with detergent, for instance).

Category 4 is water that represents a serious health hazard: strong chemicals, pesticides, or commercial washing machine water.

Category 5 is water that represents a serious health hazard: pathogens, faecal contamination, or toxic substances.

An outside tap is classified as Fluid Category 2 at minimum (because the water in the hose is no longer being maintained as drinking water) but in practice is treated as Fluid Category 3 because of the realistic risk of the hose being placed in pesticide solutions, fertiliser buckets, or other garden chemicals. Schedule 2 of the regulations (Paragraph 15.4) requires a minimum of a double check valve on any outside tap supply.

A single check valve (one disc) only provides Fluid Category 2 protection. A double check valve (two check valves in series) provides Fluid Category 3 protection. Some plumbers fit a single check; this is technically insufficient under the regulations for an outside tap.

What "double" means

A double check valve contains two independently-operating check valves in a single body, mounted in series. Each check valve is a spring-loaded disc or ball that allows water to flow in one direction only. When pressure is equal on both sides or lower on the supply side, the spring holds the disc closed, preventing backflow.

With two discs in series, both would need to fail simultaneously for contaminated water to pass through. The probability of double failure is orders of magnitude lower than single failure, which is why the double arrangement is required for Category 3 risk applications.

The valve body is usually brass, typically 15mm diameter (the same as most domestic supply pipes), and includes a test port between the two check elements. That test port allows a plumber to check whether each check valve is functioning correctly using a differential pressure gauge. Most homeowners will never use this facility, but its presence is part of the BS EN 13959 compliance requirement.

Where double check valves are required

Outside taps are the most common installation in a domestic extension, but they're not the only one.

Outside tap (mandatory). Every outside tap in England and Wales requires a double check valve on the supply side, inside the building, before the pipe passes through the external wall. This is Schedule 2, Paragraph 15.4 of the Water Fittings Regulations. It is not optional. If your plumber installs an outside tap without a double check valve, the installation does not comply with the regulations.

Washing machine. The hot and cold supplies to a washing machine require at least single check valves, but a double check valve provides better protection and costs the same as two individual single checks. Most washing machine installation kits include hoses with built-in check valves, but these are rarely to the standard of a separate in-line DCV on the supply pipework.

Dishwasher. Same principle as a washing machine. The supply hose typically has a built-in check, but a proper DCV on the supply pipe is the correct approach if you want full compliance.

Boiler fill loop (combi boiler). The filling loop connecting the mains cold supply to the sealed central heating circuit requires a double check valve arrangement. This is because heating system water (which contains inhibitor chemicals) is Fluid Category 4 (significant health hazard). The Water Fittings Regulations require a minimum of a Reduced Pressure Zone (RPZ) valve for Category 4 applications, but many installations use a double check valve. Strictly, an RPZ valve is the correct fitting for a boiler fill loop; in practice, double check valves are widely fitted and the regulations are unevenly enforced on this point. Your heating engineer will address this.

Garden irrigation systems (underground pipework). Any underground irrigation system that includes a permeable pipe or a spray head that could be buried in or in contact with soil is Category 4 or Category 5. This requires either an RPZ valve or an air gap (physical break in the connection). A double check valve is not sufficient for underground permeable irrigation. Most domestic drip irrigation from garden taps is fine with a DCV on the tap itself, but underground soaker systems connected to a permanent underground supply pipe need specialist backflow prevention advice.

Installation requirements

The double check valve must be installed:

• Inside the building, before the pipe passes through the external wall
• In an accessible location (it contains check valves that can stick and require testing or replacement)
• On the supply side of the isolation valve (some plumbers put it the wrong way around: the DCV should be between the main supply and the tap, with the isolation valve between the DCV and the tap or on the tap itself)
• Upright is preferred, but most approved DCVs can be installed in any orientation; check the manufacturer's instructions
• The flow arrow on the body must point in the direction of water flow (towards the tap)

Testing and commissioning

The test port between the two check elements allows pressure testing of each valve independently. For a domestic outside tap, most plumbers confirm the installation is working simply by:

1. Opening the supply and checking for leaks at all joints
2. Turning the outside tap on to confirm forward flow (water comes out)
3. Turning the tap off and checking there is no backflow

A more rigorous test uses a differential pressure gauge on the test port to confirm each check element holds a specific pressure differential. This level of testing is standard in commercial premises and in care settings but is rarely performed on domestic outside taps. If you want reassurance, ask your plumber to confirm the DCV is the correct type for the application and that it's WRAS-approved.

Prices

A 15mm double check valve suitable for an outside tap costs £8 – £25 depending on the brand and where you buy it.

The valve is fitted alongside a 15mm isolation valve for the outside tap circuit. The labour to fit an outside tap with a correctly installed DCV is typically one to two hours of plumbing time.

Common mistakes

Fitting a single check valve instead of a double check. A single check valve provides only Category 2 protection. An outside tap requires Category 3 protection. The cost difference between a single check and a double check is typically a few pounds. Fitting the wrong valve means the installation does not comply with the regulations.

Installing it outside the building. The double check valve must be inside the building where it is protected from frost, accessible for inspection, and cannot be bypassed by someone working outside. A DCV installed on the external face of the wall provides very little protection: it can freeze, it's difficult to access, and it's on the wrong side of the potential contamination source.

Fitting it with the flow arrow reversed. The valve body has an arrow indicating flow direction. Install it backwards and the check valves resist forward flow (water coming out of the tap is reduced to a trickle) while offering no protection against backflow (the check valves are oriented to admit water freely in the wrong direction). Some installers discover this error only when the tap flow is noticeably weak.

Not providing access. Like all serviceable fittings, the DCV must remain accessible. In a kitchen extension, the outside tap supply typically runs under the sink or behind a cupboard. The DCV should be positioned where you can reach it, test it, and replace it without dismantling the kitchen. Under the kitchen sink with the isolation valve nearby is the standard arrangement.

Omitting the isolation valve. A DCV and no isolation valve means you can't shut off the outside tap for winter without turning off the whole house. The correct sequence (from the supply) is: branch off the cold mains supply, isolation valve, double check valve, pipe through wall, outside tap. The isolation valve goes before the DCV so you can isolate the whole outside tap circuit from inside.

Where to buy

Screwfix, Toolstation, Wickes, and most plumbing merchants stock WRAS-approved double check valves. Search for "double check valve" or "DCV" with the pipe size (15mm or 22mm). For trade supply, Wolseley, City Plumbing Supplies, and Graham Plumbing all carry Pegler and Reliance Water Controls stock.

Check the packaging for the WRAS mark before buying. Budget non-WRAS valves are available from some online marketplaces at very low prices; these do not comply with the Water Fittings Regulations and should be avoided.

Alternatives

For Fluid Category 4 applications (boiler fill loops, underground irrigation), a Reduced Pressure Zone (RPZ) valve provides higher protection. An RPZ valve includes a relief port that opens to atmosphere if the downstream pressure exceeds the supply pressure, physically venting potentially contaminated water out of the valve body rather than allowing it to flow backwards into the supply. RPZ valves cost substantially more (several times the price of a DCV) and require annual testing by a certified tester. They are not needed for outside tap applications.

A simple air gap (a physical break in the connection, where the supply terminates in a container and a separate inlet is not directly connected) is the highest level of backflow protection and is always acceptable regardless of fluid category. It is also entirely impractical for an outside tap installation.

Where you'll need this

Double check valves appear at specific points in the plumbing specification:

• First fix plumbing - installing the outside tap supply with isolation valve and DCV while pipework is exposed and accessible
• Second fix plumbing - connecting washing machine and dishwasher supplies, where check valve requirements apply

The fitting is inexpensive. The Water Fittings Regulations fine for non-compliance (technically a civil matter enforced by water companies) is rarely applied to individual homeowners, but a more immediate risk is a failed building control inspection if your drainage and plumbing are being inspected. More importantly, the valve is there to protect your family's drinking water. It costs the same as a box of breakfast cereal and installs in ten minutes.