Check valves and backflow in a UK home

Water is meant to flow one way: from the water company's main, through your pipes, and out of a tap. Backflow is when it goes the other way. It sounds unlikely, but a hose left in a bucket of weedkiller, or a washing machine full of dirty water, can be pulled or pushed back into the drinking-water pipes if the conditions are right. Check valves and air gaps are how the plumbing stops that.

This page explains the idea in plain terms. It is not a design guide, and it does not tell you which device a particular fitting needs. Your water company, and an approved plumber, decide that. The wider rules behind it are outlined in WRAS and the water fittings regulations.

Back pressure and back-siphonage

WaterRegsUK's backflow page defines backflow as fluid flowing in the opposite direction to the intended or normal flow, and names two kinds:

• Back pressure. Something downstream pushes harder than the supply. A pump, or water heated in a closed space, can do it.
• Back-siphonage. The supply pressure drops, perhaps because a main has burst or a neighbour is drawing a lot of water, and water is sucked back up the pipe. A hose with its end in a pond is the classic route.

WaterRegsUK says Schedule 2 paragraph 15 of the water fittings regulations in England, Wales and Northern Ireland, and the byelaws in Scotland, requires every plumbing system to include protection against backflow. That can be an air gap or tap gap, or a mechanical device. It says the protection should be approved by the regulator, rated for the highest contamination risk downstream, and of an appropriate quality and standard. It also says to check with the local water undertaker that a device is suitable for the use.

Single and double check valves

A check valve is a spring-loaded flap or poppet that lets water through one way and closes if it tries to come back. A single check valve has one. A double check valve has two in series, often with test points between them so the valve can be checked. A double check valve is a common choice for the lower-risk uses in a house, but it is still a mechanical part: it can stick, wear, or freeze, and it needs to be accessible.

A check valve is not the same as an isolation valve, which you turn by hand to shut a fitting off. Some fittings combine the two. A plain check valve does not stop a leak and is not a way of turning water off.

Garden hoses and outside taps

WaterRegsUK's hose union tap page treats a hose on a hose union tap as a high contamination risk, because a hose dropped in a bucket, trough, puddle, drain or pond can carry that water back. It says all hose union taps must be supplied through backflow protection rated for the risk downstream. It adds that a trigger on a hose is there to prevent waste, and is not itself backflow protection.

Two points from the same source matter if you are replacing an outside tap:

• A hose union tap with a double check valve built into it can only lawfully be used, on WaterRegsUK's account, if it was installed before 1 July 1999. It suggests a replacement should ideally be a standard hose union tap with the double check valve moved inside the property and protected from freezing.
• Its fluid category 3 hose union tap FAQ says that in premises built after 1 July 1999, a double check valve used for that purpose must be installed inside the property and, if required, protected from freezing.

The kits and taps themselves are covered in outdoor tap kits, and frost in frost-free outdoor taps. An automatic garden watering system is a step further. Regulation 5 of the Water Supply (Water Fittings) Regulations 1999, as made, lists a garden watering system, unless designed to be operated by hand, among the installations that need notice to the water undertaker and its consent before work starts. Check the current text and your water company. See garden irrigation and water butts.

Washing machines and dishwashers

On appliances, WaterRegsUK's whitegoods FAQ says some but not all appliances include backflow protection that satisfies UK requirements, and that conformity with BS EN 61770 does not by itself mean they do. If the built-in protection is not adequate, the appliance must be supplied through its own suitable backflow protection. Check the appliance manual, and ask the installer. The supply valves are covered in washing machine plumbing and dishwasher plumbing.

One double check valve Wickes lists

We haven't tested this. Figures are as the maker states them (checked 13 November 2026).

Scotland has its own byelaws and Northern Ireland its own regulations. The ideas are the same, but ask your own water supplier.

Does my outside tap need a check valve?

A hose union tap needs suitable backflow protection. WaterRegsUK says that for a tap in premises built after 1 July 1999, a double check valve used for that must be fitted inside the property and protected from freezing if required. Ask your water company or an approved plumber what applies to yours.

Is a check valve the same as an isolation valve?

No. A check valve lets water flow one way only and works by itself. An isolation valve is turned by hand to shut off a fitting. A check valve will not turn the water off for a repair.

Do washing machines have backflow protection built in?

Some do and some do not. WaterRegsUK says conformity with BS EN 61770 alone does not show adequate protection, and where the built-in protection is not adequate the machine needs its own. Check the manual and ask the installer.