Backflow in Plumbing

Clean, wholesome drinking water. We can take it for granted. Turn on the cold-water tap and it is there.

Wastewater. The grey soup from washing machines, washbasins, baths, and showers. It disappears down sinks and drains never to be seen again.

It would seem quite important that the two should never be allowed to mix and be inadvertently consumed by members of the household, or indeed other unsuspecting households connected to the supply.

Yet without precautions, clean water can become contaminated with materials that can have serious implications for human health. Toxic chemicals and dangerous micro-organisms can infiltrate domestic water supplies if suitable barriers are not in place to prevent them doing so.

The importance of maintaining an effective barrier between clean (potable) drinking water and the water using devices and appliances connected to the mains supply within the home should not be under-estimated. There are thousands of domestic water contamination incidents recorded each year, with some resulting in fatalities.

The biggest cause of problems involving domestic potable water contamination results from cross-connection issues. A contaminated source of water has the potential to be drawn into the clean water supply when it is connected to it. For example, a garden hose connected to the water supply could create a cross connection. If the flow of water through the connected hose is induced in the opposite direction this is referred to as backflow.

There are numerous cross connection unions in the typical home, from dishwashers and washing machines to combi boilers and mixer taps.

Backflow can be initiated by a number of adverse conditions such as a burst water main causing a sudden drop in pressure, a high demand for water on a supply line or frozen pipes interfering with the flow.

There are two main types of backflow. Siphonage and backpressure.

Siphonage may occur when the pressure of the mains water is not great enough to overcome the tendency for water to flow to its lowest level. For example, when siphoning a liquid from one container to another, a vacuum is created within the siphon tube by removing the air. So long as the siphon tube exit is positioned at a lower level than the level of the siphon tube entrance, the liquid will flow to its lowest level. This facilitates the flow of the liquid from one container to another.

Likewise, a garden hose with one end attached to the mains water supply and the other left submerged in a garden pond has the potential to contaminate the mains supply by the process of siphonage.

Backpressure is caused where the pressure in a system connected to the mains supply is able to overcome the mains pressure supplying it. For example, when water is heated by a combi boiler the water expands. As a result, the increased pressure has the potential to overcome the mains water pressure and cause a reversal of flow back into the mains water system. The same thing has the potential to occur with central heating fluids if the filling loop is left in place in the absence of a backflow prevention device.

The Water Supply (Water Fittings) Regulations 1999 makes the fitting of backflow prevention devices and techniques mandatory. The regulations also make it incumbent on the homeowner or a competent engineer, to install and maintain systems to comply with the regulations. They must prevent contamination and also give notice to the local council of any installation work that falls under The Buildings Regulations notification requirements.

The type of prevention device or method that must be employed can be established by referring to the list of water categories set out in the regulations. This list categorises the seriousness of contamination fluid risk on a score of one to five, with five being the most serious.

The simplest method of backflow prevention is to create an air gap. This is very effective and can be seen in operation with kitchen taps, sink taps, and toilet cisterns. The distance provided by the air gap is determined by the risk as set out in the regulations.

A tundish can also act as a backflow prevention device by virtue of the air gap it produces.

For direct connection, and in compliance with the necessary precautions determined by the Act, check valves, either single or double provide an effective barrier against potential contamination risks. These are easily fitted into the pipe-work where connections are made. A garden tap is required to be fitted with a check valve between a hose connection point and the tap bib.

Other forms of backflow prevention include ball valves in water storage tanks, where the water outlet must be above any overflow outlet, and integral protectors, such as are often found in mixer taps and non-return valves.

Reduced pressure zone valves (RPZ) may also be installed where required, but must be installed by a competent person.

Although it is not always necessary to fit check valves on mixer taps that do not have integral protectors, it is advisable.

It is also important to remember that tap outlets are a prime source of contamination and should be regularly cleaned. It is not uncommon to see contaminated material being placed in contact with the outlet or splashing back into it. Microbiological contaminants can thrive on tap outlets and contaminate further supplies.

When purchasing fittings it is important to ensure that they comply with the Water Regulations. It is not illegal to sell fittings that do not comply, but it is illegal to fit them.

The Act also stipulates that any fittings are installed in such a manner that they are easily accessible for maintenance and are adequately protected against frost.

The Water Regulations Advisory Scheme (WRAS) can provide copies of the Water Regulations (Water Fittings) Act 1999 and provide advice regarding compliance and the suitability of fittings.