Backflow? What Is It And How Do We Sort It Out?
Backflow is the term used in plumbing to describe when water is sent back into the plumbing system. It essentially flows backwards mixing in with the clean water which we use. This can be quite dangerous as undesirable water such as sewage, and these fluids can go back into the pipes and cause drinking water to become contaminated and unusable. This we don’t want!

Backflow is particularly important to address when installing commercial washroom cubicles in business premises and school establishments where the volume of sewage fluid discharge is high.

What’s The Cause Of Backflow?

There are two types of back flow; Back Pressure and Back Siphonage.

Back Pressure
This is when the pressure is higher than the water supply and as a result, causes the water to flow in the opposite direction. This could occur, for example, in an unvented heating system where pressure increases due to the expansion of the water as it heats up.

Back Siphonage
This can occur when the pressure in the supply becomes lower than the pressure in the plumbing system or external source, causing the same result as back pressure and causes the water to flow in its reverse direction. Back siphonage can also occur if the water supply is interrupted such as a break in the main supply or a drainage in water.

Back flow Prevention Devices
There are a number of backflow prevention devices that can prevent backflow occurring . There are rules governing what device may be used according to various criteria, one of which is the potential risk from the water.

Fluid risk categories, as they are called, are set out in The Water Supply Regulations 1999.

This is a basic overview:

Fluid Category 1
Wholesome water supplied by the Water Company (No risk. Potable water)

Fluid Category 2
Same as 1 but where the temperature, taste, smell or appearance has been impaired for example mixed hot and cold water from a tap (Aesthetic quality affected, e.g. water which may have been heated)

Fluid Category 3
Slight health hazard for example water from baths, basins, heating circuits or washing machines

Fluid Category 4
Significant health hazard for example due to concentrations of chemicals and toxic substances (e.g. pesticides)

Fluid Category 5
Serious health hazard for example from toilets and sinks (e.g. human waste)

Air Gaps
A simple form of backflow prevention device is an air gap between the water supply outlet and the maximum potential water level in an appliance. This is readily seen with taps on sinks and wash hand basins. The gap means that contaminated water cannot be drawn back into the supply.

For a more comprehensive review of fluid category risk arrangements you can review the WRAS category profile.


If an air gap cannot be achieved, it may be possible to use a single or double check valve instead. This will depend on the risk level.

Automatic check valves are required to prevent back pressure. Regulations for these check valves specify the design capabilities of the valve used, according to the hazard.

Category 2 contamination may be prevented by a single check valve, but category 3 requires a double check valve (these are manufactured as a convenient single unit, or even integrated into tap (faucet) fittings).

Category 5 requires an air gap, not merely a valve.

A recent introduction to the UK has been the Reduced pressure Zone (RPZ) valve, a form of double check valve where the intervening zone is drained and normally kept empty.

The fluid risk here is level 5 because of the potential contamination. Sinks may be used for washing blood from meat or soil from vegetables and these represent a significant health hazard. For a level 5 risk, an air gap device must be used.

In toilet cisfluidterns, an air gap must also be used. This is achieved by mounting the outlet from the float valve above the overflow outlet. Rigid silencing tubes which feed the water to a point below the water level may no longer be used. However, collapsible silencing tubes may be used as these will not allow water to be siphoned back up.

Another, less obvious, example of an air gap being used might be with a shower hose on a bath or basin. This should be fixed in such a way that the shower head cannot be placed below the water level in the bath or basin. As this is a fluid risk level 3, we can use a double check valve instead.

There are strict rules governing the gap in various given situations. The air gap between tap outlet and the overflow point in a sink for example will need to be greater than that for a wash hand basin due to the different poten
tial risk. Water in basins, baths and showers may pose a Level 3 risk whereas the water in a sink will be classed as a level 5 risk.

Hope this information was helpful. The Diligent Team.

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