In plumbing systems it is important to prevent contamination of the water for obvious reasons. Dirty water getting back into the system poses a health risk and steps must be taken to prevent this happening. Backflow, as it is called, is when water which could be contaminated is drawn back into the plumbing system.
Causes of backflow
There are basically two causes of backflow.
Back pressure is where the water may be caused 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 can occur when the pressure in the supply becomes lower than the pressure in the plumbing system or external source
A simple example of backflow caused by siphonage:
Imagine a paddling pool in the garden being filled by a garden hose from a tap at the back of the house. The chances are that the tap on the wall is higher than the pool. When the pool is full you switch off the hose and the water stops flowing along the hose pipe to the pool. Now imagine that the garden slopes and the pool is much higher than the tap. And when you switch off the hose you may leave it in the pool in case it needs topping up later. Unless there is a device to stop it happening, the water could start to flow the other way because the pool is higher than the supply point and gravity will be pushing it downwards.
The same siphonage effect could happen with a toilet cistern, a sink or many other appliances unless some preventative measure is taken
Backflow Prevention Devices
In order to prevent this backflow due to backpressure or siphonage happening there are a number of devices which can be used. 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. See here: http://www.legislation.gov.uk/uksi/1999/1148/schedule/1/made
This is a simplified overview:
Fluid Category 1
Wholesome water supplied by the Water Company
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
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
Fluid Category 5
Serious health hazard for example from toilets and sinks
Air Gaps To Prevent Backflow
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.
Check Valves to Prevent Backflow
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.
One of the most common DIY tasks where you would have to consider all these requirements is when fitting an outside tap. This would be deemed to be a level 3 risk and meeting the regulations to prevent backflow from an outside tap can be achieved by fitting a double check valve before the hose union bib tap.
Another example might be the supply to a water softener. The potential risk here is deemed to be level 2 and therefore a single check valve may be used.
However, check valves cannot be used for all situations. If the fluid risk is greater than level 3, check valves are not acceptable. Taking this into account, you will see therefore that using a check valve to prevent backflow at a kitchen sink is not suitable. 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 cisterns, 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 potential risks. 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.