Backflow 2018-04-11T16:02:10+00:00

Backflow can be a major problem if it happens to you.

A popular belief is that water from any tap is ‘safe drinking water’. This is simply because no one has told them otherwise.

Safe water

However, safe water is regulated by The Department of Environment, Transport and the Regions (DETR).

This is the professional body, responsible for overseeing the protection of safe drinking water.

The problem is, even though there are strict regulations, incidents can occur of backflow contaminating the water supply. This could be very dangerous.

Contamination problems are critical. Therefore, you should read the section on backflow to ensure you have a constant flow of protected water.

Backflow incidents

Backflow is exactly what you’d think – when water flow is reversed in pipe networks.

There are a number of well known cases of this happening. In the ‘60s wine flowed through the taps in Cincinnati, US, when a water valve was left on after the flushing of fermentation tanks.

But there have been much worse incidents. Three houses exploded when a maintenance man connected a liquid petroleum gas tank to the water mains to wash it out. When the water valve was opened, the water supply was back pressurised with gas.

In another situation, water backflow entered an air conditioning unit in Philadelphia. This created the perfect conditions to foster the growth of bacteria that causes Legionnaires’ disease.

While the examples are extreme circumstances that are very rare, backflow is a major cause for concern. In fact 100,000 cases occur in the US every day.

So prevention is a priority.
plumbing pipes

What causes backflow?

There are two factors associated with backflow:

  • Back siphonage. This is when the supply end of a pipe loses pressure quickly. It creates a vacuum which can suck fluids back through the pipes and cause contamination.
  • Back pressure. This is when the water pressure is reversed, usually from an outlet in the water mains.
contaminated water

Backflow solutions

One quick and easy solution is the reduced pressure (RPZ) valve. It immediately lowers pressure as a backup to the check valves already in place.

If the check valve fails, the RPZ kicks in causing a relief valve to open. This will drain the system, preventing any possibility of contamination in the water supply.

RPZ valves have been in operation since the ‘70s, but only recently in the UK. Before this, plumbers installed a break tank with ball valve.

An RPZ valve has to be checked and approved by a local Water Regulations Inspector before it can be fitted. However, this is the weapon of choice for most plumbers with its ease of installation compared to break tanks.

There are also regulations in place to ensure a double valve check is installed on a tap for external use, to stop the risk of backflow.

So if you plan on fixing a water hose to an outside tap, you need to either:

  • Fit a double check valve on the pipework
  • Replace the whole tap – with a double check valve fitted
  • Or have a Nobak Triton valve installed – this is a double check valve that fits directly onto the outside tap.

Levels of water contamination

Fluid category 1:

Clean supply of water suitable for domestic use. Drinking water needs to be supplied from the mains, without any contamination.

Fluid category 2:

The water has been changed slightly in taste, temperature or odour. Water that has been heated for a bath, or treated with water softener would fall under this category.


  • Hot water from hot and cold taps mixing
  • Water that has passed through a salt regeneration water softener
  • Ice making machines, not containing additives
  • Water-cooled air conditioning units

Fluid category 3:

This water is a hazard and not suitable for drinking or cooking. This could be bathwater, for instance.


  • Water in the central heating system
  • Dishwasher and washing machine water
  • Water used in garden hoses with a fertilizer spray
  • Domestic/commercial irrigation systems

Fluid category 4:

This water is a severe hazard to health, as it’s filled with toxic substances. Not suitable for any domestic purpose.


  • Water in commercial heating systems
  • Car washing and degreasing plants
  • Water with anti-freeze solutions
  • Commercial dishwashers

Fluid category 5:

This category is the most extreme hazard to health. The liquid has a concentration of pathogenic organisms or toxic substances.


  • Human or animal waste
  • Drain cleaning plants
  • Permeable pipes in areas other than domestic gardens
  • Grey water recycling systems.