Problems with Dead Legs

I recently bought a new washing machine. Not that there is anything particularly out of the ordinary in doing so. The working life of a modern washing machine seems to be considerably shorter than that of the robust models of the past. Whether that is due to the increasing complexity of the electronics or the designed-in time-dependent failure of its components is a matter of opinion.

The main thing that did attract my attention was the fact that the new machine had just one water inlet hose. My previous washing machine had two independent water inlet hoses, one for hot and the other for cold water supply.

It seems that now one cold supply hose is the only water inlet connection required.

This naturally left a length of redundant hot water supply pipe. Although it was fitted with an inline valve to close of the hot water flow, I decided to fit a blanking cap to the hose connection end as a secondary precaution against leaks.

Unknown to me I had just created a dead leg.

That might have been the end of the story had I not read an article about the increasing awareness of the potential problems of microbial proliferation in domestic water supplies, particularly in dead legs.

A dead leg is a section of water pipe that branches from a T-junction and is blanked-off due to it no longer being required. It can also refer to a section of water pipe that services an appliance that is infrequently used.

Apparently, such lengths of pipework can become traps for silt and organic material. This provides the perfect environment for the development of microbial agents that can pose a danger to householders.

This is particularly the case in respect of dead legs on hot water systems.

Although hot water may be flowing through the domestic system at a temperature and flow rate that prevents microbial development, a dead leg remains as a sump for collecting debris.

The water temperature in the main domestic circulation is usually at a high enough temperature to inhibit microbial growth, but in a dead leg, the water stagnates at a lower temperature.

The dead leg on a hot water system may pool water at the optimum temperature to allow scale formation. The surface provided by developing scale, the presence of nutrients from collected sludge and the warmth from the water provide the ideal environment for the development of dangerous organisms.

Organisms such as Amoebae, Ciliates, Coliforms and Algae may proliferate and disperse into the main circulation. However, Legionella and Pseudomonas bacterium can also flourish.

So how do these organisms get into the hot water supply to begin with?

Mains cold water from the provider is chlorinated to destroy most potentially harmful organisms, but contamination can still occur due to leaking supply pipes or unprofessional plumbing work.

Perhaps the greatest source of contamination is a water storage tank, particularly one that is uncovered or that has an unscreened overflow pipe.

Although regular flushing of the hot water system, either intentionally as a maintenance task or by continual domestic usage, will help to prevent microbial contamination, dead legs will remain un-flushed and prone to scale formation. The scale provides a perfect material for organisms to adhere to.

Fragments of contaminated scale can break away from formations in dead legs and become suspended in the domestic water flow.

In the case of Legionella, the bacterium can survive the flowing hot water temperature and then incubate in lengths of T-pipes supplying hot water outlets when the temperature drops in-between outlet demand.

The bacterium can also thrive in showerheads in-between usage, particularly where scale buildup in the showerhead provides niches for development.

Although microbes in contaminated hot water can be harmful if ingested, bacterium like Legionella pneumophilia can be dangerous when inhaled.

The inspiration of aerosol particles can penetrate deep into the lungs.

Aerosols are minute water droplets suspended in the air. They are created by water falling onto a hard surface; such as occurs when running a tap, a bath, or having a shower. Flushing a toilet or spraying water will also produce aerosols.

Whirlpool and Jacuzzi type bath installations are now being identified as potential sources of microbial incubation and harmful aerosol formation, particularly where regular sanitation and cleaning maintenance is neglected.

Aerosol particles in the air can remain suspended and circulate on air currents for over twenty minutes.

Although Legionella infections are not passed from person to person, they do occur in clusters. The symptoms can vary from mild flu-like conditions to life-threatening pneumonia. People with compromised immune systems or pre-existent lung conditions are the most vulnerable to acquiring Legionella infection. The mortality rate can be high among confirmed cases in susceptible people.

However, it is thought that many mild cases go undiagnosed and that the incidence of Legionella infections amongst the population is much higher than the identified and confirmed cases suggest.

On the Continent, plumbing procedures encourage the installation of loop systems rather than T installations to help to prevent microbial development problems in domestic hot water supplies.

Best practice and Water Regulations now issue guidance on dead legs and associated blind ends. It is recommended that redundant T water pipes are removed and the T replaced with a standard in-line pipe connection.

It is also worth noting that landlords of rented properties must undertake a risk analysis of the potential for water-borne infectious agents to develop in services installed in properties they let. They must also take action to make safe any potential sources of microbial contamination. Failure to do so can expose the landlord to criminal action and substantial litigation issues should subsequent related harm occur to a tenant.

I have now removed my blind end and in conjunction with a regular flushing of my entire domestic hot water system, can rest assured I am doing as much as practical to reduce the chances of my household contracting a water-borne infection.