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.




Replacing a Hot Water Cylinder on a Vented System

A friend of mine has just moved into a rather splendid period property. Large and spacious, it is ideal for his rapidly expanding family. With a modern condensing boiler and a programmable zoning installation controlling the radiators throughout the house, he is confident he can keep his heating bills under control.

The previous occupants also had a large family and found an open vented hot water storage system ideal for their requirements. Where large amounts of hot water are required on-tap, storing hot water in a cylinder is a practical solution to cope with a high quantity demand.

With the property having three floors there is also a good head of pressure available from the cold water feed tank in the loft. This provides a good hot water flow and pressure rate through the cylinder and out of the taps.

However, after taking off the hot water storage cylinder insulation jacket to inspect the tank, my friend noticed that water had been leaking from the indirect heating coil flow pipe leading into the cylinder. Green and white encrustation around the external connection was quite evident along with an indication that someone had tried to stop the leak with a filler compound in the past.

Realising that the copper cylinder was quite old, my friend decided it was time to replace it, rather than try to mend the leak.

Because he has an open vented hot water system, he decided to undertake the work himself. Open vented means that when hot water is heated and expands, the surplus water volume is directed up a pipe leading to the cold water feed tank. If the expansion becomes too great within the system, surplus hot water can be discharged back into the cold water feed tank. This maintains a low and safe working pressure within the system. Any excess of water flowing back into the cold water feed tank is simply expelled from the property via the overflow pipe.

If he had inherited an unvented system, the replacement of a hot water storage cylinder would have been a specialised procedure requiring expert installation.

The original hot water storage cylinder was snugly located in an enclosed cupboard in the bathroom. Although this meant that installing a new cylinder would have to be undertaken in a rather confined space, my friend decided that this location would still be suitable. The installation would simply require the removal of the old tank and the insertion of a new one, plumbing into existing fittings.

New hot water storage cylinders are available in a variety of sizes. Smaller properties can also benefit from installing vented combination cylinders, where the cold-water feed and expansion tank is incorporated into the storage unit. This is designed to save space but can lead to problems with low hot water flow rates.

General domestic vented hot water storage cylinders come in a variety of sizes ranging from 74 litres up to 450 litres. Because the existing cylinder in my friend’s house had a 200-litre capacity, he decided to replace it, like for like, with one of the same capacity and dimensions to ensure that it would fit into the available space.

The new cylinder was coated with a highly efficient foam insulation skin and also had provision for the attachment of a backup electric immersion element in the same position as the existing cylinder. Inlet and outlet fitting locations were also chosen to be identical to try to ensure a problem free installation.

Having obtained the replacement hot water storage cylinder, my friend turned his attention to removing the old one.

Firstly, he shut down the boiler and all the other accessories, pumps, electronic diverter valves and programming installations and then, because there were no additional isolating valves, drained down the central heating system.

He then isolated the current immersion heater element electrical supply.

The existing cylinder now needed draining. This was accomplished by turning off the mains cold feed to the water feed and expansion tank and locating the drain tap, fortunately, plumbed from the rear of the storage cylinder and feeding through to the front.

Next, he disconnected the electrical contacts in the head of the electric immersion heater and then removed the external thermostat device that was secured to the wall of the cylinder.

He was then able to unscrew the cylinder attachment fittings. They were the heating coil inlet and outlet points and the hot water outlet pipe at the top of the cylinder. He was fortunate that they were easy to free and unscrew. However, he could have cut the pipes at appropriate points, ensuring that he could re-plumb when necessary. He also took the precaution of marking the pipe-work to identify which pipe corresponded to which function.

To ensure convenient re-plumbing of the cold-water inlet, in this case, located at the back of the cylinder, he cut this pipe some way above the cylinder. By doing this, he could attach fittings and a length of copper pipe to the rear of the new cylinder prior to moving it into position. This saved him having to struggle to plumb the cold feed into the base of the back of the cylinder when placed in its new location.

He enlisted my assistance to remove the old tank and carry it downstairs. We were quite surprised by the weight of the old cylinder and decided to cut the top off and examine the internal area. This was encrusted with limescale, with a thick coating covering the coils and electric heating element. The base of the cylinder had about six inches of scale and debris, almost clogging the cold water feed inlet. This indicated that the water supplying my friend’s property was particularly hard.

Attention now turned to the new replacement cylinder. To ensure a watertight fitting of a new immersion heater element, a small circular ring of foam insulation was cut from the edge of the element fitting area. The element was then screwed into position, seating perfectly into the cut area and forming a tight seal.

With fittings, and an appropriately measured length of copper pipe attached to the rear cold water inlet point, not forgetting the attachment of a new drain valve leading from the back base coupling to the front area, the new cylinder was inserted into position.

The heating coil inlet and outlet pipes and the hot water outlet pipes were re-attached. The cold-water inlet pipe extension from the back of the cylinder required coupling to the pipe from the cold-water feed and expansion tank.

To facilitate future maintenance work, gate valves were installed at suitable pipe junctions to enable the cylinder to be easily isolated from all water supplies.

The new immersion element was rewired into the mains, and a small piece of insulation foam was cut from the front of the new cylinder to allow attachment of the thermostat.

The mains water was turned back on and the entire system was refilled, not forgetting the addition of inhibitor into the central heating system. The boiler was then re-started and all contributing electrical appliances turned on.

The immersion heater was checked to ensure it was working, radiators were bled to remove airlocks and the system restored to a functioning hot water and central heating installation.

The new hot water storage cylinder and its related plumbing couplings were frequently inspected for a few weeks to ensure that no problems or leaks emerged.

With the new hot water storage cylinder installed, my friend could now relax, confident that a major cylinder failure and water leak issue was unlikely to arise.