HOT WATER SYSTEMS
Water Regulations Advisory Scheme (WRAS). WRAS also gives advice on the installation of expansion vessels used in pressurised heating/hot water systems, to avoid the stagnation or low turnover of stored water. These recommendations are for a correctly-sized expansion vessel which is bottom-fed and installed upright. Connecting pipework should be kept as short as possible, and should rise continuously.
Preventing deadlegs
Similarly, the entire system design should prevent deadlegs. These are parts of the pipework with insufficient water flow where water stagnates, and where Legionella bacteria can grow under favourable temperatures (20-45˚C). Following the guidance in HSG274, the HSE’s Legionnaires’ disease: Technical guidance publication, the temperature of cold water systems should be kept below 20˚C, and hot water stored at 60˚C or higher and distributed at 50˚C or above. In the case of healthcare premises, 55˚C is the preferable temperature. In water heaters, a built-in anti- Legionella cycle can provide protection. Performed weekly, and at a set temperature (>60˚C) for an hour, this will prevent bacteria forming in the vessel. However, high temperatures necessary to protect from Legionella pose a scalding risk to users of the hot water supply – especially to the more vulnerable such as the elderly. The Health Technical Memoranda cite guidance which advises a maximum bath fill temperature of 44˚C, due to the whole body being immersed, and 41˚C for showers, to avoid scalding. To limit this risk, thermostatic mixing valves (TMVs) should be fitted to control the actual output temperature.
Material choice and corrosion Selecting the right materials also prevents corrosion, which can damage the equipment and lead to its breakdown, but also promotes the growth of dangerous pathogens (such as Legionella). From a system standpoint, good design and operating conditions, with a well-planned pipe layout and flow velocities, are an
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A sacrificial anode – by virtue of its name – ‘sacrifices’ itself for the benefit of the cylinder. The protection mechanism is based on an electrochemical process. With water serving as an electrolyte, and the cylinder’s steel shell (less reactive metal) as the cathode, a magnesium (more reactive metal) anode is installed. The latter, once submerged in water, offers a target for oxidisation in lieu of the steel shell. The electrons that are freed during this corrosion process travel through the conductive medium (water) to the cathode (steel shell), reducing it to its elementary form, thereby protecting it. In hard water, this type of protection works well, as a higher concentration of dissolved mineral ions allow for enough conductivity for the reaction to take place.
A powered alternative for soft water areas
Powerstock calorifiers installed at Walton-on-Thames Community Hopsital. Equipment such as indirect-fired water heaters (calorifiers) requires corrosion protection if not made of stainless steel.
important first step in corrosion control. Additionally, regular maintenance and monitoring are necessary.
If corrosion is of concern due to high conductivity and/or chloride levels, chemical inhibitors that are safe for drinking water systems are available. Water treatment professionals can advise on the correct product and use to prevent corrosion.
Corrosion prevention in relation to hardness
Another consideration when it comes to corrosion is the so-called potential difference between metals. When more and less noble metals are used in the same system, the latter (lower electrode potential) can corrode, as water serves as a conductive medium. While this is not a desirable characteristic for materials used in system equipment, so-called ‘sacrificial anodes’ make use of this mechanism, and can be used to protect a non-stainless steel water heater from corrosion.
In soft water, the number of dissolved mineral ions is much lower than with hard water, which means the former has lower electrical conductivity. This adversely affects the chemical reaction between sacrificial anode and cylinder shell, inhibiting protection. When checked, the anode often seems to be in excellent condition. At the same time, however, this means it might not be working at all, putting the heater at risk. To bypass this problem, a powered anode can be used. A powered anode is non-sacrificial. Instead of the anode giving up its own electrons and producing an electrolytic current – such as in the case of a sacrificial magnesium anode – an electrical supply produces a very low current in the water. This has the same protective effect on the cylinder’s steel shell, but without corroding the anode – often made of titanium – which will theoretically last the lifetime of the hot water cylinder. However, a powered anode in certain soft water areas may still not have a protective effect, since the conductivity of the water is often even too poor for this method. In these cases, a corrosion-resistant stainless steel cylinder is a suitable alternative.
Scale prevention – maintaining efficiency
70 55 39 25 15 1.5 3.0 7.0 Scale thickness (mm) Figure 1: The impact on fuel consumption of scale in a hot water system. 10.0 13.0
Hard water, affecting around 60% of the UK, contains a high amount of calcium carbonate, and often magnesium bicarbonates. Once heated up, scale forms and deposits on equipment. This affects the system performance negatively (see Fig. 1), as heat transfer is impacted due to limescale layers inside pipes, and on heat exchangers etc. Eventually, this can lead to a breakdown. However, this is not the only risk: scale can affect the flow velocity of water through pipes, and lead to the accumulation of Legionella bacteria in those areas.
As calcium carbonate becomes less soluble with higher water temperatures,
May 2019 Health Estate Journal 61
Fuel wastage (%)
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