BOILERS & HOT WATER
BSEE
Design considerations for a safe, efficient and reliable hot water system
consideration is not only a measure to prevent breakdowns but one for general safety. Corrosion and scale are common problems that increase the risk of the accumulation of Legionella bacteria which can cause serious illnesses. Taking these dangers into account from the start ensure consumers stay safe and equipment is reliable.
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Andrew Dabin, product manager for Hamworthy Heating advises on the steps that should be taken when designing and installing a hot water system in order to ensure water quality and safety whilst minimising the potential for inconvenient and costly breakdowns
Material and equipment choice
A first step to hot water system design is finding suitable equipment and material based on your water quality and composition to prevent problems. A water analysis can help determine this, which will define the future treatment and maintenance schedule and whether the system is at a higher risk of breakdown due to site specifics such as e.g. hard water.
Simultaneously, an assessment of system design (e.g. to avoid dead legs with insufficient flow which promote Legionella) and mode of operation (e.g. what building/usage – process plant, commercial, domestic, etc.) should be carried out to provide efficiency and safety. All steps presented in this article are a recommendation. To ensure particular chemicals and procedures are suitable for your equipment, always refer to manufacturer instructions.
Corrosion – Chemical or physical protection?
From a system point of view, good design and operating conditions with a well-planned pipe layout and flow velocities build the first step to corrosion control. Additionally, regular maintenance and monitoring are necessary. If corrosion is of concern due to high conductivity and chloride levels, chemicals are available which are safe for use in drinking water systems. Other options are powered/sacrificial anodes, or vessels made from stainless steel to protect water heaters and hot water storage tanks from corrosion. When it comes to metals used in a system, the potential difference between them is also important to consider. When more noble and less noble metals are used in the same system, the corrosion risk is higher.
Considering bacteriological water quality
uGraph showing the impact of scale on water heater efficiency
Microorganisms can gather easily on rougher materials, although smooth surfaces aren’t ‘immune’ to this problem either. When this accumulation of bacteria (biofilm) gets out of hand, biofouling develops which can be a breeding ground for dangerous pathogens such as Legionella. These are of special concern, as they can cause a range of diseases, among others Pontiac or Lochgoilhead fever and a severe form of pneumonia known as Legionnaires’
disease. While everyone can develop the disease, people over 45 years, smokers/heavy drinkers and those with impaired immune systems/sufferers from certain chronic conditions are at a higher risk. [1] Microorganisms feed on organic and inorganic substances contained in the water and can multiply rapidly. When corrosion is present, they have an even better chance to thrive. [2] For this reason, the Health and Safety Executive (HSE) recommends the use of fittings, materials and appliances approved by the Water Regulations Advisory Scheme (WRAS). [3,4] When it comes to system design, dead legs, where Legionella can grow under favourable temperatures (20-45°C), should be avoided. Following HSG274, hot water should be stored at 60°C or higher and distributed at 50°C or higher (55°C in healthcare premises). [5]
In water heaters, a built-in legionella protection cycle which automatically heats up the water to this temperature at certain intervals can provide safety. However, you must consider the outlet for the hot water at these higher temperatures. To avoid scalding incidents, thermostatic mixer valves (TMVs) need to be fitted. [6]
Hardships No economies with scale
Hard water, affecting around 60 per cent of the UK [7], contains a high amount of calcium carbonate and often magnesium bicarbonates. As calcium carbonate becomes less soluble with higher water temperature, scale is especially problematic in hot water applications. Once heated up, scale forms and deposits on equipment. This affects the system performance negatively, as heat transfer is impacted by limescale layers inside pipes, on heat exchangers etc, increasing fuel wastage (see table). 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 also clog up e. g. showerheads which can cause an accumulation of Legionella bacteria in those areas. Regular maintenance is vital to ensure an efficient and reliable system. This means the installation needs to be cleaned and scale removed. If hard water becomes an issue, a physical water softener or chemical treatment can be an option, too.
Finishing off the installation – disinfection
It is recommended to carry out a disinfection according to HSG274, once the hot water system has been installed, followed by a system flush until all chemicals are removed. To protect the aquatic
Read the latest at:
www.bsee.co.uk uA water heater full of scale. Inset: Wateranalysistable
environment, a so-called consent to discharge might be required when the system is drained. To test if the disinfection was successful, a bacteriological sample should be taken a week after it has been performed.
Prepared for the future
After the disinfection and when the system is ready to take up operation, we recommend getting it commissioned by a water quality professional. Commissioning sheets and water quality certificates are then kept for future reference. Sacrificial anodes, should they be used, also require regular check-ups. They need replacing when worn down to 60 per cent to ensure continuing protection. If any water treatment equipment is installed, this also needs to be regularly looked at. Other than visual checks, regular water samples should be taken to monitor water quality, also on a bacteriological level. Designing a safe and efficient hot water system depends on many factors. Therefore, a bespoke approach to design, equipment and material choice is necessary for each individual application. Lastly, it is a matter of finding a balance to overcome all the risks as outlined. For further information, refer to the ‘Water Quality Consideration of Domestic Hot Water Systems for Commercial Applications Guide’, developed collaboratively by members of the ICOM Energy Association, available from
icom.org.uk .
www.hamworthy-heating.com Further reading:
[1] Legionnaires’ disease, the control of legionella bacteria in water systems, Approved Code of Practice and guidance on regulations, Health & Safety Executive, L8 (Fourth edition), 2013, p.6
[2]
https://www.researchgate.net/publication/ 11162443_Biofouling_in_water_systems_Cases_causes_ and_countermeasures
[3] Health and Safety Executive, HSG274, Legionnaires’ disease Part 2: The control of legionella bacteria in hot and cold water systems, 2014, p.7
[4] WRAS Product Database:
https://www.wras.co.uk/search/products/ [5]
http://www.hse.gov.uk/pubns/priced/hsg274part2.pdf
[6]
https://assets.publishing.service.gov.uk/government/ uploads/system/uploads/attachment_data/file/504207/ BR_PDF_AD_G_2015_with_2016_amendments.pdf
[7]
https://www.cambridge-water.co.uk/household/ my-water-supply/water-quality/water-hardness
BUILDING SERVICES & ENVIRONMENTAL ENGINEER FEBRUARY 2019 11
ot water is one of the daily necessities we often take for granted unless the supply is interrupted, and it becomes a major inconvenience. Careful hot water system design
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