Thank to Dan Madagwa, MEng. for the following contribution…
The supply & use of hot water is vital to the efficient function of almost all industries. From NHS Healthcare to Care Homes to food production, hot water is critical yet often an overlooked integral part of the system design is the overall ability to accurately control hot water temperature. This capability can allow you to pre-emptively tackle or avoid issues arising from poor hygiene regimes or bacteria.
The significance of hot water temperature control itself is being given a lot more attention, as seen in a CIBSE journal article entitled “Taking the temperature – domestic hot water” which looked at the potential of reducing carbon and providing modern, safer low energy solutions simply by focusing more on temperature and revising guidance (CIBSE, 2020). The importance of being able to accurately control hot water temperature will be explored in this paper.
How is precise temperature control achieved?
Temperature control is the procedure where the change of temperature of a space or substance is measured and the movement of heat energy into or out of said space or substance is adjusted to achieve a desired set temperature. In the case of hot water heaters, this process is governed by a temperature controller. This instrument simply calculates the difference between a set-point and a measured temperature by taking an input from a sensor and giving an output signal for the connected heat element.
Venturing into system design, another way to ensure the desired temperature is provided at the outlet is by using thermostatic mixing valves (TMV’s). A TMV uses an internal thermostat that blends hot and cold water to achieve this result.
Hot Water Delivery units and systems
The two main forms of supplying hot water are the conventional storage tank water heater and the continuous flow (instantaneous) water heater. The main differences are outlined below.
Storage water heaters:
• Have a limited amount store of water.
• When store depleted takes time & fuel to reheat before use.
• Require greater plant room space.
• Materials handling issues, heavy and cumbersome.
• Standing losses.
Continuous flow heaters:
• Hot water is heated and provided solely upon demand.
• Evidence and data that they are mechanically and operationally more efficient.
• Limitless supply of hot water provided constant gas and water connections exist.
• Lighter and require less space.
• Advanced models can supply very precise temperature accurate hot water.
The table below shows a comparative analysis of running costs between a continuous flow water heater and storage tank water heater.
| Continuous flow | Storage W/H | |
| Max Heat Input (kW) | 58.3 | 79 |
| Max heat output (kW) | 56 | 65 |
| Efficiency (Gross) | 96% | 82% |
| 1st hour flow rate @ 50 C rise (l/hr) | 960 | 1411 |
| Storage/Cylinder (L) | 0 | 300 |
| Continuous flow rate (l/hr) | 960 | 1111 |
| Peak usage periods (hr) | 1 | 1 |
| Number of peak usage periods | 3 | 3 |
| Heating consumption (kW) | 58.3 | 79 |
| Storage recovery time (mins) | 0.0 | 16.2 |
| Storage recovery (kW) | 0.0 | 21.3 |
| Storage loss (kWh) | 0 | 9.29 |
| Secondary return system heat loss per hour (kW) | 10 | 10 |
| Efficiency (efficiency curve of appliance) | 95% | 82% |
| Appliance input for secondary system (kW) | 10.5 | 12.2 |
| Secondary system operating time (hr) | 21 | 21 |
| Reheat of secondary return (kWh/day) | 221.1 | 256.1 |
| Consumption per peak period (kWh) | 58.3 | 79 |
| Total consumption per day (kWh) | 396.0 | 566.4 |
| Consumption per week (kWh) | 2771.7 | 3964.7 |
| Annual consumption (kWh) | 144522.7 | 206730.1 |
| Annual CO2 carbon emissions (kgCO2e) | 26,570.5 | 38,007.3 |
As you can see from the data, the storage tank option tends to be less efficient and consume more energy. Once you factor in replacement costs, a continuous flow hot water heater is the more cost and energy efficient option as well as being better at reducing the carbon footprint.
However the most important feature with regards to temperature control and what makes continuous flow the preferred option for this application is the ability of some continuous flow water heater models to constantly provide temperature accurate hot water with a precision of up to ±1°C.
Temperature control in practise – Legionella & ACOP L8
ACOP L8 is the Health and Safety Executive Approved Code of Practise that governs the control of legionella in water systems. Legionella – full name Legionella pneumophila – is a bacteria that causes, amongst other less serious conditions, Legionnaires disease, a potentially fatal form of pneumonia. Legionella bacteria are common in natural 
water sources but the conditions are rarely conducive to people catching the disease. This usually occurs in purpose-built water systems and the risk of being exposed to legionella increases in warm, stagnant water, passed to humans via breathing in miniscule, aerosol-like droplets of water. Specific guidance for the control of L.pneumophila in water systems is provided in ACOPL8 and its associated regulations, HSG274 Part 2.
When it comes to combating Legionella, precise temperature control is one of the strongest weapons available. The bacteria begins to die in temperatures above 50°C and cannot survive above 60°C Certain conditions support the growth of bacteria, such as a temperature range of 25°C to 45°C as well as the presence of certain deposits such as lime-scale, rust, sludge and organic matter can also increase this risk. This is why it is important to control the risk of exposure and do all you can to minimise it by introducing measures that restrict the growth of bacteria and reduce exposure. Traditionally this is done via temperature control, keeping stored water at least at 60°C and ensuring distributed water is supplied with 50°C water (55°C for healthcare) at outlets within 1 minute.
Scalding
When discussing temperature control of water the risk of scalding should always be considered, especially in the health and social care sector. According to the Health and Safety Executive (HSE), high temperatures over 44°C can create a scalding risk and as such water temperatures discharged from outlets accessible to those vulnerable or where there is the potential for full body immersion must not exceed this temperature. Engineering controls can be put in place in order to ensure this. These include using the aforementioned thermostatic mixing valves which should be placed as close to the outlet as possible, temperature-restricted or temperature accurate instantaneous water heaters.
This would allow water of a safe temperature to be provided continuously and reliably at the outlet. For example there could be a scenario where a water heater was set using the controller to provide water of 43°C directly to the outlets. In this case, the use of thermostatic mixing valves would not be required though there could be legionella concerns. However regimes could be put in place to help control this.
One method that could be used in addition to normal operation is a pasteurization regime. This would consist of raising the water temperature very high to over 70°C, the point at which legionella bacteria are killed instantly, and flushing the system. This could be done when the building in question is not occupied, the water is returned to a safe temperature prior to occupancy and it could potentially allow for water to be supplied as low as 43°C during regular use, reducing consumption immensely.
Hygiene regimes
The temperature of hot water is critical for many hygiene regimes across a variety of industries such as laundry for hotels, care homes and healthcare, particularly with regards to the cleaning and disinfectant processes. Being able to reliably provide water at these temperatures not only serves to properly disinfect the items in question every time but gives the end-user peace of mind.
According to the Health & Safety Executive’s HSG (95)18 Hospital Laundry Arrangements for Used and Infected Linens, “The washing process should have a disinfection cycle in which the temperature in the load is maintained at 65°C (150°F) for not less than 10 minutes or, preferably, at 71°C (160°F) for not less than 3 minutes”. This is especially important as there are a number of harmful bacteria that can survive temperatures of over 50°C degrees such as Staphylococcus aureus, a well-known pathogen that if allowed to invade the skin deeply can cause a plethora of diseases, ranging from superficial skin infections that cause boils to more systematic issues like pneumonia and blood infections. Maintaining high temperatures ensures that such bacteria are killed before they have a chance to become a problem.
Food Processing
The need of hot, potable water is essential in any food processing operation as it is a major component of the cleaning process. To reduce the risk from hazards, comply with legislation, presenting a hygienic visual image or to ensure the variety of bacteria and residue left behind during production is removed, having clean equipment is of paramount importance.
Hot water (thermal energy) plays an integral role in the effectiveness and rate of the cleaning method. Increasing the temperature generally increases the rate of the chemical reactions involved in the cleaning and so make it more effective and take less time. In the same vein, higher temperature of water may require the use of less detergent, allowing money to be saved. The goal is often to find the perfect balance between cost, efficacy and food safety and so the ability to precisely provide water of specific temperatures is hugely beneficial and allows great control.
Conclusion
It is clear to see that implementing temperature precision and control in the specification and installation of hot water systems is vital and will only continue to grow in importance, particularly in the current pandemic and the aftermath. Being able to choose between providing water hot enough to combat Legionella and disinfect laundry or 
low enough to prevent scalding is a useful tool and if it is more widely incorporated as one of the first steps in the design process, a plethora of issues can be avoided and effectively design-engineered out of the equation.
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