Safety before efficiency

Over the last 12 months the term ‘low carbon’ has become the buzz word with heat pump technology being pushed to the forefront of government’s decarbonisation of heat strategy during the 2020s.

As an ex installer I can remember many shifts by government in policy towards encouraging renewable heat technology deployment. However, I was always left uncertain and confused as to what was actually required to become a competent low carbon heating installer. The next 12 months will be a precarious transition for the low carbon education and skills sector, with a risk of a proliferation of standards and a race to the bottom. Therefore, it is paramount that both government and industry ensure the sector is joined up whilst avoiding the development of education initiatives in silos.

Positive disruptive innovation in education is needed

The education sector is traditionally slow moving, meaning it sometimes fails to maintain pace with current technology and methods being deployed by technology innovators such as CIPHE industrial associate manufacturers, so the ongoing collaboration between the education sector, employers, installers and the technology innovators is critical moving forward.

The low carbon technology of today

There has been endless debates about what will be the heat source of the future, and the holy grail for decarbonising our heating systems as we embark on an ambitious net zero journey. However, whilst we need to encourage and accelerate innovation, we need to also get the basics right in the first place. If I could have a pound for every time I have read about the importance of heat loss, system balancing and oversizing over the last two years I would be a millionaire. Nevertheless, whilst carrying out a room-by-room heat loss assessment, and ensuring that we do not oversize equipment for efficiency is critical, we cannot ignore safety and the other critical elements in the room, such as the domestic hot water system. This article focuses on the importance of ensuring the consumer needs are addressed whilst ensuring the safety, health and welfare of the public is considered when specifying a domestic hot water system.

As we transition towards much smaller heat generator sizes with lower flow temperatures, the relationship between the manufacturer, consumer and installer has never been so important.

What are the requirements for the temperature regime for a safe hot water storage and distribution system?

Figure 1 illustrates the requirements laid out in HSG 274 part 2, water regulations guidance, ACOP L8 and the CIPHE PESDG (Plumbing Engineering Services Design Guide) which highlights the domestic hot and cold water temperature requirements. In the Health and Safety at Work etc Act 1974, section 15 and 16 states that you have to comply with equal to or better than the guidance notes, such as ACOP L8: 75 Designers, manufacturers, importers, suppliers and installers of water systems that may create a risk of exposure to legionella bacteria must:

a) Ensure, so far as is reasonably practicable, that the water system is so designed and constructed that it will be safe and without risks to health.

The reasons for following these simple rules are to reduce the risk of legionella growth in both the storage and distribution of a hot and cold water system whilst preventing the risk of scalding at the outlets. In any system during peak demand there will always be a time where the temperatures fall below the minimum temperatures, so it is important that pasteurisation of the hot water storage and pipework is carried out where required, such as once a week. There are also some simple installation tips that will also help to protect the system from legionella.

• Keep the cylinder maintained at a temperature of 60-65°C.

• Insulate the cold water pipework to prevent the transfer of heat from other services such as heating pipes.

• Ensure that hot water pipes are installed above cold water pipework.

• If there is a cold water storage cistern in the loft space, then ensure it is fully insulated to avoid the water from reaching temperatures above 20°C with suitable ventilation in the area.

• Install a pumped domestic hot water return circuit in long pipe runs and ensure the return temperature never drops below 50°C at point of return to the cylinder.

• Insulate the hot water pipework on the hot water return circuit to maintain the higher return temperatures.

• Install a destratification pump on the cylinder to ensure the whole cylinder is evenly heated.

Ensure the DHW control strategy, heat exchanger type and volume mitigates the risks of small heat generators.

Now we know about the temperatures needed to keep the water safe, how do we ensure consumer comfort?

One of the most obvious reasons for installing sufficient DHW storage volume is to ensure consumer comfort is maintained, but, an undersized cylinder will increase the frequency of temperatures falling below the safety limits at peak times. The first key step is to understand the consumer’s domestic hot water needs by potentially one of two simple measures:

1) Calculate the hot water requirements based upon occupancy and the property.

a) Number of bedrooms plus 1

b) Number of occupants

(Caution required)

Low usage = 45 litres per person

Medium usage = 55 litres per person

High usage = 65 litres per person

The designer should use the highest number out of a) and b) to avoid the risk

of the system being undersized in situations where the number of occupants are less than the number of bedrooms plus one. Below is an example demonstrating the assumption for a four-bedroom house.

a) (4 bedrooms + 1) x 45 l/p = 225 litres per day DHW usage

Safety before efficiency

The important thing to understand is that this calculated number may be a useful strategy for calculating the cylinder size for high output, high temperature heat generators; nonetheless, it is advisable to specify additional volume when sizing a cylinder for a small heat generator with lower flow temperatures. Figure 2 illustrates an example of a 9kW boiler and a 210 litre hot water cylinder installed within a 4 bedroom house using the 225 litres of daily hot water from the example above. The daily usage profile has been taken from EN 50440 and is also stated within the latest BS EN 12831:2017 part 3 standard as a means of assessment in the absence of any national annex.

Figure 2 illustrates a limitation when applying the rule of thumb method to sizing a cylinder for a low temperature heat source without understanding the system requirements and the consumer needs. The above example assumes the use of a priority hot water control system that takes priority over the heating load upon demand. It is also worth considering that when matching the heat generator size to the actual peak heat loss of the building without any response capacity, then the DHW storage capacity needs to be sized sufficiently to cope with demand during the colder winter months. Historically, heat generators have been oversized as high as twice the peak heating load of the building for this exact purpose. This has helped cope with the high usage periods demonstrated in the blue shaded area in figure 2. Nonetheless, the future low carbon heating system output will need to be matched as closely as possible to the actual building heat loss with heating temperatures lower than 55°C, so correct DHW sizing is essential. There are many different DHW technology solutions supplied today that help solve these challenges as we transition, so it is paramount that the CIPHE members consult the manufacturer first for advice before purchasing a cylinder or heat pump. Whilst using this simple method of calculating daily hot water usage can be effective, it is advisable to follow method 2 especially when faced with a property with high occupancy and multiple bathrooms.

2) Calculate the hot water requirement by an assessment of the actual consumer’s hot water behaviour.

When assessing the DHW requirements of the consumer it is advisable to fully understand their peak day DHW usage profile similar to the importance of calculating the peak heat loss of the property. It is important to understand the type of outlets, duration of use and the cold water inlet dynamic pressure and flow rate before specifying any equipment. This can appear laborious, but it will lead to a safer, happier and healthier consumer.


Using the same 210l cylinder as used above, a large family with a single bathroom may consecutively use a 12l/min shower for 30 minutes in the morning before work totalling 360 litres of water at 40°C. During this peak usage period there is also a heating requirement of 9 kW per hour for two hours after a night setback of 17°C to maintain the internal temperature of 20°C in the morning. As many consumers are increasingly searching for that powerful showering experience, situations with such peak demand illustrated within figure 3 can become a reality, albeit as the blue dotted line drops below the minimum temperature the user would stop showering. In a situation with such high demand of hot water, I would install a cylinder with a destratification pump that charges the whole cylinder to the required temperature.

Understanding some basic types of domestic hot water systems

There is a wide variety of different domestic hot water cylinders and stores available on the market today, many of which fall into one of two main DHW system types; Mixed and loading hot water systems which are defined within BS EN 12831 Part 3:2017. One of the limitations of the traditional indirect mixed hot water storage cylinder (figure 4c) is the effects of stratification and the amount of usable hot water within a given storage volume compared to a fully de-stratified and charged hot water storage system such as the one in figure 4d. Due to the mixing effect within an indirect coil system during both the charging and discharging phase, the the amount of stored water needs to be sufficient to avoid any disruption in supply.

System type A – Thermal storage with internal mains pressure coil

The volume of water within this thermal store is heated directly by the primary heat source which heats the integral DHW main pressure coil running throughout the cylinder. The pressure drop across the thermal store is minimal on the primary side, which is perfect for high flow rate systems such as a heat pump. However, care would need to be taken to understand the pressure drop across the DHW coil to ensure a dynamic pressure of no less than 1 bar (check manufacturer requirements) is achieved at the outlets such as the shower.

TIP: Ask the manufacturer for their pressure drop graph of their cylinder coil and check the pressure drop across the coil with your DHW design system flow rate, for example 12l/min. If the pressure differential across the coil is too high, this will have a huge impact on reheat times.

System type B – ‘Tank in Tank’

A tank in tank hot water cylinder contains a tank within a tank as the name suggests. The inner tank which is typically made out of stainless steel includes both a mains pressure cold water inlet and DHW outlet. The outer tank that surrounds the inner tank contains the primary heated water directly from the heat source itself. This type of cylinder can provide a quick recovery solution with the whole surface area of the inner tank acting as the heat exchanger, however, the performance of system type C with a larger finned coil surface area at the base of the cylinder will deliver greater performance in general.

System type C – Traditional indirect DHW storage cylinder

One solution for dealing with lower flow temperatures is to specify a cylinder with a larger surface area coil. This improves the effectiveness of the coil surface area at the base of the cylinder and heating up times, however, there are many different types of cylinders on the market so it is important to discuss your requirements with the manufacturer before purchasing, and highlighting the type of system you are installing such as a low temperature heat pump.

TIP: If you are purchasing an indirect type C cylinder for a heat pump installation, make sure you purchase a cylinder with a coil that has an increased surface area and larger diameter – you will get a more effective heat transfer when the coil is at the lowest point of the cylinder. This reduces the degradation effect of the coil’s ability to transfer heat. The coil at the lower part of the cylinder will have its surface area immersed within the colder water for longer, hence being able to deliver its full capacity over a greater Δt. (Figure 6 illustrates this principle.)

TIP: When buying an indirect cylinder to be used on a heat pump system with much higher flow rates, always check the manufacturer’s pressure drop across the coil to ensure your primary pump has the available head of pressure to deliver the heat required to heat up the water. If the primary circuit is undersized then the consumer will receive a system that may not meet their expectations.

System type D – DHW storage cylinder with external plate heat exchanger and loading pump.

The DHW loading storage system enables the whole volume of the DHW to be heated to an even temperature throughout the tank by the use of a bronze pump that circulates the higher temperature throughout the whole cylinder. This system offers an increased amount of usable hot water compared to an equivalent volume of the water within system C, as the primary coil within system C experiences a reduction in heat exchanger effectiveness during the charging and discharging phase. This system increases the amount of available hot water whilst improving protection against the risks of legionella by avoiding cold spots between 20 and 45°C within the cylinder.

TIP: Ask the manufacturer for their pressure drop graph of their plate heat exchanger (HEX) and check the pressure drop across the HEX at the design flow rate against the available head of pressure of the primary pump. A 9 kW heat pump could deliver 26 litres per minute.

System type E – (PCM) Phase change material thermal battery.

Thermal batteries use phase-change materials (PCM) to store the energy needed to heat water. They store the same amount of energy in a much smaller space (3-4 times smaller) than water. The energy is stored by melting the PCM through energy input from a primary heat source or an internal element. This energy is then used to heat the water flowing through the secondary coil. Because there is both a primary and a secondary coil involved, it is important to take the pressure drop across both these coils into account when designing the system. It is worth noting that from a safety perspective legionella risks are greatly reduced when using these systems, as no water is being stored. For the same reason they also don’t require a Pressure and Temperature Relief Valve with low water content coils. Understanding the pressure drop across your hot water heat exchanger as well as all of the components on your priority hot water circuit such as pipework, fittings, filters, zones etc will dramatically reduce your risks.

Most manufacturer instructions will display the pressure drop across the cylinder coil at a given flow rate, so it is paramount you understand how this might affect your system design and specification. In properties that have a high demand for hot water at peak periods, it can be common to find that the DHW priority circuit may have the highest pressure drop and hence becoming your system index circuit. Figure 5 shows two different cylinders of the same volume being applied to a 16 kW heat pump installation. Cylinder 1 is a standard unvented hot water cylinder with a 0.5m2 coil and cylinder 2 is a heat pump ready cylinder with a larger diameter and surface area coil.

Key points to remember

1 Always follow manufacturers’ instructions and if unsure talk to their technical department.

2 When comparing products, convert everything to a single common unit such as kPa or Pascals.

3 If you are designing your heat generator to be exactly the same as the design heat load, ensure you have specified a sufficient volume of water to meet the consumers needs during the colder winter months.

4 When you calculate a heat loss of a building before designing your DHW system, ensure you have selected a suitable outside temperature based upon variable risks.

5 Make sure you install a priority hot water system with a suitable DHW heat exchanger that meets the needs of the consumer and does not starve the space heating for long periods of DHW usage on the cold days.

6 When installing a high flow rate heating system with low ΔT’s, ensure that the primary heating pump has sufficient available head to deal with the pressure drops across the primary circuit and all components on the DHW circuit.

7 Fit a destratification pump to increase usable hot water and to prevent cold areas within the DHW cylinder.

8 Insulate all pipework in the system to maintain the safe water regime and to save energy.

9 Ensure your primary pipework feeding the DHW cylinder is sized sufficiently below 1.5 m/s to avoid noise and large pressure drops. High velocities running through a cylinder coil may cause noise and even detach the internal coil. Remember excessive noise in your heating system is a sign you are wasting energy.

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