Module 74: Direct-fired storage water heaters for hot water generation

This CPD article considers the application of direct-fired storage water heaters for supplying domestic, sanitary, potable hot water

The performance of building envelopes improves incrementally as the demands for low energy – and low carbon – building operation become more stringently defined in national and international codes, standards and guidelines. As the absolute carbon impact of a building’s fabric is reduced, the proportion of total energy consumed in a building by the generation of domestic (potable) hot water (DHW) becomes ever greater. It is estimated that providing DHW consumes about 3% of the total EU gross energy consumption (which, to provide context, is roughly the annual energy consumption of Sweden)1 and approximately 24% of global residential – and 12% of global commercial – buildings’ energy consumption.2

The energy consumed in generating hot water is tied to the simple relationship that: heat energy to water = m x Cp x Δ θ, where m = mass water (kg), Cp = water specific heat capacity (kJ·kg1·K1), and Δ θ = water temperature increase (K). However, the gross amount of energy required to deliver that heat to the water will depend on the system configuration, as well as operational conditions and maintenance procedures.

In the UK, the ‘tradition’ has been to install a boiler that indirectly supplies heat through a separate, indirectly heated ‘calorifier’ (a domestic hot water storage vessel with internal heat exchanger) for the production of DHW, as shown in Figure 1. Irrespective of the boiler type that supplies the primary heating, this system will experience losses from the boiler, the primary circuit pipework (the pipework that carries water that passes through the boiler) and the storage calorifier. Potentially, when there is little demand for DHW – and no demand on the heating circuits – the boiler can experience short cycling periods that will increase the proportion of ‘standing’ losses, reduce firing efficiencies and increase nitrogen oxide (NOx) and carbon monoxide (CO) emissions.3

More recently, installed indirect systems would have a condensing boiler as the heat source. This relies on return primary water temperatures below 55°C to ensure condensing (and, so, high-efficiency operation), but during periods of low DHW consumption – and particularly when there is no space-heating load – this may not be possible. A direct gas-fired storage water heater can replace the calorifier from the system in Figure 1 and remove the need for that part of the primary pipe loop.

Figure 1: A simplified sketch of the arrangement that has traditionally been applied for providing both heating and domestic hot water

The principle of direct-fired storage water heaters

High-efficiency, gas-fired storage water heaters are designed to heat water by using energy directly from the combustion process, and from latent heat collected by condensing water vapour in the combustion gases. The heat exchanger is located directly within the storage cylinder and, therefore, heat is transferred directly to the stored potable domestic water.

There are various high-efficiency, condensing, direct-fired storage water heaters available. For example, single-burner models are commonly available that deliver a high rate of heat exchange to the water – this is known as the ‘recovery’ rate, since it relates to how swiftly the hot water is replaced, or ‘recovered’, after a period of hot-water use. Single-burner models have a long internal flue, with heat baffles within the flue that provide an extended surface area for the water vapour in the flue gases to condense, as shown in Figure 2.

Figure 2: A representation of the heat exchange path in a single-burner, direct-fi red condensing water heater

Hot-water heaters with multiple burners are typically equipped with two or four modulating modular burners – as shown in Figure 3 – with external stainless-steel heat exchangers located outside of the storage cylinder. This storage cylinder would usually have a capacity of 200 to 300 litres. Because of thermal stratifi cation, the lowest-temperature water is drawn into the heat exchangers from the base of the vessel, allowing the unit to act in condensing mode for longer periods. This low-temperature feed water ensures the burners remain at full output for longer periods, producing short recovery times. The multiple heat exchangers can work independently, providing built-in system redundancy and greater opportunity for modulating control.

Figure 3: A representation of heat exchange in a multipleburner, direct-fired condensing water heater

Sizing direct-fired storage water heaters

It is important to size the direct-fired water heater properly, to suit the specific application within the building. Most manufacturers have sizing calculation tools – such as that shown in Figure 4 – readily available to aid selection . These should be based on hot-water requirements as enumerated by CIBSE Guide G Public health and plumbing engineering.4 It is important to consider the profile of hot-water use across a whole day and, in many cases, different daily profiles, depending on the assumed building use. Once the daily usage is determined, the more critical peak demand can be assessed.

Traditionally, hot water peak storage has been based on a two-hour recovery period. When calculating hot water storage volumes, an availability factor should be applied, assuming stratification of 80% unless otherwise stated or known. This implies that 80% of the storage capacity will provide usable hot water.4 It is important to consider extraordinary circumstances. For example, a health club offering an open day that encourages more people than usual to turn up and use the showers could – if not properly considered – result in the building running out of hot water. Sizing packages would normally allow such diversity to be included.

Figure 4: Example of manufacturer’s sizing tool (Source: Andrews Water Heaters)

Typically, water heaters will be selected to provide the building’s peak demand. For example, a particular four-burner, 300-litre storage, high-efficiency, gas-fired storage water heater can readily supply hot water (at a 50K temperature rise) at a rate of up to 1,920 litres per hour. So, considering a typical CIBSE Guide G recommendation of a two-hour storage recovery period, such a direct-fired, gas-fired water heater is able to produce 3,840 litres of hot water over the two hours. Many installations specify multiple water heaters to be installed to allow for maintenance or breakdown. Those models that have multiple burners may effectively have built-in redundancy that is controlled by the unit’s software, allowing water to be provided by remaining burners when one fails.

Take, for example, an annexe to a business hotel, with 45-person occupancy, equipped with showers and wash-hand basins in each room. Assume each person has an hourly hot-water allowance of 25 litres for a shower and 2.5 litres for hand washing (reasonably generous hot-water allowances, based on the mixed water allowances in Table 2.12 from CIBSE Guide G).

Installation of direct-fired storage water heaters

Direct-fired storage water heaters can be installed in many different locations within commercial premises. Manufacturers supply many different models that offer the designer of the hot-water services flexibility of installation. Models that are equipped with room-sealed flues and that have long flue runs can be installed within smaller, dedicated plantrooms – often close to the point of use. This lowers the amount of energy required to supply domestic hot water over a long pipe run, and reduces heat losses from distribution pipework. This also helps lessen the length of any dead legs in the system, and more readily allows for compliance with the Health and Safety Executive’s Legionnaires’ disease – Technical guidance5, which suggests the distribution pipework should be designed to enable the water to reach all outlets at 50°C within one minute of opening an outlet. The direct-fired storage water heater has been shown to have a low incidence of colonisation by legionella bacteria.5

When considering the installation of gasfired storage water heaters, careful thought should be given to: •Flue location
•Provision of gas pipework
•Space requirements for maintenance
•Type of cold water supply – tank-fed system orunvented system (direct from a mains supply).

Water treatment

Water treatment for the cold water supply is an important consideration, especially in areas of hard water. Water quality is variable according to geographical location – for example, in England, water is typically harder towards the south and east, and softer towards the north and west. But there are local variations, depending on the physical source of the water. Hard water contains dissolved minerals – mainly calcium, magnesium and associated anions bicarbonate, sulphate and chloride. When hard water is heated, bicarbonate decomposes and calcium carbonate is deposited into the waterheater tank and associated pipework. Mineral deposits can dramatically reduce the efficiency of appliances and, eventually, cause the unit to fail. For example, a water-heater tank with a limescale coating of one-millimetre thick can equate to approximately 7% loss in efficiency.6 Water treatment should, of course, be properly considered with all types of hot water services, including continuous-flow water heaters and water heated indirectly using calorifiers.

Required storage water heater efficiencies

Legislative requirements will often set minimum performance requirements for DHW systems. For example, the England Non- Domestic Building Services Compliance Guide 20137 requires a minimal thermal efficiency of 90% for natural gas-fired storage water heaters larger than 30kW, and 73% for smaller units (LPG system efficiencies are 92% and 74%, respectively).

In England and Wales, when assessing the expected performance for compliance purposes, an additional 2% heat efficiency credit is given for decentralisation of the heating and hot water services (not for new buildings); 1% for the use of direct-fired water heaters with integral combustion circuit shut-off devices; 0.5% for fully automatic ignition controls; and 2% for the use of manufacturers’ water-heater sizing guides and/or technical helplines.

To test compliance with the Energy Performance of Buildings Directive (EPBD) by employing the SBEM National Calculation Method (NCM) software, these credits are used in conjunction with the manufacturer’s heat generator seasonal-efficiency figure – or, in the case of direct-fired water heaters, the manufacturer’s thermal-efficiency figures.

For example, if a direct-fired, multiple-burner, gas-fired condensing water heater is equipped with fully automatic ignition controls (installed as standard) with a gross thermal efficiency of 98%, selected by the use of the manufacturer’s selection tool: Water heater thermal efficiency 98% Fully automatic ignition heating efficiency credit 0.5% Sized in accordance with manufacturer’s selection-tool heating efficiency credit 2% Total to be entered into NCM software 100.5%

Direct-fired storage water heaters and low carbon technologies

Renewable or low carbon technologies can be readily integrated into a scheme using direct fired
storage water heaters to provide pre-heat feed water, as shown in Figure 5. The additional pre-heat cylinder will be sized to maximise the use of the low carbon heat source economically. This cylinder should be located as close as practicably possible to the gas-fired storage water heater to reduce losses in transmission.

Figure 5: Integration of domestic hot water pre-heating from a low carbon energy source

As an example of the benefit of application, a well-designed and applied solar thermal system in the UK may well be able to satisfy around 30% to 40% of the annual hot water load – this is known as the ‘solar fraction’. It is important that the solar cylinder is sized correctly to maximise the amount of pre-heated water, meeting the needs of the water system and so reducing the building energy usage and carbon emissions. It is equally important to size the solar collector array to match the size of the solar cylinder, mitigating the risk of stagnation at times of low hot-water demand.

© Tim Dwyer and Jonathan Tedstone, 2015.


  1. Commission Delegated Regulation (EU) No 812/2013, 18 February 2013 – energy labelling of water heaters, hot water storage tanks and packages of water heater and solar devices.
  2. Ürge-Vorsatza D., et al Heating and cooling energy trends and drivers in buildings, Renewable and Sustainable Energy Reviews Volume 41, January 2015, pp85–98.
  3. CIBSE AM14 – Non-domestic hot water heating systems CIBSE, 2010, section 5, p3.
  4. CIBSE Guide G – Public health and plumbing engineering CIBSE, 2014.
  5. HSG274: Legionnaires’ disease – Technical guidance, UK Health and Safety Executive, 2014
  6. Andrews Water Heaters, internal research document
  7. Non-domestic Building Services Compliance Guide (England) NBS, 2013.