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www.heatingandventilating.net Electric water heating: beyond the immersion
The increasing urgency to decarbonise commercial buildings has placed centralised electric water heating systems at the forefront of modern building services. While the simplest approach—direct immersion heating—off ers a straightforward path to electrifi cation, it is fraught with critical limitations concerning effi ciency, reliability, design complexity, and maintenance. Successfully implementing a high-performance electric hot water design requires moving beyond basic direct heating towards indirect systems utilising advanced components like electric boilers.
C
entralised electric water heating systems often begin with the concept of using immersion heaters within a large storage
tank. These systems typically fall into two categories: prewired electric water heaters or bespoke buff er-with-immersion designs. Pre-wired electric water heaters off er the
advantage of having multiple heating elements prewired across three phases, providing an inherent degree of redundancy. This is crucial for commercial reliability, as a single element failure does not necessarily cripple the entire system. However, these prewired packages face signifi cant hurdles. Generally expensive for the technology off ered, tank size is also limited, typically to 300 litres due to EcoDesign regulations, thus restricting their use in large-scale commercial applications. While prewired for basic control, they often have limited compatibility with sophisticated Building Management Systems (BMS), hindering overall building effi ciency management. Given their limited availability of pre-wired water
heaters in the UK, and that they fundamentally suff er from severe scale problems in hard water areas, designers often prefer bespoke systems due to the versatility they off er in selecting components - a standard cylinder coupled with one or multiple immersions. But this versatility is often undermined by the complexity and cost of the required controls and switchgear, something frequently overlooked in the tendering process. Creating a sophisticated bespoke system requires the expertise of a controls designer, a panel builder, and an installer, leading to signifi cantly high costs. Crucially, the responsibility
for this work often falls into a contractual grey area. The system is usually tendered by the mechanical
contractor, who may not fully appreciate the necessary electrical control work. The specifi cation for the water heating is often missing from the electrical section of the tender, causing the electrical contractor to miss it entirely. This ambiguity leads to project delays and costly variations. Furthermore, the resultant one-off design is often
overly basic, lacking essential modern features such as soft start/stop, modulation, and BMS compatibility. Unfortunately, getting this complex electrical/mechanical integration wrong without ample experience is all too easy. For high-demand domestic hot water (DHW) demanded by commercial buildings, designers frequently will default to large immersion heaters. This is not the optimal technical response due to high costs, high electrical load, and a lack of redundancy. Also, a severe operational problem arises from the combination of large immersion heaters and internally mounted thermostats. When the sensing point of the thermostat is in close vicinity to the element where emitted heat rapidly increases the temperature, the internal stat will turn the immersion off before the bulk cylinder temperature has been raised signifi cantly. The heat then rapidly dissipates into the surrounding water, causing the stat to immediately turn the immersion back on. This rapid cycling leads to two catastrophic failures. Localised overheating occurs due to the stacking, while the constant cycling of the contactor can eventually cause it to weld shut, creating a dangerous safety issue. A key design rule is that the contactor should never be used as the overheat safety shut-off method. The recommended approach is to use a separate packaged thermostat located approximately 300mm above the immersion heater. A far better direct option is to
employ multiple small immersions with multiple stats, as this confi guration provides inherent redundancy and actively prevents large load spikes, mitigating issues like voltage instability, damage to sensitive electronic equipment, and lighting fl icker. However, this approach necessitates the use of special cylinders with many ports, once again leading to increased system cost and complexity.
28 February 2026 The case for indirect heating
All direct electric water heating systems, whether prewired or bespoke, share the same fundamental drawbacks. They are basic, expensive for the level of technology, and highly susceptible to scale formation. The best technical answer to the scale problem is to fi nd an alternative, indirect way to heat the water. Indirect hot water cylinders are well understood and long proven to not build up scale at the same rate as immersion heaters. In these systems, scale formation over a long period does not cause the failure of the heat source - a heat exchanger coil. Furthermore, indirect cylinders are not constrained by the 300L EcoDesign limit, provided the standing loss limits are met, enabling the design of large commercial systems. The optimal approach is to deploy an electric boiler which utilises immersion heaters within a vessel acting as a heat exchanger, creating a primary loop. When paired with an indirect cylinder, this approach off ers signifi cant advantages, as the primary loop separates the immersion heaters from the mains water. This virtually eliminates all scale buildup on the heating elements. The electric boiler also comes with built-in controls, including all necessary switchgear, BMS communication (enable, fault relay), soft start/soft stop, and modulation. This confi guration creates an electric hot water
system providing a high level of performance and reliability. It is easy to install, use, and maintain, often resulting in low initial outlay costs when compared to complex bespoke direct systems and helps eliminate the complex and error-prone bespoke electrical design process. While an electric boiler and cylinder system
consumes nominally the same energy as a direct electric system, the benefi ts of integrated control, reliability, and maintenance outweigh the marginal energy diff erence. This combination represents a limited space consumption response that provides a robust, reliable, and high-performance electric hot water core. The fi nal step in creating a truly sustainable and
low-cost-to-operate system is to add a low-carbon preheat source, such as an air source heat pump (ASHP) or solar thermal system. This integration utilises the electric boiler as the high-temperature booster and backup, leveraging the low-carbon source for as much as 70% of the energy input, maximising carbon savings and minimising operational costs.
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