The Pitfalls Of Decentralising Electric Water Heating With Point-Of-Use
Point of use (POU) electric water heating in public sector applications was created for single outlet demands, usually far away from the plantroom that would be difficult to supply in other ways. Typical examples of its application might be an outbuilding, very small buildings with low hot water demand, a tea point in a community centre or a small washroom at the end of a long dead end corridor.
Increasingly POU is being specified in buildings without a gas connection to decentralise the hot water system. But is this the right way to use the technology?
Centralised hot water systems have existed in public sector buildings for generations, where the pumped secondary returns ensure hot water is available at the tap quickly. But secondary pipework does lose energy, and this is seen as an argument for using POU, but is this just increasing complications and costs?
We also need to look at how centralised systems work and understand if all energy loss is a waste or a necessary function of the system, and if eradicating energy losses will it improve the system? The answer is not as straightforward as you might think.
Under BS 5422:2023, the minimum level thickness of insulation (25-40mm for 17 to 60mm external diameter pipework) is established for non-domestic hot water service areas to control heat loss. Under this regulation, which gives an average of 9W/m maximum permissible heat loss, secondary losses from centralised systems have reached acceptable levels.
Under this current condition if we were to apply the maximum permissible heat loss to a 50m run of secondary return pipework, operating for 16 hours per day the secondary losses would amount to 7.2kWh/day. So, the aim of POU is to replace the centralised system and entirely eliminate the secondary losses. As a conservative response we could propose a replacement POU application employing 10 x 15 litre over or under sink units. But you must remember that POU also have their own standing losses, which can easily be 0.85kWh/day. Times this across the specified 10 units and losses from POU are going to be 8.5kWh/day. These are averages, so there will be variance, but overall, the energy losses of both systems are going to be similar.
If energy losses are part of both centralised and POU-based systems, and are acceptable within the specification what are the other considerations?
Let’s take school buildings as an example, where the Department for Education (DfE) currently recommends a variety of approaches to water heating, with centralised systems for catering functions which represent a large, single ‘point of use’ of hot water, while “design and installation shall prioritise the use of local non-storage (or low storage) ‘point of use’ electric hot water heaters. This is to reduce standing losses from centralised systems and to prevent pipework heat loss increasing the risk of overheating.”
This approach is driving the specification of heat pumps to support the ‘centralised’ kitchen and then large numbers of POU electric water heaters. A recent example for a primary school with 350 students incorporated more than 35 POU heaters for washbasins located throughout the school. Although units are individually low-cost, at these numbers, the capital investment starts to climb, especially when factoring in the high cost of cabling in each unit. POU electric water heaters are also basic, with no connection to the Building Management System (BMS), so there is no option to highlight a fault, or enable the unit or apply time control. That’s compounded by a lack of individual miniature circuit breaker (MCB) to prevent damage or potential fire from a overload or short circuit. Crucially for water heating, which is a business critical service, there is no redundancy with a POU water heater. If it breaks, there is no hot water at the tap until it is repaired or replaced.
Regular maintenance of hot water systems is a critical part of efficient operation, with regular replacement of anodes and descaling of tanks and in particular electric immersions where high temperature encourages its formation. This is wet, dirty work, that in centralised systems is contained within the plant room. While the issues of limescale can be virtually eliminated in centralised systems such as Adveco’s award-winning FUSION package electric water heating systems, POU water heaters cannot avoid it in hard water areas will exhibit scale problems with some variance in time due to intensity of use. The problem is that building operators do not maintain point of use water heaters, so maintenance, or more likely replacement, will be necessitated as the cost of a new unit at £150 does not warrant a £100 maintenance charge.
For this reason, POU systems will be on a rolling replacement program which will then increase a building’s embodied carbon and landfill waste generation. This also brings maintenance personnel into the occupant’s space. Public sector organisations do not want technicians working in their toilets, whilst many buildings such as schools, care homes, prisons, or hospitals have safeguarding concerns with technicians in occupied spaces, who are then carrying out wet, messy works.
With the drive to eradicate energy losses not stacking up, as POU standing losses are likely to equal or even supersede secondary losses of well-designed centralised systems most public sector buildings should have a centralised hot water system with secondary return and not POU on every floor of shell and core constructed properties. The centralised system is also able to take advantage of low carbon preheat, whether in the form of heat pumps or solar thermal, that can continue to evolve the systems ability to reduce dependency on energy, reducing carbon emissions and energy costs across its lifetime. On paper POU electric water heaters appear to be a quick, easy way to distribute hot water around a building. But the reality is that over the course of the operational life of the building, POU is, unless correctly employed as a terminal fixture away from the centralised system, ultimately expensive, problematic and limited in options for future developments.
