Designing Better Water Heating For Commercial Buildings

Designing a domestic hot-water (DHW) system for a commercial building is a little like planning the plumbing for a busy railway station: it is not enough to know how many people will pass through each day. You need to understand when they arrive, where they congregate, how demand peaks, what happens when part of the system is unavailable, and whether the infrastructure can cope without wasting energy during quieter periods. Get the assumptions wrong, and the consequences can range from poor user experience and high energy costs to water-hygiene risks, operational disruption, and expensive remedial work.

What’s Needed For Better Water Heating?

The first question should therefore be: what problem is the client actually trying to solve? A DHW project may be driven by plant failure, insufficient capacity, rising energy costs, decarbonisation targets, poor temperature control, Legionella concerns, or a change in building use. These are not interchangeable objectives. Replacing an old calorifier may restore operation, but it may not address excessive distribution losses, poor system control, or an outdated design. The consultant’s role is to establish whether the project is replacing an asset or correcting a wider system failure.

Demand is the foundation of the design. It is not enough to apply a generic litres-per-person allowance or size the system from the number of sanitary fittings. The critical issue is the pattern of use: peak demand, simultaneous draw-off, peak-hour consumption, shower use, catering, cleaning, occupancy, and operating hours. In a new build, these demands must be modelled from the intended building operation, with reasonable allowance for future changes. In a retrofit, actual water-meter data, BMS trends, energy records, and user feedback can provide valuable evidence. However, historic consumption should not be accepted uncritically if occupancy or building use is changing.

The next question is whether the existing DHW architecture remains appropriate. Projects often assume that a central calorifier should simply be replaced with another central calorifier. That may be the right solution, but it should not be taken for granted. Depending on the building, a combination of central generation, local boosting, point-of-use heating, or decentralised systems may reduce pipework, circulation losses, and water-hygiene risks. This is particularly relevant in retrofit projects where long or poorly performing distribution routes may be more problematic than the heat source itself.

Addressing the Needs of Energy & Hygiene

Energy and carbon performance must also be assessed at system level. A heat pump may have an attractive coefficient of performance, but that does not automatically make the overall DHW system efficient. Storage losses, secondary circulation, pipework heat loss, pumping energy, electrical boosting, and part-load operation can materially affect real-world performance. In a new build, the DHW strategy should be integrated with the wider energy and carbon approach from an early stage. In an existing building, the practical constraints may be more significant: electrical capacity, plant-room space, external locations, noise, structural loading, and the need for high-temperature boosting may all influence the preferred solution.

Water hygiene must be embedded in the design rather than treated as an operational issue to be addressed after completion. The system should support appropriate storage, circulation, return, and outlet temperatures while also providing effective scald protection. Dead legs, redundant pipework, low-use outlets, poorly balanced secondary returns, and inaccessible sections can all create risks. A new build provides the opportunity to minimise unnecessary pipework and design suitable monitoring and maintenance access from the outset. In a retrofit, the existing network should be surveyed carefully. Installing new generation plant while retaining defective or poorly arranged distribution pipework may leave the underlying problem unresolved.

Countering Constraints to Deliver Resilience and Practicality

Physical and utility constraints require equally careful scrutiny. The proposed system must fit within the available plant space and be capable of being installed, maintained, and eventually replaced. Electrical and gas capacity, water pressure, drainage, ventilation, flue routes, heat-pump air paths, acoustic requirements, structural loading, and access routes should all be verified. In a new build, these requirements can be coordinated into the architectural, structural, and services design. In a retrofit, they may become major cost and programme risks. A technically attractive solution is of little value if it cannot be delivered within the constraints of the existing building.

Resilience is another important consideration. The appropriate level of redundancy depends on the building’s use and the consequences of losing DHW. A commercial office may tolerate a short interruption, while a hotel, healthcare facility, leisure centre, or specialist building may not. The design should consider plant failure, maintenance, isolation, zoning, and temporary provision. Retrofit projects must also address how DHW services will be maintained during the replacement works, particularly in occupied buildings.

Finally, the system must be practical for the client to operate. Controls, BMS integration, monitoring, alarms, maintenance requirements, water-hygiene procedures, and staff competence should influence the design. A highly sophisticated system may perform well in theory but become inefficient or unreliable if the facilities team cannot understand or maintain it. Early engagement with the operational team is therefore valuable.

The commercial assessment should extend beyond equipment cost. The full project cost may include utility upgrades, structural works, builders’ work, temporary services, asbestos management, controls, commissioning, water treatment, access constraints, and disruption to building operations. Whole-life costs should also account for energy, maintenance, replacement, and future adaptability.

The central distinction is that new-build DHW design is largely an exercise in optimisation and integration. In contrast, retrofit is an exercise in evidence, constraints, risk, and defining the right intervention boundary. In both cases, the key question remains the same: does the proposed solution address the building’s real operational needs and improve the performance of the entire DHW system, rather than simply replacing the most visible piece of equipment?

Talk to Adveco about support for DHW application design. 

Click for demand metering in existing buildings 

Or visit our training portal to review our CIBSE-accredited CPDs and to book a presentation at your office.