Using less energy to keep hot water hot
Keeping hot water hot in an extensive DHW system can use a lot of energy — and water. Jonathan Jones gives an insight into the many benefits of trace heating.
Current Building Regulations require architects and building-services engineers to actively seek opportunities to reduce energy or to use renewables. Initially the main focus of these reductions concentrated on reducing energy consumption for space heating and cooling — with the inevitable result that the energy required for generating domestic hot water now represents a higher proportion of the current energy needs of a building. Any energy saving that can be realised on the hot-water distribution will therefore have a larger proportional impact on the total energy reduction of a building than most other services.
The conventional means of distributing hot water throughout a building is a flow-and-return system that constantly circulates hot water from the central hot water plant around a building, returning to the water heater for re-heating after it has cooled in the distribution pipe.
It is a classic example of pragmatic engineering design, albeit with an increasingly complex range of valves and controls aimed at achieving a hydraulic balance across the pipe network. The main design criterion of the flow-and-return solution is simply the temperature of the water at the tap outlet, with no consideration of energy efficiency. In fact the simplest synopsis of the re-circulation system is that it is actually designed to lose heat!
This is in contrast to more modern solutions — specifically electrically trace-heated single-pipe distribution systems. The trace-heating solution addresses the key need of a hot-water-distribution system that the water temperature is delivered at the desired level at every outlet — whilst also reducing the overall energy requirement for doing so. The result is a system that has as much as 50% less pipe work to deliver quality hot water to every outlet. Significantly, this is achieved reliably at all outlets — regardless of whether they are on the main distribution circuit or on lengths of pipe known as dead-legs.
Raychem’s HWAT system is specifically designed for this application and has been shown to realise energy reductions of about 60%.
The temperature of hot water is maintained by attaching self-regulating heating cable along the hot-water pipes to compensate for heat losses. Water temperatures can be maintained at any selected temperature up to that of the HWS flow temperature, typically 55 to 65°C in standard maintain mode, with setback temperatures down to 45°C and boosted temperatures to 70ºC for legionella prevention cycles.
The self-regulating heating cable continually adjusts its heat output at all points along its length in direct response to the ‘local’ varying heat loss conditions at all points throughout the pipe network.
A trace-heated system delivers hot water instantly; water is not run off and wasted waiting for the warm flow, thereby satisfying the requirement of Part G of Building Regulations in respect to waste or undue consumption of water.
While hot water is flowing, the HWAT system’s thermal output is reduced; the more the hot water taps are used the less the system for maintaining temperature needs to be activated. The system utilises an easily-programmable temperature control device that monitors boiler temperature and maximises the efficiency of the whole system by incorporating setback temperatures for times of low usage. As with recirculation systems, minimising heat losses with good thermal insulation is an essential element in lowering operating costs.
Trace heating to maintain water temperature in a single-pipe system offers particular benefits in large buildings like hospitals, schools, hotels and commercial premises where a significant proportion of the overall energy consumed is by hot-water services across many tapping points. In addition to energy efficiency the HWAT system also provides space, material and installation time/cost savings and reduces water wastage.
The energy savings can be attributed to a number of factors
• A single flow pipe removes the heat losses incurred along return pipes.
• The boiler functions more efficiently without return pipes and the continuous build up of heating layers.
• The reduced water volume in a single-pipe system can offer opportunities to use a smaller boiler.
• During times of heavy water consumption when the water from the boiler is flowing, temperature maintenance is unnecessary, and the trace-heating system can be shut down.
• Heat loss is compensated with greater efficiency.
The HWAT system fully satisfies the requirements of the Department of Health – Health Technical Memorandum 04-01: The control of Legionella hygiene, ‘safe’ hot water, cold water and drinking water systems. As the single-pipe system has half the pipe length, less water volume, less heat loss and the pipe volume is completely renewed with each tapping, the danger of bacterial problems is significantly lower. The system can also be boosted to 70ºC for legionella prevention cycles.
Trace heating is flexible, maintenance-free, space saving and has low investment costs without components like pumps, control valves, double water meters etc. The absence of secondary recirculation pipework minimises the risers, shafts and openings and frees up space for other services.
An extra storey or extension to an existing building can be connected to the HWAT system easily, quickly and economically, without needing any hydraulic compensation.
With a system heat-traced to the tap point it has been calculated for example that the water saved every month at a 200-room hotel could be sufficient to fill an average swimming pool, simply by not having to run off cold water. This would be water that you had already paid to heat in a recirculation system.
Among the key benefits of an HWAT system are reductions in energy consumption, water usage and materials. Of particular significance, though, is lower carbon emissions. With typical energy savings up to 60%, an HWAT system can demonstrate a carbon reduction compared with recirculation systems, even allowing for Standard Assessment Procedure (SAP) fuel factors.
Jonathan Jones is European product manager with Tyco Thermal Controls.
Project planning and cost comparisons
On its website www.hwat.co.uk Tyco Thermal Controls has SaveWatt calculation and design software which compares the cost effectiveness of its Raychem HWAT hot water temperature maintenance system with that of a recirculation system. Comparison encompasses:
• investment costs;
• energy consumption;
• operating costs;
• amortisation (lifetime cost)
Visitors are invited to submit project data to obtain an application-specific comparison. The program contains modules with which the optimum temperature settings can be calculated. The software also creates a bill of materials for the project.
