Driving energy efficiency in HWS

Jonathan Jones, Product & Marketing Manager – Europe for Pentair Thermal Management (Formerly Tyco Thermal Controls) explains why self-regulating heat loss compensating cables play should be playing an integral role in reducing the energy consumption of hot water.

According to government research the built environment accounts for about 40% of the UK’s energy consumption and approximately 50% of CO2 emissions. It is therefore no surprise that our lives as mechanical and electrical engineers, product developers, manufacturers and building designers are engulfed by the need to reduce consumption and to drive the development and use of energy efficient technologies forward.

Over many years, this shift in direction (which I would liken to an oil tanker changing course) has become increasingly pronounced – and it is encouraging to see. I have never seen the building services industry so eager to embrace new technology and best practices to lower energy consumption of systems; notably in hot water distribution. 

It is no secret that by improving the fabric of the building and increasing the insulation and mechanical ventilation systems you are increasing the comfort levels. However the energy consumption associated with hot water is a more significant percentage of a building’s energy requirement. It therefore needs to be tackled in the same way as any other energy using product (or system) to drive down the carbon footprint for safe hot water.

When considering centralised hot water production in large buildings, the traditional distribution system is the recirculating one. In essence, such recirculating systems are designed to accommodate energy inefficiency. Why? Because half of them (pipes, insulation, tees, valves etc.) are installed only to take cooling water back to the source for re-heating, having not been used the first time it was heated and distributed. The industry has spotted this opportunity for improvement. Utopia would be that water did not cool in the distribution network and when the water was drawn off, it had lost none of its heat. Well, whilst better insulation certainly reduces the heat loss, we cannot reduce this to zero, so we need to compensate elsewhere.

This is where self-regulating heat loss compensating cables play an integral role in single pipe DHWS.

Reduce the heat loss

Designing out the return system allows focus on the supply pipework and the reduction of transfer losses throughout the pipe network. Of course, given the space saving associated with removal of the return pipe, there is an opportunity to increase insulation thicknesses to reduce heat loss. Then, by adding a heating cable to compensate for the remaining heat loss, hot water can reach the draw off point without temperature loss in the pipe.

With the removal of the return pipework, the amount of hot water production is also reduced.  The inefficiency of recirculating colder water back into the boiler is removed, alleviating stratification issues and allowing the boiler and hot water storage to function more efficiently.

An additional safety benefit is improved thermal disinfection capability. Both hot water production, storage and distribution temperature can be raised simultaneously to ensure disinfection by temperature across the entire hot water system.

Effective control

In other mechanical and electrical engineering disciplines, the movement towards usage-driven control and monitoring systems is clear. Early adopters are following similar best practices for single pipe DHWS solutions.

In a complex network, pipe runs, branches, and even draw-off points experience different usage patterns. A hotels distribution system for example will have hot water supplied to kitchens, guest rooms, communal WCs, and other cleaning and housekeeping facilities. Modern control and monitoring devices for single pipe DHWS allow multiple pipe circuits to be controlled independently opening the possibility of economy settings to be applied by function. An example would be the kitchen hot water supply with setback mode during the night, standard maintain mode before busy periods and the possibility to switch off the compensating heat loss cable during times of high water usage (when there is minimum opportunity for water to cool in the pipe.)

So far the energy saving comes from hot water production and supply, but we must not forget usage. It is still common for recirculating systems with 3-5 metre dead legs (unheated and un-circulated pipe runs) to cool prior to usage. In such instances the cooler water is run off until the hot water reaches the draw off point. Remember, single pipe DHWS do not have dead legs, therefore, this water wastage is prevented.

Accumulation of benefits

So what do we achieve? Well the simple answer is huge energy savings!

We reduce transmission losses massively; we improve boiler efficiency and offer the opportunity to reduce boiler size, provide better control and monitoring of our distribution network with heat loss compensation when and where it is needed. We also reduce wastage of heated and then cooling water by removal of dead legs.

But let’s be clear, saving Kilowatts is important, but reducing primary energy consumption (with consideration to fuel factor) is critical. Whilst some buildings enjoy electricity from low carbon CHP district heating schemes, we have to consider the worst case scenario; today’s grid supplied electricity and current fuel factor. Well, even with this consideration, single pipe DHWS with heating cables for heat loss compensation (supplied from grid electricity) demonstrate a primary energy saving of approximately 10-15%. This varies from system to system but a recent independent study, conducted by Dresden University in Germany, clearly highlighted a net benefit even allowing for primary energy values.

So we’re starting with a net benefit and the knowledge that fuel factors for grid supplied electricity will improve as more renewables enter the grid and we move to smart grid supply. Thus the benefit of single pipe DHWS will expand.

So going back to my earlier comment, it is so reassuring to see the willingness to embrace new ideas and new design philosophy, particularly in regard to energy reduction in hot water supply. We must not forget that today’s installations will immediately benefit from the carbon reduction of tomorrow’s grid supplied electricity in a hot water systems which are as lean on energy loss as it can be.

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