Looking to the future of domestic hot water

Published:  12 May, 2008

The Park Central apartment development in Birmingham uses central plant to provide domestic hot water and heating via an Elson Coral CB unit in each flat to provide individual control.

Mike McDonald explains how the flexibility and efficiency of central plant in developments such as apartments can be combined with individual control in each apartment.

Providing hot water in large-scale projects, apartment buildings, mixed-use developments and specialist housing projects such as care villages, presents some new challenges in the current era of reducing carbon footprint.

First among them is that the primary source of heat has to be used as efficiently as possible and is likely to be, now or in the future, based on renewable energy. That implies central plant, which raises a number of questions.

• Where central condensing boilers are used, how can these be run at optimum efficiency (using weather compensation controls) and give the tenants control over their heating?

• Where combined heat and power (CHP) is used, how can each apartment be connected and also give individual control?

• Where low-temperature renewables are used (such as heat pumps or solar) how can the domestic hot water be brought to useable temperatures?

Elson, which has been in the business of designing water heating systems for over 90 years, has focused its business on these new challenges.

Central boiler plant

SAP ratings and gas safety requirements are increasingly driving developers and designers towards central boiler plant rather than individual boilers in each apartment.

In such projects the Coral CB Si can act as the interface between the central boiler plant and each apartment. A storage vessel contains water that is heated by the central plant and used to heat incoming cold mains water (see diagram). The Coral provides the hot water supply and can be designed to be a prefabricated airing cupboard, even including space for a washing machine with the water and waste connections ready plumbed.

A central heat source means that renewable energy inputs, especially solar, become viable as there is no need to feed the solar input into each apartment. This approach is already in use by some of the largest UK developers, and others are at the planning stage.

CHP

Large-scale building projects with shops, flats and offices within the same complex offer opportunities for the major investment required in CHP. Since the power side of the plant is designed for continuous running, control is required between the heating mains and each apartment.

Here Elson can offer the APV Compakva. Within each apartment the constant pressure and temperature heating mains interface with the apartment using a plate heat exchanger. These compact wall-mounted units provide individual control of heating and hot water in the same way as if a boiler were installed in the apartment. The coupled heat exchanger delivers both heating and hot water, reducing piping complexity and space. The plates of the exchanger are bolted together and can be cleaned easily.

Rapid response to demand for hot water is provided by two thermostatic valves controlling heating and hot water at a consistent temperature. The hot water temperature is held at ‘idle’ when not in use, so running temperature is reached in seconds when a demand is made. A pump with a built-in frequency converter reduces noise within the unit and cuts energy use by 40%.

Renewables

Another interesting new development is the use of renewables, usually heat pumps, to serve schemes of groups of buildings — mini district heating in a way.

The requirement is to provide domestic hot water at usable temperatures and ensure that conditions for legionella do not arise — whilst taking maximum advantage of the green energy from the heat pump.

EXTRA PICTURE

The thermal store in a Coral Si unit is heated by the central plant and heats incoming cold mains water to deliver domestic hot water.

One scheme we have worked on has ground-source heat pumps providing water at around 45º C. That temperature is fine for an underfloor heating system, but DHW requires 60ºC. For such schemes the Coral Eco has been developed from the well established Coral E electric water heating system.

The Coral E tank comprises a highly insulated copper thermal storage vessel, heated by immersion heaters using cheaper off-peak tariffs. This thermal store is at atmospheric pressure and is topped up by its on-board feed-and-expansion tank. The coil within the thermal store is connected to the cold mains. As it passes through the store, cold mains water picks up heat giving mains pressure hot water. Its temperature is controlled by a thermostatic blending fail-safe valve. There are three sizes of thermal store. Installation is simple, and specialist certification to meet Building Regulations is not necessary.

The Coral E becomes an Eco unit when a plate heat exchanger and controls are added. The plate heat exchanger preheats the incoming cold mains water using heat-pump energy. On a recent scheme the overall power saving, including the power used by the heat pumps, is typically 53% — representing a reduction in carbon emissions of around 200 kg a year for each dwelling.

For more information on this story, click here: May, 08 105

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