Decentralised continuous flow hot water heating systems – an alternative to central plant

Centralised plant rooms for heating and hot water systems are a traditional solution and have been so for many years. But we live in different times where economic and energy efficiency – and legislative compliance – are demanded by users, premises owners and managers in both public and private sectors.

An alternative solution that marries high efficiency and wide ranges of modulation with simplicity of installation and design is the gas fired continuous flow hot water heating unit. These can be manifolded into a hot water system capable of coping with virtually any size of site or application. This system is a process of delivering a continuous flow of hot water at a constant temperature, without the need for storage. Continuous flow will only ever utilise energy whenever heat is needed i.e., the turning on of a tap in a washroom or a kitchen. When the units are in operation and heating water – gas is used – when the user is finished, so is the energy usage. With continuous flow systems, the user can set the required temperature of hot water to suit the needs and demands of the site. So, vulnerable users, such as children, the elderly or less able, can be safeguarded from the risk of scalding. Also, where the site has a secondary recirculating system the temperatures can be set at a level to further minimise the possibility of the proliferation of Legionella bacteria.

Condensing continuous flow hot water solutions qualify for additional energy efficiency points, boosting building energy performance as they far exceed energy efficiencies called for in Part L Building Regulations (Conservation of fuel and power). The systems are also ‘A’ rated under ErP.

Current thinking on the design of systems is moving towards separating the hot water production from the heating system completely as it can give better efficiencies all round. Buildings need hot water all year round. However, better insulation in most buildings, and a greater awareness of energy efficiency, means there is a reduced need for heating input.

In large commercial buildings, centralised heating and hot water systems have traditionally adopted the thermal storage design. These systems are frequently over-sized at the design stage in relation to actual usage on the not too scientific basis of ‘just in case’. This ‘hedging of bets’ approach to system design often leads to excessive safety margins, which in turn adds to the expense of purchase, installation and running costs.

A storage type, conventional system of hot water that must be heated 24/7, 365 days a year to ensure hot water is always available is an anachronism in today’s energy and cost-conscious world. Even if no hot water is drawn off, a standard cylinder, just for example, can lose a maximum of 3Kw of heat a day as a standing loss.

All cylinders suffer from this to some extent and it equates, over a year, to 1095Kw of energy wasted. If you also look at the gas saving alone it equates to 101.8m₃ of gas used.

Decentralised hot water provision was recently installed at a luxury country club and spa in the leafy and very affluent Cheshire countryside. The site operators, having both the contractor and the consultant make calculations on usage and life cycle costings, was adamant that a stored hot water system was not in the frame for the new fit out as it would be “inefficient and expensive”. The company went on to specify 19 gas fired condensing continuous flow units over the whole site to fulfil its huge demand for 20,000 litres per hour of safe, temperature controlled hot water. The heavy-duty units feed executive soaker showers, poolside showers and all changing room ablutions.

Within this complex, the water heaters feed the showers directly so there is no need for re-circulation around the building. With no centralised boiler room having to be incorporated into the plans.

Elsewhere, one local authority in North Wales is currently in the process of replacing its stored hot water systems with manifolded continuous flow systems. The council is set to make considerable savings on running costs. When the water isn’t running the continuous flow, system is not using fuel, which is ideal for the schools’ shut down during the holidays, for example.

These claims and assertions have been borne out by independent scrutiny. Recent independent testing – by AECOM – of continuous flow hot water systems showed a clear advantage in terms of initial capital costs, as well as life cycle costs over 20 years, when compared with indirect stored and stored hot water systems.
The AECOM findings, after extensive deliberation, found that the brand’s continuous flow water heaters are 7.5% more economical than ‘traditional’ stored hot water systems.

Decentralised water heating saves energy and money, but it also offers flexibility. If, for example, a hot water store cylinder should burst or break down, the whole system is shut down – no more hot water until the problem is fixed. With a manifolded system one unit may be at fault but this doesn’t stop the whole system shutting down.

The problem of potential legionella proliferation is also better maintained in the design of a manifolded continuous flow system. In a continuous flow unit, water temperature for distribution is accurate to ±1˚C. The fact that there is no stratification as there is no storage ensures permanent even temperature and because of this there is no requirement to heat a volume of water just to ensure the elimination of legionella. Another plus is, if demand grows, additional plant can easily be added in a modular fashion at a later stage if required.

In addition, smart controls can be added to commercial condensing continuous flow systems to boost and govern the performance of the hot water system.