Why two is better than one
Published: 01 April, 2016
While energy efficient products can have an undeniable impact on the performance of a building’s heating system, the way that the system is designed can play an equally important role. Chris Meir of Andrews Water Heaters discusses how separating heating and hot water generation can be the key to greater savings.
With the recent Paris agreement underlining the global commitment to reducing carbon emissions and the UK Government set to publish its fifth carbon budget some time before June, the pressure on non-domestic businesses to cut carbon has never been greater.
Of course, the specification of highly efficient products can help to deliver these savings — but so too can separating heating and hot-water generation at the design stage.
Traditional systems where the boiler both supplies space heating and heats a calorifier for the production of domestic hot water can be large and unwieldy. They can waste energy and money — both when demand for hot water is low and because of heat losses from the primary circuit pipework, the boiler and the calorifier.
For the application above, the boiler’s output is normally sized to meet peak demand, meaning that in summer the domestic hot water load may only be a fraction of the total system capability. During periods of low hot-water draw off, the boiler can also experience short cycling and inefficient firing.
Even when using a modern condensing boiler, the overall efficiency will be limited, not only due to heat losses, but also because of the lack of potential for condensing during periods when the consumption of domestic hot water is low.
A much more efficient approach is to install individual heaters where the hot water is needed, with separate boiler plant solely for space heating. In fact, the Energy Performance of Buildings Directive (EPBD) compliance guide offers heat-efficiency credits for decentralising heating and hot water services.
Separating heating and hot water enables the hot-water energy load to be more appropriately matched to the water heater output, which means lower running costs and carbon emissions. In addition, installing water heaters close to the point of use will reduce the amount of energy required to pump hot water around the system and heat losses from distribution pipework.
Furthermore, water is heated from a low mains temperature of around 10°C to a 60°C supply temperature, allowing for high levels of condensing. The electrical consumption of boilers, primary pumps and secondary pumps can be reduced, and heating-system boilers can be turned off in warmer summer months.
If separate water heating is chosen, direct gas-fired storage water heaters are ideal for separate water heating. As the heat exchanger is normally located directly within the storage cylinder, heat is transferred directly into the stored water. To further improve efficiency, they are designed to heat water via energy from the combustion process and to collect latent heat from the condensing of the flue gases within the heat exchanger.
Some gas-fired storage water heaters have one burner, which have high rates of recovery, whereas some models have multiple burners, which also offer built in redundancy. In the latter, each burner can work independently if the others fail, alleviating the need for multiple units to be installed.
Products with multiple burners are equipped with two or four modulating modular burners, which use the thermodynamic stratification of water by drawing it into the heat exchangers from the coolest part of the vessel. This increase in temperature differential allows the unit to condense over a greater range — increasing real-life efficiencies. Due to the high differential, the burners remain on high fire for longer, resulting in shorter recovery periods.
As with all heating systems, it is important to size the direct-fired water heater correctly. Leading manufacturers will provide sizing calculation tools, and the CIBSE ‘Public health and plumbing engineering’ guide part G1 states that the building’s total daily hot-water usage is relevant to the assessment of the peak demand. Once the daily usage is determined then the more critical peak demand can be assessed.
Traditionally, the peak use of hot water has been based on a 2-hour storage re-heat period, and this has generally proved to be a satisfactory benchmark. A diversity factor should be taken into consideration for accurate sizing and to allow for unusual circumstances.
To increase efficiency and reduce the risk of legionella, direct-fired storage water heaters should be installed close to the point of use. Consideration should, of course, be given to flue location and the provision of gas pipework, space requirements for maintenance and whether the cold-water feed is to be provided by a tank or an unvented system.
With an increased onus on non-domestic buildings to cut carbon emissions, designing a system with separate space heating and hot water, with the latter powered by direct-fired gas water heaters, is a helpful solution for those who want to save carbon and costs.
Chris Meir is sales director with Andrews Water Heaters.
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