CPD PROGRAMME
need to be sized to cope instantaneously with peak demand, whereas storage systems can spread the fuel demand over a longer period. A condensing continuous flow hot water
heater additionally attempts to recover the heat in the flue gases through the flue by deliberately inducing condensation (recovering the latent heat in the flue gas). This is achieved by adding a ‘secondary’, or extended, heat exchanger in the flue that exchanges heat with the incoming water, as shown in the example in Figure 3. The dew point of the flue gas from the combustion of natural gas is around 57°C; hence, condensing systems operate most efficiently with incoming water temperatures below 57°C. Condensation will potentially release approximately 3.5MJ per m3
natural gas and increase
overall gross efficiencies to more than 90%. Since the resulting condensate is slightly acidic (typical pH 3.6 – similar to orange juice), the materials for the heat exchanger are frequently manufactured from stainless steel. All the materials and construction that is exposed to the condensate must be able to properly resist the acidic atmosphere. The additional performance of a condensing water heater is due not only to the recovery of latent heat, but also to the lowering of the flue gas exit temperatures. As the flue gas exit temperature falls, there will also be greater amounts of condensation – hence, again, increasing the overall efficiency. The resulting flue gases are typically 50-60°C and, depending on the external conditions, can result in considerable pluming from the flue as
The secondary heat exchanger pre-heats the system water by absorbing latent heat energy from escaping hot flue gases
The primary heat exchanger further heats the pre-heated system water via sensible heat energy transfer
the remaining water vapour in the flue gas condenses into small entrained liquid droplets. The low flue gas temperature is likely
to mean that a fan is needed to remove products of combustion – this would be an integral part of the condensing water heater. By applying fast response digital control to the fan – using sensors in the flue gas to monitor the excess air – it can be automatically regulated to provide close to stoichiometric combustion, while ensuring that the flue gases are safely removed through the positively pressurised flue. Since the flue will be subjected to
low temperatures, it may be made from plastic, stainless steel, ceramic or glass – the key quality being its resistance to the slightly acidic flue gases. A fan-assisted flue will also provide greater flexibility for the positioning of the system (i.e., it does not need to be sited adjacent to an external surface), as well as having a small diameter, allowing simpler routing through the building; significant lengths of PVC or ABS pipework (more regularly associated with drainage applications) can be used. A drain is required to remove the condensate and, again due to the low condensate temperatures, this can be run in plastic (though in a position that is not at risk of freezing). It is important that the condensate drains freely from the heat exchanger, as any residual liquid will reduce the heat transfer and potentially increase the acidity adjacent to the heat transfer surfaces. When a condensing continuous flow
water heater system is constantly fed with water from the mains supply – typically under 10°C and well below the required 57°C – it will work in condensing mode throughout the whole year. However, there are many applications where the feed water may have a higher temperature – for example, in larger systems where there is a requirement to provide circulation in the hot water system, or in systems that are providing instantaneous top-up to water pre-heated via a renewable heat source (such as solar thermal or heat pumps). These systems will still condense if the feed water is below 57°C; however, it is important that the selected heater is designed to accept such pre-heated water.
Cold water Hot water Gas Condensate Figure 3: An example of a condensing water heater
Conclusion The challenge of continuing to reduce carbon emissions from buildings will become greater as the ‘easy wins’ provided
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Figure 4: A commercial continuous flow condensing hot water heater
by improving the building fabric become more complex to implement. The decarbonisation of the energy supply is still some 40 years (at least) in the future and, hence, the benefits of using lower carbon systems are increasingly important. More than 9%8
of energy consumed in
non-domestic UK buildings is by gas water heating and, compared to traditional oil, gas and electric powered storage systems, the application of condensing continuous flow water heaters (as shown in Figure 4) can reduce the energy required to supply hot-water, and so reduce operational CO2 emissions. © Tim Dwyer, 2012
References
1 Meeting Carbon Budgets – 2012 Progress Report to Parliament, Committee on Climate Change, June 2012.
2 Directive 2009/125/EC of the European Parliament and of the Council of 21 October 2009, establishing a framework for the setting of ecodesign requirements for energy-related products.
3 The Carbon Plan: Delivering our low carbon future – Department of Energy and Climate Change (DECC), December 2011.
4 Eco-design water heaters, September 2007,
www.ecohotwater.org
5 Directive 2010/30/EU of the European Parliament on the indication by labelling and standard product information of the consumption of energy and other resources by energy-related products, 19 May 2010.
6 2012 consultation on changes to the Building Regulations in England – Section two – Part L (Conservation of fuel and power). Proposed changes to technical guidance, DECC, January 2012.
7
http://www.guardian.co.uk/money/2005/apr/02/ consumerissues.jobsandmoney accessed 11 July 2012.
8 The Future of Heating: A strategic framework for low carbon heat in the UK, DECC, 2012.
August 2012 CIBSE Journal
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