Boiler replacement is only part of the story

Delivering the potential of replacement boilers — John Bailey
Replacing a boiler will not in itself fully maximise the efficiency of the heating system. Engineers must take all aspects of the system into account when upgrading commercial heating, says John Bailey.The growth in the use of high-efficiency condensing boilers in commercial applications has undoubtedly led to an improvement in the energy efficiency of many existing buildings. However, replacing a conventional boiler with a condensing one will not, in itself, be enough to meet the energy targets we are now expected to achieve. Heating engineers will have to optimise the whole system. In an ideal world, engineers would be given a blank sheet of paper to design a high-efficiency system from scratch, but the reality is that we have to make the best of what we already have. Over 70% of the commercial buildings that we will still be using in 50 years’ time are already built. Therefore, we have to develop a refurbishment culture that can, step by step, improve the performance of existing buildings. Ripping out and starting again is rarely an option — and is unlikely to be the most sensible or sustainable solution anyway. Modern commercial boilers can deliver around 95% operating efficiency in most circumstances, which is why replacing an aging boiler with a newer version is usually the right place to start. However, we then have to make sure the whole system is configured to make sure the boiler does operate close to its optimum for as long as possible. Future versions of Part L of the Building Regulations — due in 2010 — and the forthcoming European Energy Using Products Directive are aimed at producing a further 20% increase in energy efficiency in new commercial buildings. Insisting on more incremental efficiency gains for already highly efficient boilers will not deliver this figure; legislators have to focus on the building as a whole. Integrating boilers with low- and zero-carbon technologies like solar thermal systems and heat pumps along with the use of new fuel sources, such as biomass, will give us a major efficiency boost. First, however, the system designer should consider more basic measures like improved insulation and upgrading controls, along with setting up initial and on-going commissioning. ‘Building efficiency is not just about the new heating plant used, it’s about the interaction of the building and the plant and also the maintenance of that optimum efficiency year in year out,’ says David Hughes, technical director of the ICOM Energy Association. ‘A fantastically efficient building will only stay that way if there is a requirement for monitoring and full maintenance — otherwise all the savings will be lost over time. We need to keep pressure on the authorities in the UK and Europe to make sure that the legislation focuses on the areas where the largest efficiencies can be made.’ This is not just about legislation, however. The commercial heating industry also has huge responsibilities. For example, efficiency will be seriously affected if the boiler is not sized correctly to match the heating load; oversizing has been a serious problem in the past. Similarly, the heating engineer must establish correct boiler sequencing. Carefully matching which boiler(s) operate to meet a particular load can improve overall efficiency. At Vaillant, we also seek to ‘tune’ the boiler by making sure its internal settings are optimised for the conditions it will experience in operation. This means we must be given clear and accurate information about the project in hand and likely future demand patterns. The controls strategy is also critical. Contractors face a considerable technical challenge when trying to achieve full system integration and proper boiler sequencing to ensure optimum performance. This is why Vaillant has developed the VRC 630 cascade controller. This unit offers built-in weather compensation and load control to automatically smooth out demand peaks and troughs while closely matching ambient conditions to the operating temperature of the system. It can control up to eight boilers in series, with all their associated system components, and up to 15 separate heating zones in a building.
Benefiting from the 95% efficiency that most commercial boilers can achieve these days requires detailed attention to the heating system they will serve.
By harmonising the operation of the entire space and hot water system, the control system should be able to optimise the performance of the boilers, spread the heating load to minimise energy consumption and so lengthen the operational life of system components. Weather compensation control is an ideal addition to existing systems and is coming into its own as more condensing boilers are deployed in commercial buildings. By keeping the flow temperature as low as possible, the boiler is almost constantly in condensing mode and so is operating as energy efficiently as possible. Condensing boilers are at their most efficient close to the dewpoint temperature of 57°C when flue gases condense and latent heat can be extracted. Weather compensation takes full advantage of this theory by adjusting the flow temperature to the heat emitters. The aim of weather compensation is to work the boiler closer to dew point during the average days of the year and only run the boiler at higher temperatures during the coldest periods. The warmer the day the more efficiently the boiler will operate by making full use of the fluegas condensation. This leads to impressive energy savings. With this in mind it makes absolute sense to range rate the boiler to the system heat load; the boiler will modulate itself depending on the difference between flow and return temperatures. Systems using weather compensation do not need room thermostats, but they must have thermostatic radiator valves (TRVs) fitted throughout to compensate for other heat sources or solar gains. The only thermostat fitted externally to the boiler is an outside temperature sensor. End users can also be persuaded to adapt their behaviour slightly and get used to moderate changes in comfort conditions. For example, with weather compensation they will find that radiator surface temperatures will be lower than they may be used to, but comfort levels will be maintained and energy efficiency greatly improved. In any case, internal temperatures should be allowed to ‘float’ to some extent these days. To achieve the necessary energy savings, it is no longer practical to maintain temperatures at a single set point throughout the year; this requires too much energy to run mechanical heating and cooling working flat out to produce an artificial and unnecessarily restrictive comfort regime. We should not be seeking to design our buildings to be ‘immune’ from changes in outside conditions,. Rather they must adapt to them and take advantage of ‘free’ heating and cooling when it is available. A new boiler on an old system will immediately improve the performance of that system, but the addition of modern controls and full system commissioning will raise that performance to a whole new level. John Bailey is commercial heating and systems director at Vaillant.
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