Understanding the requirements of Part L for boilers
Combining boilers in cascade operation can be more effective than a single boiler, especially where demand for heating varies significantly throughout the year.
PETE MILLS comments on the latest changes to Part L and their effects on commercial boiler specification, with particular reference to the application of controls in refurbishment projects.Energy efficiency has been an increasingly important factor in Government environmental policy for a good number of years. In 1984, the first energy-efficiency rules were introduced within the Building Regulations. They were extensively tightened in 2002 with the introduction of the SEDBUK (seasonal efficiency of domestic boilers in the UK) rating system within Part L1 and the introduction of Part L2 for non-dwellings. Further strengthening has occurred in the 2004 updated regulations and precipitated the market’s rapid transference to condensing boilers. SEDBUK is, of course, only relevant to domestic heating boilers, but it has led the way to the new Building Regulations Part L:2006 which, for the first time, introduces specific energy-efficiency criteria for heating systems in non-domestic buildings. These regulations affect both new and existing buildings. Complex equations are required to calculate the efficiency rating for a central heating boiler, but these are undertaken by boiler manufacturers to strict criteria to obtain figures for publishing. The figures take into account gross boiler efficiency measured at 30% load and at 100% load. The minimum seasonal boiler efficiency for a single boiler in a new commercial building is 84% (gross). The minimum efficiency for a single boiler in a multi-boiler system is 80%, with overall efficiency of the whole multi-boiler system required to be 84%. A minimum control package is also obligatory, providing time and temperature demand control. Control must be zone specific for buildings with a floor area of more than 150m2, with weather compensation unless a constant-temperature supply is required. For boilers between 100 to 500 kW output, additional controls are required, comprising optimal start/stop control, night setback or frost protection, 2-stage firing or multiple boilers, measures to limit heat loss from non-firing modules and sequence control for multiple boilers. Fully modulating burner controls are additionally required for boilers over 500 kW output. The situation for existing buildings is a little more complicated. The minimum seasonal boiler efficiency for natural-gas boilers is 80% (gross) (LPG 81% and oil 82%) with obligatory minimum controls to be on-off zone control where the building is more than 150 m2 and room thermostat with time control through a diverter valve. This does not on its own achieve the 84% required for new build. This same 84% is also a required minimum for existing buildings, so there may be a 4% gap to be bridged to comply with Part L. This can be achieved by gaining heating-efficiency credits which are applicable depending on other efficiency measures. Credits range in value from 0.5 to four percentage points and can be combined with boiler efficiency to reach the required 84%. A full building-management system gains the maximum four percentage point credits, optimised start/stop and directing weather compensation each give two credits, weather compensation with mixing valve and optimise start each give 1.5 credits, sequential control of multiple boilers, monitoring and targeting, TRVs, full zoned time control, decentralised heating system, and multiple boilers each give one credit, and room thermostat or sensor control of boiler water temperature receives 0.5. The resulting percentage figure gives the effective seasonal efficiency for generating heat. Specifying boilers to meet these new efficiency standards is best achieved simply by choosing non-domestic boilers which have published figures which meet these criteria. Once a decision is made on the type of boiler that best meets the needs of the end-user, it is worth considering additional measures to increase the overall efficiency of the system — in anticipation of ever-increasing fuel prices. Controls can significantly improve the efficiency of the heating system, but selection of the right controls is paramount. Weather-compensation, for instance, is widely specified in Europe yet infrequently at present in the UK — even though it improves efficiency by reacting to actual weather conditions rather than flow and return water temperatures. Given the efficiency of condensing boilers, a cascade multi-boiler system may be a more effective option and a good alternative to a single larger boiler especially where demand for heating varies significantly throughout the year. For example, a Buderus modular cascade system combines four GB162 boilers in just a square metre of floor area to give 400 kW output and is automatically controlled to modulate for the longest possible period to maximise energy efficiency. Extra boilers are fired as and when it is most efficient to do so. The GB162 can modulate down to 19% of total output. It is, of course, extremely important that all boilers are specified and installed correctly to ensure they remain in modulating mode for the optimum time, which means return temperatures need to be below 50ºC.
By mixing return water with hotter water in the boiler, these Buderus cast-iron boilers overcome the issue of minimum flow volumes or return temperatures — eliminating the need for bypass circuits and shunt pumps.
Choosing the best boiler or control system is not always straightforward, and there may well be more than one obvious solution. To help heating engineers make their decision, Buderus offers substantial technical consultation services and has over 25 years’ experience in condensing-boiler technology gained throughout Europe. The right heating technology can limit the effects of rising fuel prices. To ensure these cost increases are minimised in the long term, businesses should aim to exceed the minimum requirements set by new regulations and invest now in the most efficient boiler and control systems so that future fuel price increases will have less impact on the business and the initial investment payback period will be shortened. At first glance Part L and its effects on the refurbishment of commercial properties may well seem rather complex. In fact, and based on experience of the domestic market, it could well be seen to simplify options by eliminating those boilers which simply do not perform to satisfactory levels of energy efficiency. Pete Mills is technical manager for Buderus commercial boilers, Cotswold Way, Warmdon, Worcester, WR4 9SW.