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Commercial Heating: Condensing boilers must be part of an integrated system

Steve Cooper, managing director of Armstrong Integrated Systems, which supplies off-site manufactured integrated plant rooms, looks at how condensing boilers can help to achieve the energy efficiency targets
Commercial Heating: Condensing boilers must be part of an integrated system
WITH the recent implementation of the new Part L of the Building Regulations, engineers are increasingly looking to condensing boilers to help achieve or even exceed the seasonal efficiencies stipulated.

However, whether it is new build (Part L2A) or refurbishment (Part L2B), it is essential condensing boilers aren’t just considered in isolation as an intelligent sub-system but as part of a fully integrated system that matches pumps, controls and hydraulic system design.

We all know that for a boiler to operate in condensing mode, return temperatures need to be below 550C. Too often though, designers adhere to the tried and tested 820C supply, 710C return, particularly on refurbishment projects.

In new build the answer is easy; designed around a flow of 650C supply and 450C return for optimum performance. Radiators and underfloor heating systems as well as air handling units and other constant temperature emitters can be selected to work efficiently at these temperatures.

In multi-residential (community heating) projects instantaneous DHWS can also be generated via plate heat exchangers in satellite or substation units. However, for larger commercial DHWS applications where storage is required, it is better to generate this via stand-alone units for maximum efficiency and to prevent the possibility of legionella.

In refurbishment projects, selection of the optimum supply and return temperature is not quite so easy. Invariably, the system will have been designed for 820C supply, 710C return (a mean temperature of 76.50C) and the existing pump will have been sized accordingly.

However, the building will require a supply temperature of 820C on only the coldest days of the year when the outside temperature is around -20C or below. So if we resized a new pump on a temperature difference of 200C and incorporated weather compensation, we could operate the condensing boiler at its peak efficiency for most days of the year by reducing the supply temperature. On the rare days that the outside temperature is around -20C, the control system could then ramp the supply and return up to 86.5/ 66.50C to achieve the mean temperature of 76.50C. Of course the boilers won’t condense on those days but in reality they are so few and far between that it hardly matters. The comments for DHWS in new build also hold true for refurbishment where, if storage is necessary to cope with peak requirements, separate stand-alone generation should be used.

Another important aspect that many people forget when using condensing boilers is efficient sequencing of multiple boiler installations. Condensing boilers are at their most efficient at low load which is the opposite to conventional boilers. So a conventional boiler sequence philosophy that switches boilers on at or near to full load is not achieving the best efficiencies. It is much better to run all multiple modulating condensing boilers in unison to reach the required load.

This ramping up and down in unison not only saves on energy but also reduces maintenance costs as there is strong evidence that constantly switching equipment on and off increases wear. A major manufacturer has estimated that eight minutes is taken off the life of a piece of equipment every time it is switched on or off.

Funnily enough, Part L2B penalises anyone who over-sizes boilers by more than 20% yet, from an energy perspective at least, it could be argued it is actually better to oversize condensing boilers.

The final piece of the jigsaw is the hydraulic system design. Most current system designs incorporate a constant flow primary circuit.

While this ensures boilers are never starved of water, it wastes energy by mixing flow and return circuits and reducing the ∆T in partial load conditions. Whenever heat is generated, we must strive to dissipate it usefully in the space. Any control devices such as 2-port zone valves or thermostatic radiator valves should ideally be used as high-limit devices with the space temperature regulated, as far as possible, by modulating supply temperature and flow rate.

So, how do we protect modern, low water content boilers and still achieve excellent overall system energy efficiency?

One answer is to incorporate intelligent inverter-driven pumps configured to provide variable primary flow. Although relatively new to the UK, variable primary flow has been used extensively in North America, albeit almost exclusively on chilled water systems (for further information visit and click on Knowledge Exchange). When applied to integrated condensing boiler systems, variable primary configuration automatically modulates flow rate to match building load while maintaining the minimum flow rate across the boiler. By reducing flow rate and supply temperature, terminal control valves remain open reducing system pressure drop and wasted electrical input.

If all of this sounds a little scary to implement, help is at hand. A fully integrated heating solution combining all of the features and benefits described here, is about to be launched in the UK market.

The Armstrong MBS is a modular solution ranging from 120 to 320kW incorporating fully modulating boilers, variable primary pumps, automatic fill/pressurisation unit, integrated controls designed for rapid site assembly.

The MBS optimises both the gas and electrical input to achieve significant overall savings. Individual modules will pass through a standard doorway and quickly bolt together to form the complete system. All power and control wiring is plug and play. A 320kW system can be positioned, assembled and ready for commissioning in just a few hours. As the analysis shows, the resulting cost-savings of a MBS compared to a traditional approach could be in the region of 25%.

By rethinking how we incorporate modern condensing boiler and variable speed pumping technology into new build and refurbishment projects, significant energy savings and carbon reductions can be achieved.

Part L2 calls for minimum overall boiler efficiencies of 84% in new build and 80% in existing buildings. Fully integrated heating solutions, such as the Armstrong MBS, can achieve overall seasonal efficiencies of 94% and will have a significant impact on carbon reduction initiatives.

1 October 2006


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