New boilers for old buildings

Published:  10 September, 2009

Domestic hot water is now usually separated from space heating to avoid having to size a boiler for both loads. This is the NEOflo stainless-steel condensing storage water heater from Andrews.

Yan Evans discusses the benefits of bringing modern boiler technology to existing buildings and the issues involved when integrating new plant into old systems

Condensing technology has changed the landscape of the heating industry. Part L2B of the Building Regulations specifies a minimum efficiency of 84% in replacement boiler installations. That means any newly installed boiler generally has to be condensing, except in rare cases where this technology is not feasible. Some pressure-jet boilers can also achieve these levels of efficiency, but atmospheric-combustion type appliances cannot reach these figures alone. The minimum efficiency stated under Part L2B of the building regulations refers to the boiler system, so the required efficiency could be achieved using a combination of non-condensing and condensing products. Boiler controls and other measures also qualify for efficiency credits, helping to raise overall system efficiency to the minimum regulatory level. Commercial heating and hot-water loads are now generally separated to avoid sizing a boiler for both heating and hot water, as it will operate less efficiently in summer to satisfy just hot-water demand. When a boiler provides domestic hot water via a calorifier requiring a stored-water temperature of, say 60°C, the boiler will not operate in condensing mode. Where beneficial, hot-water loads are also decentralised to minimise heat loss from long pipe runs. Consequently, modern commercial boilers generally have reduced in size, being more compact, and with floorstanding models having a much smaller footprint. Outputs can be as low as 30 kW, with lightweight wall-hung boilers having outputs up to about 120kW. Such a variety of choice is essential within the refurbishment market, where issues with limited space, existing pipework configurations, doorways and flueing requirements dictate precise criteria for replacement applications. Most appliances have relatively sophisticated integral electrical controls, which can be linked to a BEMS, further optimising efficiency and operating cycles. A recent development utilising commercial wall hung modular condensing boilers is the packaged prefabricated system. This consists of one or, usually, more boilers designed to operate in ‘cascade’ to share the load efficiently and mounted on a prefabricated frame(s) complete with pumps, pressurisation system and heating system controls with all gas, hydraulic and electrical connections. These solutions offer a variety of advantages, not least because downtime is dramatically reduced, by maximising off-site fabrication and assembly with the system arriving factory tested and ready for a ‘plug and play’ installation. Saving downtime is important for businesses as disruption may result in lost revenue. The consideration for properties such as residential care homes and schools, is the limited time available for boiler plant refurbishment. For example, a prefabricated modular heating installation commissioned as part of a refurbishment project in a social housing facility in Alnwick was completed in only half a day. Prefabricated packages also eliminate the loss and consequent replacement of components They also reduce on-site work, saving on installation costs. However, simply installing one or more condensing boilers will not immediately guarantee the system will work at peak efficiency. Several factors apply when integrating new technology into an existing system to ensure optimum performance. Water-borne debris not removed from the system, would lead to loss of efficiency, performance and eventual boiler failure. The normal solution of power flushing may give rise to other issues such as corroded leaking pipework. A plate heat exchanger could be installed between the existing heating system and the boiler circuit, effectively creating a hydraulic barrier. Critical when using condensing boilers on an existing system are the water flow and return temperatures for which the original heat emitters were selected. Radiators are likely to have been selected for flow/return temperatures of, typically 82/71°C. However, boilers will not condense at return temperatures above 55°C, so performance will not be optimised. Water heaters are also being developed to overcome challenges faced by the retrofit market, one of which is low water pressure. This can occur for a number of reasons, such as variable pressure in the mains water supply or the building having been extended and hot-water outlets added, lowering the dynamic water pressure when all outlets are open during peak-demand periods. However, condensing water heaters are now available with low internal water resistance that can operate at pressures as low as 0.2 bar, making them perfect for refurbishment projects. There is no doubt that a modern high-efficiency boiler or water heater, when connected to an effective control system and, ideally integrated with LZC (low to zero carbon) technology, preferably all from the same manufacturer with the expertise to provide full technical support, can offer significant energy savings and reduced running costs for existing buildings. Yan Evans is technical director for Andrews Water Heaters and Potterton Commercial, both part of Baxi Commercial Division.

Bringing modern boiler technology to existing buildings — Yan Evans.


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