With fuel prices taking off and larger organisations starting to feel the effects of the Carbon Reduction Commitment scheme, the efficiency of heating boilers is coming under increasing scrutiny. Employing an effective controls strategy can result in a significant reduction in energy use and carbon emissions. Jeff House examines the opportunities.
Driven by Government policy and incentive schemes such as the recently announced Renewable Heat Incentive and Feed in Tariffs, interest in renewable technology is burgeoning. Clearly, with correct application, renewable technologies have much to offer and they will form the backbone of all building services systems in the future.

However, around 80 per cent of today's building stock is connected to the UK gas network, employing natural gas fired appliances to deliver space heating and hot water requirements. Ensuring that these appliances are operating as efficiently as practicable makes sound financial sense to businesses while also aligning with broader UK Government policy objectives of reducing harmful emissions and reliance on fossil fuel.

Boiler manufacturers quote efficiency figures for product in a number of different ways. However, in all cases the figures will be derived from closely managed laboratory tests in controlled, optimum conditions. In order to achieve high efficiency and low energy consumption in a real installation, good control of both the boiler and distribution circuits is essential.

Even with boilers closely matched to the building load, inadequate or incorrect application of boiler control can increase fuel consumption by 15 to 30 per cent. Over the course of a boilers' life this can stack up to many times the original installation cost in wasted fuel.

Approved Document L2 of the Building Regulations 2010 (England & Wales) requires a minimum level of control for boilers in new buildings as well as for replacement boilers. Full guidance is detailed in the Non-Domestic Building Services Compliance Guide 2010 which is a supporting document to the Building Regulations. Controls requirements for boilers in new buildings are reasonably robust and, provided system design is carefully considered, the in-use efficiency of product should be to an acceptable standard.

Less stringent standards

However, replacement boilers are subject to less stringent minimum standards, requiring only a time control, demand control (room thermostat) and zone control for buildings in excess of 150 sq m. This is designed to allow flexibility and direct like-for-like replacement for older, low efficiency atmospheric boilers with minimal work on site.
When appraising the whole life costs of an installation, however, this approach is short-sighted.

In many cases, condensing boilers are now being installed as replacements for older non-condensing appliances, on the basis that system efficiency will increase and fuel costs will reduce. However, this will not be achieved without appropriate control. Many existing heating systems will have been designed around flow and return temperatures of 82 and 71 deg C.

If a condensing boiler is installed onto such a system the boiler will rarely operate in condensing mode as it requires a return temperature below 55 deg C and the manufacturer's claimed efficiency will not be achieved. In such cases, the system will require high temperatures outside the condensing range to achieve comfort conditions at the coldest times of year. However, this only represents a small proportion of the entire heating season.

With the introduction of direct weather compensation the boiler flow temperature is lowered as the external temperature rises, therefore improving efficiency and allowing condensing boilers to operate in the condensing range for at least part of the year. Other simple additions to the system are 'night set back' and 'optimum start', both of which offer efficiency improvements at minimal outlay.

Modern condensing boilers are most efficient when operating at part load so it makes sense when considering a boiler replacement to employ a number of condensing boilers with cascade control. This approach has many benefits. In the event of a boiler failure the remaining boilers should continue to run, thereby keeping the building operational.

In addition, the cascade controller will share the system load between the maximum number of boilers, ensuring they work at part load for most of the year and, thus, optimising the efficiency of the system. (This should not be confused with the older principle of step control whereby multiple boilers are controlled so that individual boilers are bought on line at full load in turn; this will not offer the same efficiency benefit as cascade control).

More functionality

In new installations, a full BMS system can offer this, and more, functionality if appropriately applied and monitored. However, this approach can be expensive to implement and is not necessarily appropriate for older buildings. Leading boiler manufacturers provide controls options designed to complement their boiler ranges with cascade, optimisation and zone control available at a competitive cost, thereby affording advanced control to even the most basic of systems when replacing boilers.

• Jeff House is applications manager for Baxi Commercial Division