Meet the dual boiler installation, a heating system that operates a bit like a tag team – a condensing boiler does most of the work, but when things get too cold a standard cast-iron boiler comes to the rescue to get the right result
Heating-system technology for larger commercial and industrial premises has come a long way over the past three or four years - yet without a great deal of publicity.

In 2006, part L2 of the Building Regulations introduced energy efficiency requirements for non-domestic buildings for the first time. These require a minimum of 84% boiler-operating efficiency, together with additional minimum -control packages.

The regulations apply to both new build and to replacements for heating systems. With the next stage of the Energy Performance of Buildings Directive requiring energy-labelling certificates for commercial and public buildings, the focus and visibility of energy- saving measures will be further highlighted.

Changing energy legislation across Europe has driven the development of new, technologically advanced and high-efficiency boilers. In turn, this has meant a fresh look at the most efficient heating solutions for commercial property.

Buderus has been involved in condensing-boiler design and controls technology for 27 years. The company has introduced the multi-boiler cascade concept, which, it says, has had a dramatic effect on heating-system specifications.

Working with contractors and heating engineers, the Buderus technical support team has been able to assist in creating new system designs.

The cascade system is built on the Buderus GB162 wall-mounted 80kW and 100kW condensing gas boilers. It is suitable for installations where heat demand varies significantly throughout the year. The basis of this system is a number of linked boilers with individual boilers coming into and out of operation as heat demand changes. For instance, in summer months only part of the system needs to operate as heating demand is low. Each boiler automatically modulates to enable it to remain in energy-saving condensing mode for the maximum amount of time.

A similar idea is the use of dual- boiler plants, but this should not be confused with a cascade. A dual-boiler installation uses two different types of boiler, rather than multiples of the same boiler being operated sequentially.

This is especially relevant where weather-compensated heating circuits are used. With these types of system, for most of the year only lower flow temperatures are required with only a few days each year requiring high temperatures when outside conditions are very cold.

In these circumstances, Buderus advocates splitting the capacity at about 50% to each of the two boilers, of which one is a condensing model and the other will be a standard-efficiency type - most probably a high-efficiency cast-iron model.

The result is an installation where the condensing boiler operates for the majority of the time below the dew point to meet the lower flow temperature demand. It is therefore operating in condensing mode for maximum efficiency.

The cast-iron boiler only comes into use when high temperatures are called for and when condensing is not possible.

Because the cast-iron boiler only operates at high temperatures, there is no disadvantage in energy consumption on the occasions when both boilers are in use.

The Buderus system can be programmed to operate in conjunction with weather-compensation controls, so that it switches over to the cast-iron boiler as the main boiler when very cold weather is identified.

The dual-boiler plant delivers cost savings in these circumstances. While two Buderus GE cast-iron boilers would usually operate at 94% of net calorific value (NCV) efficiency, two Buderus SB stainless -steel condensing boilers would be expected to operate at around 106% NCV efficiency.

Dual-boiler efficiency achieves a figure surprisingly close to condensing boiler levels at 104%.

This is because the boiler operates, most often, in condensing mode as a result of the design of the system.

The dual-boiler option is cost effective for replacement installations because many systems operate on high water content and there may be a requirement for boilers that offer lower hydraulic resistance.

The capital costs of a dual-boiler installation will generally be lower than for two condensing boilers.

The decision on which system offers the most cost-efficient solution for each commercial heating installation can be complex.

Each installation is different as it needs to meet differing usage patterns and specific criteria for each building.

The Buderus technical team can offer assistance in identifying all the issues.