Summary

It could be argued that storing water at high temperatures over extended periods in a cylinder used heated by a system boiler would increase energy losses; however greater inefficiencies are likely to be caused by inappropriate design, poor control and usage, and inefficient installation of either system. Each system when applied appropriately has its merits. On a like for like layout, with like for like usage, there's little to choose between them.

Introduction

Combination boiler

A combination 'combi' boiler will provide heating and hot water 'on demand' without a storage cylinder. They are designed for smaller properties where space is limited and the hot water demand is small and only drawn from one location at a time. When they are installed in larger properties the tendency is to install an over-sized boiler, maximising the hot water output. However being over-sizing for the heating demand causes a boiler to cycles more frequently than necessary causing in additional energy losses.

Combination boilers are ideal for filling an adjacent basin, however long lengths of pipe between the boiler and the point of use result in much hot water going unused. Each time a combination boiler is fired to produce hot water it must first warm the heat exchanger before heat can be transferred to the water. These losses are relatively small for the occasional use of drawing a bath, however with the frequent use of a wash hand basin these losses can add up and often water is drawn without reaching its point of use. An under-powered boiler could take a long time to fill a bath; accumulating the energy in a cylinder can speed the process.

When a combination boiler is converted to a system design, a storage cylinder is added on a controlled heating zone. The surplus 'on demand' hot water heat exchanger remains warm within the boiler and without it being raised to full temperature there is an increased risk of Legionella. Installers advocate a single unused 'on demand' hot water tap, however it is unclear whether this satisfies regulations.

Most combination boilers must not be supplied with pre-heated water, as already hot water will be raised a further 40º C with danger of steam flash. More advanced boilers are available to do this, however it is unclear whether they are appropriate for British regulations.

System boiler

A system boiler will provide heating and hot water via a storage cylinder which can accept a variety of heat inputs: a stove or range fitted with a back boiler, or solar hot water panels etc. Designed for larger properties, they satisfy a greater demand for hot water drawn from multiple locations, so there no drop off in pressure when using the shower. The controls are designed bespoke for the application, so it is  easier to expand or modify the system at a later date.

Conclusion

To satisfy the hot water demand for a family of four in large three-bedroom house with potential to expand, a system boiler with well-insulated water storage cylinder would be most appropriate and efficient.

A system boiler design could be installed for maximum efficiency, with short pipe length to the kitchen, family bathroom and attic bathroom if expanded. Additional heat sources from a solar hot water collector or wood stove could be added later to further increase efficiency.

A combination boiler design could also be installed, however, further investigation would be required to confirm its appropriateness for later conversion to a system design with regard to the Legionella risk in an unused heat exchanger.

A variant of both system could be designed, however further investigation would be required to confirm whether it is possible to do so within the current regulations.

Note

A storage cylinder should be raised in temperature above the risk of Legionella but below the risk of lime scale.
 

Simon Lewis

Fairfield Road