Is a back boiler on a traditional log stove a good idea?

In most cases, using a traditional type of radiant stove with 'back boiler' (a water jacket) to feed many radiators is not a good idea, as it will not operate efficiently - using lots of wood and causing lots of smoke. The more hot water is required (e.g. if trying to supply a few radiators and a hot water cylinder), the lower the efficiency will be.

An example of the lowered efficiency is that with a water jacket back boiler added, no traditional wood burners qualify as 'exempt appliances' under smokeless zones legislation. This is because by making the stove less efficient, the water jacket will cause more smoke to be given off, including particulate pollution.

Also, the traditional kind of back boilers on log stoves will increase the probability of tar condensing in the chimney, and so of the risks this then poses. it will also exacerbate any problems of low fuel quality.

However, there are (at the time of writing) two log-stove-type appliances that have been designed in a different way, to be more efficient when heating water as well and so do meet smokeless zone standards. These are the Dunsley Yorkshire Stove/Boiler (about 7kW heat, 8kW water) and the Broseley eVolution 26 boiler stove (10kW heat, 16kW Water). Note that these appliances are relatively big, so may be too much for a small house.

These high-efficiency appliances move the water heating part of the system well away from the fire box, to extract heat after the gasifying stage of combustion. With this kind of improved design (and also much better control, for example of air inlets), modern ‘batch’ log boilers or pellet-fired appliances can produce hot water at 90% combustion efficiency when operated at maximum output.

With traditional back boilers you'll also get a ratio of water to space heating that is not ideal - perhaps 60/40 but often 50/50. If half the output is to space heating it can be difficult to get enough hot water without overheating the room the stove is in. This reduces the effective efficiency more, as you'll have to open windows to get rid of the excess heat. Pellet stoves have a much better ratio (80/20 or 85/15, water/space heating), while log or pellet boilers only produce hot water so the problem does not arise. You can see a video of a pellet stove in a small house back at http://info.cat.org.uk/biomass

A standalone stove with no water jacket might be a good supplement to either a heat pump or a larger biomass (wood fuelled) central heating boiler, as then the boiler can be sized to meet the likely space heating demand, with the stove as an additional space heater only if there is a cold snap. Many standalone log stoves (with no back boiler) do qualify as exempt appliances for smokeless operation, as they can be run quite efficiently. See http://smokecontrol.defra.gov.uk for a list.

Traditional log stoves are not to be supported under the Renewable Heat Incentive (RHI) scheme, as it will focus on the highly efficient 'batch' log boilers or a pellet-fired boilers and stoves. These systems have been in use in mainland Europe for many years and will become more common in the UK as the support mechanism kicks in.

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