Flu Gas Heat Recovery

Wood Stove Flue Boiler

We are looking for investment to development a prototype wood-burning stove flu boiler. We have the design and the theory ready; we just need to test the principles in practice.

Flue boilers have always been conceptually flawed. The idea is great in that a vast amount of heat is lost through the flue of a wood burning stove and in theory, a flue boiler would recover a good proportion of this heat.

Have you ever seen one though? No, because in reality, existing flue boilers stop the stove from burning efficiently. They cool the flue gases which causes precipitation within the flue and the cooling also reduces the draw through the stove, decreasing the efficiency even further. This causes soot build-up and eventually complete failure of the whole stove system.

Here are the benefits that the UFL flue boiler offers:-

  • no change (heat output or fuel consumption) to the existing stove performance

  • free hot water via the central heating system

  • drastically reduced flue emissions

  • significantly reduced carbon footprint by displacement of fossil fuel consumption

  • no longer any need to clean the chimney
  • rapid return on investment


Everybody loves an open fire and the flames within a wood-burning stove. It is the heart of the welcoming home.

Burning wood for heat is also very environmentally friendly. Wood is carbon-neutral because to grow the wood, the trees have to absorb carbon dioxide from the atmosphere. This makes wood the most attractive of heating fuels and in a world of climate change and increasing greenhouse gases, its use must not be squandered.

Wood stove manufacturers are therefore feeling the pressure to improve stove designs to increase their efficiency. This must, of course, be good news for the new stove owners, but is it?

A sensible interpretation of what ‘efficiency’ means would be the proportion of chemical energy within the wood that makes it into the home. An efficiency figure of 75% is deemed good, and it would be nice to think that 75% of the chemical energy in the wood contributed to heating the home.

The real definition, and the one that stoves manufacturers use, is the proportion of hydrocarbons within the wood that get completely combusted into CO2 and H2O. This means that if you put 4kgs of wood in a 75% efficient stove 1kg of unburned hydrocarbons leave the house via the flue.
That is a pretty dismal thought but the story gets worse.

In order to increase a wood-burning stove’s efficiency, the designers need to increase the temperature of the stove’s combustion chamber. This is done using two strategies simultaneously:-

  • the combustion area is insulated, meaning that less heat makes it into the home, and

  • more air needs to be drawn into the combustion area. This can only really be done by employing the draw from the gases rising in the flue. This can only be achieved by keeping sufficient heat within the flue – removing heat to warm the house will decrease the ‘efficiency’ of the stove.

The conclusion therefore is that if you want more heat in your home from a give amount of wood, don’t choose a high efficiency stove, and accept the fact that a good proportion of the chemical energy within your wood is going to leave your flu as unburned hydrocarbons.

The benefit of this is that you displace more fossil fuel consumption because more wood heat is being utilised in the home, but you are contributing significantly to gaseous pollution.

What we need is a device that can easily be fitting into the flue that:-

  • recovers some of the waste heat

  • reduces hydrocarbon emissions

  • does not degrade the performance of the stove

This device has been designed and the design is now ready to become a first prototype.

Market size

In the UK 200,000 new wood burning stoves were sold last year. The market has been growing significant every year for the past 5 years.

There are 1 million wood-burning stoves currently installed in UK homes. That is one in every 20 houses has one.

For further reading, take a look here. Saving money by installing wood-burning stoves, read here, and here.

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