…The rationale behind installing a rocket mass heater in Cooksbridge – keeping warm, keeping fossil fuels in the ground, keeping alive an old technology for a modern age. By Doreen du Boulay

  Rocket mass heaters are old technology; they have regained popularity in continental Europe as they use a renewable source of energy – wood – and can be adapted to fit the available space. My house has low ceilings but a brick fireplace to accommodate the rocket and its bench.  Ianto Evans is the main guru behind the renaissance (manual and plans are available through the internet for a small fee) and trained my Dutch rocket engineer, Sjang van Daal.  You can indeed do it yourself – make a heater with bench – with basic brickwork skills, but if you want your heater in a house or in an integrated system, as I did, you need some experts to do the chimney lining and any plumbing etc. to become HETAS certified. 

So why not just have a pellet boiler or wood stove?  Rockets are less polluting; smoke is circulated round the system twice, so practically nothing comes out of the chimney.  You also get more heat from your burn.  They can adapt to the available space as long as there is a working chimney and also store slowly released radiant heat in the bench, over several hours, once the fire has gone out.  This was a big point for me – having a warm house in the morning.   The heat from the firebox in my rocket goes to heat water in a heat store upstairs (it’s a partnership arrangement with solar panels, which even in winter, keep the water above freezing and reduce energy use) and /or radiators or just the bench – great for aches and pains – when the fire is lit.  Levers help you direct the heat to where you want it.  At night, the chimney is closed and the remaining heat goes to the bench for storage.

What does it run on?  Rockets are picky and wood must be dry, ideally below 15 on a moisture gauge; most logs sold are far too wet and need storing for a couple of years.  You also have to chop logs up. The firebox is narrow and anything longer than about 12 inches or wider than 2 or 3 won’t fit.   Ironically, rockets do best on soft wood waste off-cuts, which can produce a quick, hot burn for bath water in about an hour.  Wood for pizza ovens is good and usually dry enough. Kiln dried wood is fine but expensive.  Hardwood waste is easier to come by and it takes a little longer to ‘rocket’,  the hot burn noise you want to hear at the start, but is fine.  There is no griddle bed as in multi-fuel wood burners, so pellets would work in principle – I haven’t worked out the cost.  Skips can be a source of free wood, but most wood used in house-building is polluted with chemicals.  Garden waste wood and branches will eventually come into their own – checked with the moisture gauge.  I also get a thrill from recycling waste wood.  My house is recycled.  It used to be in Offham, but the main structure was moved to Cooksbridge around 1730, so just keeping up the tradition.

Building from scratch, I’d have a central rocket with room high storage (higher ceilings) and a bench – plenty to look at on Sjang van Daal’s website. The rocket as is, is the best adaptation for my old house eschewing oil or coal – keep it in the ground.  We love the central fire and the top keeps liquids warm samovar-like and potatoes can be roasted in the firebox – or pizzas – when the fire is low.

How did it all happen?  Sjang runs workshops to train would-be rocket builders in the skills they need.  I was the host, providing food and accommodation over a 4 day week-end and the basic materials like firebricks, house bricks, cement and clay for the adobe covering locally.  Sjang designed the project and brought with him the heat exchanger, fire door and metal top – all made by his local blacksmith – the tools needed and his expertise.  My rocketeers came from Holland, Wales, Spain, England and Argentina  – architects, masonry stove installer, interior designer, plumbers, mini rocket bean can maker, ‘green people’ and all worked very hard.  Zoe is putting the final touches to the decoration and the bench has been tiled with natural stone, but the system is working well.  It’s not a low cost option because it s a whole system, which is regulated and so has to achieve required standards, but is cheaper than a pellet stove and we enjoy having the fire in the house (and warm bottoms).  We learned a lot in our build and are happy to pass it on.


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