The ETA SH biomass log boiler

This is the heart of our system. The ETA boilers are made in Austria, and I have to say that having looked at a number of boilers, this one seems to be the best and most efficient that I could find. Ours is the 60KW model, although an innovative switch to the rating means that it is officially known as a 40KW boiler, and therefore comes under the domestic RHI rating. 

Official ETA website

It's computer controlled.  You open the middle door (the bottom one is for ash removal, which is stupidly easy) and put some sticks and a bit of cardboard in the middle - I hit it with a blowtorch to light it for a few seconds - easier still if there's charcoal left from the last burn. The fans start up automatically and draw volumes of air through it - so the flame rapidly becomes a furnace. Chuck a load of wood on top through the top door, and its away and running. Officially, you shouldnt use anything else. Unofficially, we burn most household rubbish in it (in moderation - a few bits of plastic veg wrapping just helps the thing get going..)  - and the high temperatures (up to 185 degrees Centigrade) reduce everything to harmless carbon. After a full season of burning, we had a bit of trouble with carbon coating the inside of the flue - this was sorted with a quick rodding with flue brushes - which produced a bucket of lovely greyish carbon that was burned in the next firing. We use the ash to fertilise the garden. A weeks burning produces a few litres of fine greyish granular ash. 

During this last winter, temps were down to zero, und often only 6 degrees in daytime.  Our house is 3 storey, about 15 rooms, solid walled. Windows are now all double glazed, in oak frames.  The system is so efficient that we've had to turn most of the radiators down, and the house is too hot at night, so we turn the heating off most nights. 

The Numbers:

People have asked about numbers, of course - and how does it compare with what we had before. Well here goes.  The old system was a 55KW Keston LPG gas boiler. It cost about £150 a week to run the heating full bore, and it never got the house warm enough anyway - we had to keep refilling the LP gas tank. So... our heating cost us about £3,000 a year, and didnt really work.  Our decision to go Biomass was influenced by several factors - first, we had woodland, so log availability. Second, it bought us independence - no reliance on other suppliers, oil and gas prices etc. Third - it was partly grant funded.

So... we looked all over the place for suppliers. I have to say that finding the right supplier of a system like this is hard. There are not very many who know what they are doing, and a lot of fly by night operators who supply sub standard boilers and can't make them work. You MUST do a lot of research - and to be fair, I can't go past the ETA kit. We used a company in Shropshire, called Edge Renewables - who were superb - their team are incredibly professional, clean, tidy and so pleasant to deal with. 

Our estimated annual usage was 44,000 kWh's.  This was related back to an LPG cost of about £2,600 a year at 35p / litre. Theres a cap of 25,000 kWh's for domestic RHI payments, so we were told the RHI would be around £1,700 a year and inflation linked for 7 years. This meant that we'd get back around £12,000 of the total outlay for the system of £24,000.

With this usage, Edge worked out we'd use about 13 tonnes of logs at moisture below 20% - that's wet wood to me - we generally get our moisture down to around 12% by using the Norwegian methods, and storing carefully. The system runs at over 90% efficiency, except, as we found, when there was too much ash in the flue and it needed sweeping. This equates to an annual saving of the LP gas cost (£3,000) plus for the first 7 years, of another £1700.  Doing the Edge Renewables calculations, with fuel inflation at 4% and a cpi of 2.5%, total savings of around £32,000 - of which £13,000 is RHI payments. Based on this info, payback is around 5 years.

Using split logs, it's estimated that they are about 60% cheaper than using oil. I'd say this is conservative.

They are theoretical. Of course they are.. But dont take into account fuel and equipment cost of cutting wood, tractors, chainsaws, and so on. But... who can put value on the woodland lifestyle - going down the woods on a misty morning, watching the changing seasons, the carpets of bluebells, wood anemones, foxgloves, wild garlic by the stream, and the wildlife.. Our land has gone up significantly in value already, and we derive immense pleasure from both being there, and physical fitness from lugging timber around and planting trees. There is something incredibly satisfying in planting an oak tree that you know is going to be there in 100 years - long after you are gone, but a legacy of careful woodland management that you can't put money on.  

Heating Value of Wood

A lot has been said about the heating value of wood - most people think it needs to be dried for years, and if you don't dry it, you get no heat.  Let's have a look at this properly:

The moisture content of split logs intended for a boiler should be below 20% air dry. Damp wood with a moisture content of above 20% creates a moist environment within the fuel chamber and can rust it through. Unsplit logs, left in the woods will take at least 2 summers to dry out. However - if we use the Scandinavian methods, timber can be used in the same year. The timber is felled in winter - when there is minimal sap in the tree. Logs are cut to 1 metre lengths, and immediately split to half or quarter - aiming for a size of around 4" across. These, stacked in the woods so wind can pass through the stack, mean that a moisture content of 15% can be easily achieved by September just through air drying. Split logs also burn far better than unsplit logs.  The best method, according to the Scandinavian authorities, and the ETA experts, is this:  Cut all logs to 1 metre.  Immediately after felling, split in half for a log diameter of up to 15cm.  Quarters for diameters up to 20 cm, and 6ths for up to 25cm.  8ths for up to 30cm.  Essentially the split pieces - called billets - need to be around 4" in size. If wood is stored outside, cover the top of the stack to keep the rain off it.  Wood that is stored inside for over 3 years will lose around 1 to 3% of heating value per year, due to changes within the wood akin to decay. Heating value changes by only 2% between 20% and 10% moisture content - and its easy to achieve this by splitting early in the year, stacking under shelter to air dry through the summer, and then bringing into store and cutting to 500mm ready for the boiler in winter. 

It's interesting to note as well that heating value does not vary between timber species by weight - in other words, a kilogram of spruce will produce the same heat as a kilogram of oak.  This doesnt mean to say you buy it this way - generally, people buy by volume - so if you are buying a 1 tonne bulk bag, better to buy split oak than split spruce!

The very best book to buy on the subject is this - its a classic!

 

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