Location: In the Sierra de Bazas, Andalucia, Spain
posted 1 year ago
Hi You Wonderful Forum People,
My husband and i have had the seminal book on rocket mass heaters by Ianto Evans sitting on the bookshelf for 9 years now. Finally we have, well he really, has built one. I know he read the book at least 3 times and has a handle on all the important dimensions and built it to them. One exception, we have used the stratification chamber idea for the bench. BUT we have a smokey dragon. It smokes in the house, it smokes out of the exit chimney and last night it set off the carbon monoxide alarm closest to it. We closed the feed tube almost completely before going to bed last night. Could the weakest link be the not-entirely-dry wood (one year of seasoning but rained on over the summer....something we are not used in the arid high plains of Granada, Spain where we live ) we are using ? I'm thinking that not-so-dry wood will make the fire will be cooler therefore not burn all the smoke and carbon monoxide off AND create more ash and maybe a blockage in the system.
Any thoughts would be soooo welcome.
Linda and George
Hi Linda. It may be worth obtaining a moisture meter. In the UK you can get one for around £20. Split a piece of your fuel and check the level on a freshly exposed section of the wood. Ideally your wood should have a moisture content of 20% or less.
If your wood isn't dry enough, try to get hold of some clean, unpainted and non-treated pallets. Cut them into small sections and make sure they are dry (they should 'ring' when hit together, not 'thud' )
Fire up the beast and once the 'dragon' is up to temperature, if your stove stops smoking your original fuel may well not have been dry enough. If the smoke persists then lets look at some other possible issues.
If you can safely suspend wood near/above the barrel to help dry it (while watching it the whole time to avoid chance of combustion), you could try again with dried wood. Measure the top of the barrel down to a fixed point, then remove the barrel and measure from the top of the heat riser to that same fixed point. At a minimum you want 1/4 of your system size, but go larger if you can so any ash buildup doesn't cause an issue. If you aren't trying to cook on the barrel or boil water (where the smaller gap will focus the heat to a smaller area), you could make that gap larger to reduce the chance of bottleneck or blockage.
If you are just starting to use the heater for the first time, there will be some time as the mass dries out and it will behave like a cold start with poor draft. As the mass dries out this part of the equation should lean more towards your favor.
How tall is your chimney, and is it inside or outside? What material is it made from? An indoor chimney made of metal that starts near the barrel (which will help heat the chimney with radiant heat) and goes straight up and out the roof will improve the system draw compared to a masonry chimney (which will suck the remaining heat from the exhaust and can stall it) or a metal chimney outside where it is kept very cold by outdoor temps. The kind of cap on the chimney can also help reduce issues caused by wind gusts that could cause smoke back. A tall, warm chimney will help keep exhaust flowing the correct way.
Can you tell how hot the exhaust is based on the feel of the chimney (assuming it's metal), or by using a digital thermometer? The size of the bench bell should be matched to system size, so remove enough heat but not too much. People report good results if their chimney exhaust is in the 100-150F (~38-66C) temp range.
I'm not quite a lumberjack, but that's OK, I sleep all night and I dream all day; I'll coppice trees, I'll grow my food, and compost poo and pee! With a well and off-grid solar, it's a permies life for me! https://www.youtube.com/watch?v=FshU58nI0Ts
I don't have one of those RMH but when my woodstove smokes abnormally at start up it's someone has a fart fan on in a bathroom or in the laundry room. For this case it is from a vacuum before the draft is established. Starting those same fans after it's up and running doesn't cause it.