First thing is do NOT be intimidated! Just believe in yourselves, have good reasons for doing it, invest in some sound knowledge and get stuck in.
These are thoughts I wrote up after we completed ours in 2015 and are just my personal opinions!
1 If you have never built a RMH before, it could well take you longer than you think, to get it right, so if you want it for winter, start early. It'll take a while to dry and there's a lot of moisture given off. Much better with doors open. Don't ask how I know this.
2 Before you start, invest 18$ and download Ianto Evans book which is the bible for rockets. E &E's book wasn't available when we built, but we backed their Kickstarter and recommend their book too. It's money well-spent, considering the amount of graft and money you will put into the build.
3 Read 'Rocket Mass Heaters' book/ Erica and Ernie's book from cover to cover and make notes in the margin so you can find the bit you need later.
4 Read Ianto's book and Erica and Ernie's book from cover to cover again, so you can understand the postings you'll find in this forum, and the questions you'll ask show that you have done some research and understand the basics.
5 By all means get fired up about the many rockets you'll see on YouTube, but build your first rocket using the measurements in one of 'The' Books, they work, they are safe.
6 Paul, Erica and Ernie have produced a set of DVD's Seriously consider investing in them. They explain a lot of the theory and they convinced my significant other that this was a GOOD IDEA. I was psyching up for the hard sell routine, but he said 'we 're rebuilding' before we finished the videos ;o) Plus they are fun.
7 Watch the videos on the permies site and E&E's website to get a better understanding of what you're contemplating doing.
8 Before you post a question in the forum, take the time to search the forum. Others have probably asked your question. You'll learn a lot of information this way, and i got answers to questions I hadn't,t even thought of.
8 Bear in mind that not all of the the rocket designs on YouTube show you how they didn't work very well in the long term because of basic design flaws (because their builders hadn't read THE books?)
9 IMHO I reckon the chances are slim that you will build your rocket for 'nothing'. It is possible, if you are lucky enough to track down everything you need, but we decided that we,d have to dig into our wallets if we were ever to start it, let alone complete it. We bought about €350 of flue. ... And then two weeks later were given some free second hand stainless steel parts. But not everything we needed though.
10 Since the Ianto book was first written, new ideas have been tried and tested and are worth incorporating: lose the feed box ash pit (it burns out) and lose the small barrel (it can fill with smoke) Erica recommends leaving them both out. Also take a look at the additions Peter de berg has made (at www.rocketstoves.com) We added his steel plate to the feed tube. It allows for ventilation and protects your first fire brick from damage when loading wood. we found it works. We also made the transition bigger by cutting a rectangle out of the oildrum. (Search for transition(?) plenum(.) a great thread has been recently started on that. I found that part of the build the most challenging, but not impossible.
11 Before you build, find out exactly what the adjacent walls are made of. If they are of combustible material, it is good sense to move the rocket to a safe distance, and/or make the wall safe by adding appropriate cladding. Same for the floor.
12 Check the building codes for your area, working to them may help you to make your installation safer.
13 It has been suggested more than once on this forum, that if you have a fire in your house, (even if the rocket didn't cause it) your insurance company may not honour a claim if they see you have a non-standard, non-code compliant rocket installed. If this is likely to cause you problems, perhaps you should find out ahead of time or build to code compliance? We made the decision that if it came to it, we needed a RMH more than insurance. Our barn is an old milking parlour, with 18" thick granite walls so no fire hazard there. We showed our finished Rocket to our insurance man, showed him build pictures and explained fully how it works. He said he was satisfied, thought it a great idea and classifiable as a woodburner, which reminds me, I should get that in writing!! The local firefighters were impressed too - I hasten to add that they were on a fundraising visit not here to put out a fire.
14 I didn't build a mock-up and fire outside, but in 2007 we'd started one inside and got as far as fitting the oil drum, so I did understand what I was doing and we built to Iannto's exact dimension. Even so, be prepared to rip your build apart before it's even complete, you may make mistakes. With that in mind, I'd recommend using materials that will come apart easily. I thought I was doing the right thing in using refractory mortar for my burn chamber. Until I had to take it apart because we weren't happy with the placement. Chipping off the mortar damaged the (expensive) firebricks. I used clay mortar after that. The second strip down was a cinch.
15 I recommend you get an understanding of how materials expand and contract when they are heated, Build expansion joints into your burn area. I couldn't,t find definite answers to where, how and what. So I ended up using more firebricks ( I reduced the number of different materials, as they each expand at different rates) I added thin cardboard to the vertical and horizontal mortar. No idea if it worked/was even needed! The one house brick I used in the feedbox (ran short of firebricks) cracked and had to be replaced.
16 Do not underestimate how much digging and mixing of cob you'll be doing. I humbly disagree with a previous posting that it doesn't take much! 'How much' is subjective, the fact I'm nearly sixty and the old man hurtling toward seventy, may have coloured my perspective. (Even with the help of a cement mixer.) Halfway through I began to think I'd bitten off more than we could chew. I couldn't tell him though and we just kept makin' that cob. LOL the amount you need will depend on your bench size and how many rocks you can build in.
17. It's definitely more fun if you do this with a crowd - I posted on a local forum before our first build in 2007 and 14 people turned up none of whom had ever seen a RMH let alone built one. Me included. IMHO You'd need to be very organised and have a well conceived plan of action. I spent the weekend moving from group to group facilitating, and didn't get to play much! Feedback from everyone was that they'd had a great time, but I was a teacher in a previous life which helped. When it came to the 2015 build, we decided to go it alone so we could work at our own pace. It was hard work but we had a real sense of achievement when we fired her up.
This is just my four pennies worth. I'm not an expert. I have started and ripped apart 3 and completed the fourth. Love it! No regrets, life in winter in Europe is more than bearable now. Would I do it again, Absolutely, no question: we use less wood; have downsized the ash bucket; we can now use our tree prunings; no more tarred woodburner door; no more cold mornings; we have 24hr warmth on two 2 hour firings each day; always starts easily; easy to repair/make cosmetic changes... we raise seedlings on it ... I tuck my clothes under the cushion so they're warm in the mornings, and, as a result of digging up the clay, we have the foundations for a traditional Normandie Wood-fired bread oven.
My advice is just go for it, I reckon anyone who can read and is fairly able-bodied can do this, don't try to reinvent the wheel, stick to what works. (Says she who added an oven on the front!)