Proper siting and structural reinforcing for an RMH

Alright folks, today I had the unhappy experience of looking at my credit card statement and seeing a $528.00 charge from our fuel oil provider. We are supposed to be on a monthly flat rate fee that averages our use out over the year. Turns out their system hand an "error" and they "can't" issue a refund or credit for this erroneous charge, but even if the charge had been the correct amount of $195.00/month, that still comes out to $2,340/year that we are burning just in fuel oil. We also have a Buderus gas furnace for the upstairs part of the house. Bottom line: thousands of dollars down the drain and tons of CO2 up the smokestack. Bad time for our wallets and bad time for the Earth.

SO! I am now more motivated than ever before to build a Rocket Mass Heater. I've known for a while this is a project I want to undertake before I get too old and crotchety (I think I'm just crotchety enough right now). But whereas a few months ago I was like "Oh, you take some cinderblocks and bricks and slap cob over them plus also a 50 gallon steel drum because why not, and then you burn stuff and MAGIC HEAT HAPPENS", now I realize that the amount of weight we are talking about concentrating in one small corner of the home is substantial and may need structural reinforcing etc. etc.

So...first and foremost, I recognize that this is not Legal Home Advice Experts, LLC, so I'm not trying to ask for anyone's legally-binding architectural or structural engineering advice on how to put one of these babies in. What I am asking for is personal anecdotes from when you built one, success and failure stories, tales of floors that have caved in under the weight, tales of floors that have NOT caved in under the weight, and the criteria you used for positioning and designing your build.

Specifically, I live in a home built in 1860. It is a fine piece of architecture and made with extremely sturdy felled timber that I think will be here many generations after I'm dead and composted. My house is done in an old-school Colonial style, so the main part of the house is a large rectangle that is subdivided into 4 more or less equal dimension rectangles sort of like tihs:


                           Rear of home (South), larger yard with deciduous trees for windbreak in winter and shade in summer (prevailing wind in our spot is sort of south-westish especially in winter)
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                         Front of home (North), faces road, small front yard



Ok....so what I'm trying to figure out is first of all, where is the best place to put an RMH generally speaking? In the most recent dailyish email they were suggesting that seating arrangements ono the INTERIOR walls of a home are generally the warmest because that's where your heat ducts generally are located...is it generally a good rule to put an RMH in the CENTER of the home and allow the heat to radiate more evenly throughout? But then what is to be done about venting the exhaust? I assume you don't want oodles of CO2 just piped into your home, though it would be a fun way to have a cult go out with a bang! So I assume if I put it on an interior wall, I have 3 options that all kind of suck:

1. run a big, ugly vent pipe across the room and out an exterior wall
2. run a big, ugly vent pipe up through the ceiling and out the top of the house which means putting a hole in the roof which means dealing with leakage and such
3. run a big, ugly vent pipe down under the ground through the unfinished basement and out a vent by one of the basement windows

Might all 3 of those options require a fan to ensure ventillation? I know the rockety-ness of the RMH offers some degree of force on the exhaust to push it, but that force is, I assume, not capable of infinitely resisting the pull of gravity, so for a ground floor vent system trying to pump out 2 storeys...I don't know...I guess chimneys do that and they are a hell of a lot colder than RMH exhaust, so maybe I'm making much ado of nothing here.

Point is, venting would be harder on an interior wall, no?


So...what if I put it on an EXTERIOR wall then? Am I going to lose tons of the efficiency of the RMH system because the mass is now in close proximity to the outdoor temperature extremes? I assume I'd be better served putting it on a south-facing wall so that I can leverage some passive solar gain even in winter from the scant amount of oblique sunbeams? I note that all of our gas furnace radiators on the upstairs are located on exterior walls rather than interior, and I remember the guy who services them saying that you want that because it ensures drafts from windows are not particularly awful as there is a lot of warm air radiating from the vicinity.



Ok...so let's say you've humored me this far in the post and don't think I'm a total nutter. Cool! Thanks for being here. So by now we've picked out interior vs. exterior wall and north vs. south (or east or west, I suppose). Now I need to know what I need to do to keep this RMH from murdering my house from sheer mass strain on the floorboards and what-have-you.

As I said, I'm in an OLD house: she's 160 as of this August. And the foundation is possibly older still because there was apparently a tavern/inn/coachhouse here that burned down and the foundation for it *may* have been reused to build this house. Point is, it's been around a bit and the basement is a bit of a hot mess: dirt floors, newer dimensional lumber boards that have been sistered to older timber that had some historical wood boring insect damage (no active infestation at this time and everything is structurally sound per the opinion of several contractors and building inspectors). But the foundation walls have settled over time and definitely need re-pointing in the next 5-10 years. BUT, previous owners have put in place something like 15 different metal poles footed in cement that goes down 4 or 5 feet into the ground, and the front of the home has several HEAVY duty cement buttresses keeping the front wall of the foundation from caving at all. So to my eyes as a non-architect, non-structural engineer, the place seems pretty shored up and reinforced.

But you know what I *DON'T* want to do? What I don't want to do is pile up 2,000-pounds of bricks and cinder blocks in a corner of what is otherwise a very nice parlor and then find that within a few weeks I have a Wile E. Coyote-style Rocket Mass Heater-shaped hole in the floor and a new Rocket Mass Heater feature in my basement. That would suuuuuuuuuuuck because the cost of fixing the wood floors alone would be astronomical and I'm like 99% certain homeowner's insurance doesn't pay out for "dumbass homeowner broke his floor with a DIY project".

SO! Is there anything to look for as I'm designing this build? Are there warning signs that I'm putting too much weight for the floor to handle that could tell me "STOP NOW, YOU FOOL!" before I do irreparable damage to the floor? Like I'm looking for a tell-tale sign that something is amiss before boards have loosened, nails have bent, foundations have shifted, etc.

I have a ton more questions, but this is probably more than enough for one post. Thank you for reading, thank you for sharing your experiences, and thank you helping me do the practical magic of learning from someone else's mistakes and successes. I will absolutely pay it forward by teaching my local group of permies how to do this, thereby making the world a more carbon neutral place, I hope!

Hi D.W.    
Always lots of options. First option is not to see the exhaust pipe as a big ugly thing. If you see it rather as a thing that saves you money and keeps you warm, then that's a good thing right?
I'm assuming your house does not already have its own chimney? If not, then through the roof is going to be your best bet. Through the wall chimneys is a legacy left over by Ianto Evans (one of the RMH pioneers) who had specific goals in mind but often compromised having to deal with poor draft and smoke back issues.

- Properly installed, roof chimneys don't leak which is not a hard thing to accomplish.
- I don't see interior walls having any effect on draft.
- Using fans should not be necessary to power draft. Fans can be good to spread the heat around in the house though.
- General recommendation for RMH placement is to put it in a place where you spend the most time (particularly with a J Tube) and as centrally located for even heat distribution. If your place is too big, then 2 or more smaller stoves can be considered. They just can't share the same chimney stack.
- Best place to install is closest to a wall that has no joist span issues vs in the middle of a room.

D.W. Stratton wrote:Alright folks, today I had the unhappy experience of looking at my credit card statement and seeing a $528.00 charge from our fuel oil provider. We are supposed to be on a monthly flat rate fee that averages our use out over the year. Turns out their system hand an "error" and they "can't" issue a refund or credit for this erroneous charge...

This response has nothing to do with what your post is asking about, but I want to point out that you can call the credit card company and request a charge back as far as the fuel company's billing error. I have never heard of a company claiming that they had a mistake in their system, overcharged, but can't/won't credit you or refund you. I believe that would be called theft. Credit card companies are very helpful if you call and tell them what happened, then show them whatever other proof they need so they can take the charge away from your bill.

Chimney and through the roof, chimney, chimney and chimney. That is the only answer. Nothing else. Venting through walls, window is just asking for trouble, or death. If you could take pics of your interior, it would be helpful.

You have wooden floors/joists?

But first, read this.

http://batchrocket.eu/en/

If you plan to heat, a batch rocket is cleverer i think.

Hth.

A batch box combustion core would be better if you can't put it where you will easily tend it for a couple of hours a day, and for more heat output for a given size of system (likely important in a good sized old house). A J-tube core is decidedly easier to build for a beginner. If you are mechanically inclined and fairly handy, go for a batch box.

A rule of thumb for bench-style RMHs is that if you could safely install a king-size waterbed, you can build an RMH in the same spot. If you do a vertical masonry bell as mass, that will be more concentrated and require a dedicated footing and posts/pier(s) down to the basement. Code requires continuous masonry all the way to the ground, but if you are not bound by that, steel posts and a good thick reinforced concrete footing will work fine.

Old (and new) radiators were placed on exterior walls to make heating more even, at the cost of wasting a lot of heat warming the exterior wall. The radiant warmth of an RMH will give good comfort from the center of the house. It might work well to remove part of a wall between rooms and put the bell mass in the gap, directly warming both sides. Photos of the areas involved would let us help assess the feasibility of particular locations and layouts. The location of major interior beams in the basement would also be important information.