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Can I build a RMH here?

 
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Hello! I would very much like to build a rocket mass heater in my home for all of the lovely conveniences that come along with them. I recently bought Erica and Ernie’s book and have been reading through it. I’m here trying to collect information for my specific situation to see if one of these gems is a viable option.

My situation is as follows:

The main floor of my house, and where I’d like to put the RMH, is suspended over a basement with wood subfloors. The tentative plan is to install earth floors (because I love them) and also to build a rocket mass heater where the current wood stove is. It is important for me to admit that I am not a construction worker, or an engineer, or an architect. I’m pretty generally inexperienced at the ripe age of 22. I do have access to family members who have worked in construction their whole lives so I’m not at a total loss.

I understand that all that earth is going to be heavy. I also understand that wood is combustible. The Wisner’s book outlines a similar example where the homeowners built the actual stove part of the heater up away from the wood floor that they wanted to preserve on “feet or air channels”, and I believe also reinforced the infrastructure of the wood floor. With an 8”x8” beam under the stove, and piers 🤷‍♀️

Looks similar to my situation in a couple ways. Even though we want to put earth floors over the wood subfloors, is 3/4” to 1” of clay, sand, and straw enough of a barrier to keep the wood underneath it cool, or would it be a good idea to put the stove up on air channels? I have observed the structure holding up our floor and I see two huge beam and lots of smaller beam things running across the big beams. I’ll add pictures so you can see. There’s also a wall directly under where it would be very convenient to put the mass part of the heater.

Thanks for taking time to read my situation, any advice is welcome, and I’m happy to clarify!
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This is what’s happening directly under where we want to put the stove
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there are two of these large beams running length-wise all the way across the floor, one of which is about 2 or 3 feet from the front side of where the heater would be.
 
gardener
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What is the floor under all those joists and pipes?
 
steward
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Hi Abigail, the beam (second picture) should be very strong so the floor right above that should be able to hold a lot of weight.  The smaller boards running perpendicular to them are the joists.  They look kind of small to me but it depends how long they are.  How far is it from the beam to the next beam or outer wall?

I guess irregardless of that, the two joists underneath where you want to put your RMH have been severely weakened by the plumber (or DIYer) who ran the PVC drain through them.  Those huge holes they drilled in the joists have pretty much ruined them.  I would be very worried about even having a bath tub above those two joists.  Empty tub would be ok but not filled with water.

I'd be tempted to have that PVC drain line moved down so it runs below the joists and then add a second joist alongside each of the existing ones.  It's called "sistering" when you add a joist to an existing one to strengthen them.  Sometimes it's easy to do, other times it's hard, depending on if there are wires or other things running through the joists.  If you can strengthen them, the floor should be strong enough for a bath tub, dance party, or earthen floor.  Maybe a RMH but it's hard to say..
 
pollinator
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Everything Mike Jay said, and...Oh my gosh! That pipe drilled through the joists!!
So the rule is the hole can only be 1/3 of the joist depth and not within 2 inches of the top or bottom of the joist.
 
Abigail Abts
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Mike Jay wrote:Hi Abigail, the beam (second picture) should be very strong so the floor right above that should be able to hold a lot of weight.  The smaller boards running perpendicular to them are the joists.  They look kind of small to me but it depends how long they are.  How far is it from the beam to the next beam or outer wall?

I guess irregardless of that, the two joists underneath where you want to put your RMH have been severely weakened by the plumber (or DIYer) who ran the PVC drain through them.  Those huge holes they drilled in the joists have pretty much ruined them.  I would be very worried about even having a bath tub above those two joists.  Empty tub would be ok but not filled with water.

I'd be tempted to have that PVC drain line moved down so it runs below the joists and then add a second joist alongside each of the existing ones.  It's called "sistering" when you add a joist to an existing one to strengthen them.  Sometimes it's easy to do, other times it's hard, depending on if there are wires or other things running through the joists.  If you can strengthen them, the floor should be strong enough for a bath tub, dance party, or earthen floor.  Maybe a RMH but it's hard to say..



Invaluable perspective, thank you so much! Upon inspection, the big PVC goes to an old vacuum system that doesn’t work anymore 🙄 so that can definitely be removed easily, and behind it there are also pipes that run from the water heater, so that maybe a bit more challenging, but I’m sure we could make it work!

So the joists run all the way across the floor, which I would say is about 25 feet. The big beam is about 7-10 feet from the exterior wall and about the same to the next big beam in the opposite direction (under the middle of the house).

I’m sure we will do some beefing up of those compromised joists either way, not the most comforting thought to have a weak infrastructure even if we aren’t adding weight upstairs. In the process of reinforcing those joists, could we do so in such a way that would make it more likely to accommodate the weight of a RMH?
 
Abigail Abts
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Kenneth Elwell wrote:Everything Mike Jay said, and...Oh my gosh! That pipe drilled through the joists!!
So the rule is the hole can only be 1/3 of the joist depth and not within 2 inches of the top or bottom of the joist.



Yikes! Someone definitely did a number on these floors, huh. I hope I don’t ever find myself in a situation where I have to DIY my way through some plumbing, but you never know! If I do, I’m happy I know this! Thank you!
 
Mike Jay
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Abigail Abts wrote:So the joists run all the way across the floor, which I would say is about 25 feet. The big beam is about 7-10 feet from the exterior wall and about the same to the next big beam in the opposite direction (under the middle of the house).

I’m sure we will do some beefing up of those compromised joists either way, not the most comforting thought to have a weak infrastructure even if we aren’t adding weight upstairs. In the process of reinforcing those joists, could we do so in such a way that would make it more likely to accommodate the weight of a RMH?


So if I'm understanding you, there are two big beams that run the same direction.  And they roughly evenly divide the span of the joists?  So each joist is only going about 7-10 feet before it gets to rest on a beam or the outside wall?  If so, then they probably aren't undersized for the floor.  They're smaller than mine, but my joists have to support 14' of floor.

It's great you can remove the pvc, that will make sistering joists a bit easier.  If you can take more pictures of the joists in that area, we could give you some more ideas about how easily the floor can be beefed up.

Keep in mind, an earthen floor is adding weight so you may not want to do that until the floor support is fixed up a bit.  And in fixing the joists, the floor might move up or down a bit which could flex the floor coverings.  

Don't worry, we're here to help you figure this all out!
 
Abigail Abts
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Mike Jay wrote:
So if I'm understanding you, there are two big beams that run the same direction.  And they roughly evenly divide the span of the joists?  So each joist is only going about 7-10 feet before it gets to rest on a beam or the outside wall?  If so, then they probably aren't undersized for the floor.  They're smaller than mine, but my joists have to support 14' of floor.

It's great you can remove the pvc, that will make sistering joists a bit easier.  If you can take more pictures of the joists in that area, we could give you some more ideas about how easily the floor can be beefed up.

Keep in mind, an earthen floor is adding weight so you may not want to do that until the floor support is fixed up a bit.  And in fixing the joists, the floor might move up or down a bit which could flex the floor coverings.  

Don't worry, we're here to help you figure this all out!



Thank you for the support!!

I’m back (finally) with some pictures.

Also, yes you understand correctly. I’ve just measured and it’s a little less than 7 feet between the exterior wall and on beam, about 9 feet from beam to beam, and then again about 7 feet from the second beam to the exterior wall.
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The wires running through here are also to the old vacuum cleaner, so they can be removed
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This seems like it will be the problem child.. wires run through these holes and they are not very easy to trace. These pipes go to the water heater.
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I’ve found that this room has this issue and...
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And this room. The wiring and plumbing for hot water run through 5 joists here and three joists in the laundry room with the water heater. If I am correct, this section is either under or nearly under walls.
 
Mike Jay
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Ok, that's a big pile of fun you have there  Wires are always the problem....

I'm thinking that it would take a construction gal (or guy) to look closely at it to see if they can sister joists or not.  I'm guessing so (even with the wires there) but it's really hard to tell.  For instance, a person could support the floor, cut a slot out the bottom of the joist to let the wires out the bottom of the joist, then sister on a new joist and leave the wires running under that new sistered joist.  

I had one thought for RMH support.  If the weight was predominantly above the shower curtain, you could add a beam that runs under both joists and across the top of the curtain bar.  It would be supported by a post at each end (either side of the shower bar opening).  That would give a ton of support to those joists at that point (near where the vacuum tube ruined them).  And you wouldn't need to sister them.  And/or you could sister them but just from the ends of the joist to that new beam (meeting on the new beam) to avoid the wire issue.

This probably isn't making much sense.
 
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From the look of the diagonal floorboards (not plywood) sitting on the joists, I would guess the house is 70-100 years old. The joists look like maybe 2x8s, which is good for the spans you describe, and the pipe holes are just barely within the "1/3" rule, but it looks like the joists are at least 2' apart, maybe more. What is the actual joist spacing? If the big holes are somewhere near the center of the span, the bottoms will be in tension and the tops in compression, with relatively little shear (which tries to rip the hoist apart along its grain, and is what you get most of near the supports.)

Sistering a joist that is not endangered by shear can be simple; you can just add 2xwhatevers along the top and bottom of the joist, ideally glued with carpenter's wood glue (Titebond II) and screwed. Adding a 2x3 to each side of the top and bottom of each joist, flatwise like an I-beam, will give you plenty of strength without messing with pipes or wires or temporary shoring (unless a joist is sagging now).

I have 2x10s at 16" on center under my living room, and when I changed my plans to include an RMH, I needed to support a 3' x 6' mass bench parallel to the joists and separate from the bell foundation. The footprint is only supported by three 12' long joists, but I was able to cut a hole in the exterior wall and a couple in the basement ceiling, and snake several 2x4s and 2x3s in, missing the wiring running through the joists, and glue and screw them to the joists to reinforce them.

With all the joists that run beneath the RMH footprint reinforced, I think you will be able to safely support it for the long run. An additional beam where the shower curtain runs would be good insurance.
 
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