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Posts: 8
Location: SW VA, climate zone 4
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Been reading all I can for a while. We are in process of renovating a home we just purchased. Pretty excited due to the prospect of being able to have a wood burning something. Planning on putting a RMH in our basement as a place to hang out around, read books, and gather when friends come over in the winter. Plans are pretty much "by the book". Fire brick stove, 55 gallon drum, insulated riser, cob mass bench, and feeding that into the existing centrally located chimney. Chimney is pretty normal at probably 30 feet ish. Planning on putting the J style burn box and barrel near what used to be the exterior wall made from cinder block as an added mass for heat absorption. That will feed into a bench of about 8 to 10 feet long and then exit into the chimney. Fire brick will be sourced from a local refractory (45 minutes) supplier for the main feed, chamber, and riser (price is about $2.50 per brick, 2700 degree brick). Have a friend who is an excavator and says he hits clay all the time and can give me all I need and then some. I have no idea the quality of the clay, but it's free clay and a good starting point for my mass. I am currently building a "proof of concept"flue size in our back yard out of regular masonry brick that was demo'd from inside our house (gas fireplace surround) just to see if I get the idea of building the guts.

Current questions are:

Would an 8' truck bed of clay be enough for the whole mass?

Can the guts be built well using clay slip instead of mortar? (Donkey on that forum stated just dipping each brick into clay slip during the build)

I think my flue size entrance to the chimney is only around 8", so would 8" flue in the mass be sufficient or should I use something like 10" and neck it down to 8" right before the chimney.

I have taken university physics a year ago so the concepts of the laws are still somewhat fresh. (Changed my world BTW)

FULL DISCLAIMER: I am aware of basement RMH being frowned upon. It's the best I can do. It won't be for whole home heating and not really supplemental heating, but more so heating to keep it toasty warm in the winter months.
 
gardener
Posts: 2581
Location: Upstate NY, zone 5
92
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The quantity of clay you need will depend on how big a mass/bench you plan to build, and the character of the clay. Figure the length x width x height of your planned bench (subtract the volume of the ducts if you want to be precise) and compare this cubic foot answer with the cubic feet of the truckload of clay. Also, if the clay is relatively pure (not much sand already) you will want to add sand to make cob. This could double the volume of your original clay, more or less, depending on the results of your testing of bricks made from varying proportions of clay and sand.

Clay slip, or refractory cememt, is recommended for your brick areas. Regular (portland cement based) mortar that you can buy at the hardware store will not work at all; it will disintegrate from the heat if used in any of the combustion core.

The system should maintain a relatively constant cross-sectional area. If the combustion core is the equivalent of 8" diameter (e.g. 7" x 7 1/2"), you should maintain that except in the barrel downflow area where it can be larger, and the manifold where the gases exit the barrel to the ducting, where it must be larger (2 to 3 times the CSA).



Look at the new thread by Erica Wisner on "Top Questions" for a lot more answers and information you will want to know.
 
Josh Whited
Posts: 8
Location: SW VA, climate zone 4
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That makes too much sense to measure and compute how much mass I will need.

I have heard it is better to have the burn chamber slightly smaller than the feed tube and riser.
 
Glenn Herbert
gardener
Posts: 2581
Location: Upstate NY, zone 5
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Only slightly. You should probably shoot for the same dimensions, and round down if necessary for the burn tunnel.

BTW, you said "used to be the exterior wall" Hopefully it is now an interior wall or heavily insulated on the outside, or you will be pouring heat into a bottomless heat sink. An interior masonry wall is ideal.
 
Josh Whited
Posts: 8
Location: SW VA, climate zone 4
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Well at some point in time the built onto the back of the home. So the back wall in the basement is about in the middle of the house. There is crawlspace behind it so the heat would go into the crawlspace and seep up into the kitchen and master bedroom. The RMH is just for supplimental heat. While yes, it would be giving heat away it's not a complete loss. I want to do a RMH to burn for maybe an hour when I get home from work and let the mass keep it nice and cozy in the basement and keep my gas bill down. I am installing a hybrid heat pump water heater that will be close to the RMH as well so the heat from the RMH will also help heat my water in a sense. The heat pump water heater will be very efficient extracting the heat from the air during the winter and putting that into my water. If the basement is 75 to 80 degrees then the water heater will thrive extracting that heat. I will most likely follow Ianto's book very closely to build a tried and true RMH.
 
pollinator
Posts: 4154
Location: Northern New York Zone4-5 the OUTER 'RONDACs percip 36''
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books fungi hugelkultur solar wofati woodworking
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Josh : Go for it! Please take lots of pictures and keep us informed ! Big AL
 
Josh Whited
Posts: 8
Location: SW VA, climate zone 4
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Thanks. There's a local scrap company (think American Pickers-ish) that I have an email out to to see if they might have any used fire brick from where they might have demo'd or scrapped something like an old pizza place, or a boiler somewhere. Hoping to score some cheap fire brick. The book states 60-70 bricks for everything. Is this about right? I have been asking for 100 at various places.

Any advise on where to find cheap fire brick?
 
gardener
Posts: 318
Location: Buffalo, NY
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Hello Josh,

Perhaps you can ask construction/remodeling companies. If they have done a tear down and rebuild of a fireplace they might give you some of the broken or damaged bricks. Yet, broken or damaged bricks will not last long in a RMH. Perhaps you can consider that if a good brick will last 10 to 20 years then you are only paying pennies per year per brick for warmth!

70 bricks is a minimum for me. If you break one or two during a wet saw cut or masonry split then it is nice to have a spare.
 
You'll never get away with this you overconfident blob! The most you will ever get is this tiny ad:
five days of natural building (wofati and cob) and rocket cooktop oct 8-12, 2018
https://permies.com/t/92034/permaculture-projects/days-natural-building-wofati-cob
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