Rocket mass heater in existing timber frame construction

Is it possible/worthwhile to build a rocket mass heater in existing timber frame construction. I recently bought a drafty older home on 19 acres. No heating system and it turns out propane (used previously) may be as prohibitive as electricity.

Square footage, about 1200, give or take. Saw a YouTube video with Erica and Ernie Wisner in which they mention building a stove in a rental unit.

My house has a substantial crawl space underneath. At the back of the house, you can walk straight in and for several feet, I don't have to crouch. I'm 5'7."

Now towards the front of the house where the heater would be the crawl space is substantially less. I'm guesstimating 3.'

So, my question is this. Boulder the rocket stove, given its Weight, have to have good support—say firm earth—directly underneath? Or is it possible to have enough support by beefing up framing and jousts where stove will be?

Yvonne; You will want to support the floor underneath your mass. pier blocks on firm earth (below frost point) 4x4 or better for added support. Depends how stout it is currently built, how much you might need to beef it up. Average weight for a mass is similar to a king size waterbed. Protecting your wooden floors from the heat of the mass is also an important consideration.

You can also cut the floor, and make proper foundations for the stove too. It's been done before, and there's some pics somewhere here.


Does your place have a fireplace?

No, no fireplace. Only real winter warm comes from south-facing concrete front porch. Sadly, no way of capturing that passive solar for building interior.

thomas rubino wrote:Yvonne; You will want to support the floor underneath your mass. pier blocks on firm earth (below frost point) 4x4 or better for added support. Depends how stout it is currently built, how much you might need to beef it up. Average weight for a mass is similar to a king size waterbed. Protecting your wooden floors from the heat of the mass is also an important consideration.

Please forgive me. I am a neophyte. don't know what "pier blocks" are? Or if "4x4" references lumber or something else.

i was entertaining two possibilities. The first was to cut out the floor where the stove and mass would sit. Then pour and ram layers of gravel into a form. After that either use rubble up to floor height or rammed earth up to floor height. from floor heigh up, would use rock, rubble or cob as per any other installation.

The second possibility initially seems rather shaky to me.(I am after all a neophyte I. The business of supports and reinforcement). Provide additional support in the stove area by sistering existing joists, add additional joist and add cross members yeilding a stronger, more stable base, capable of carrying a heavier load.

As a neophyte the built up earth seems infinitely more stable and reliable, but it does require substantial labor.

Hi Yvonne; Both possibilities that you mentioned, are good. They are in fact , the suggestions that max & I made. Cutting your floor making a proper foundation is hard labor , but more permanent , you would want to insulate your rmh from the earth . The pier blocks are small pre poured concrete blocks that normally are used to support and reinforce a floor from the ground up. 4x4 is a reference to lumber that would be used by placing on top of a pier block vertically to support a cross member spanning multiple joists. If you're not comfortable with your knowledge of carpentry have a friend who is come by and give it a look . After all it's not that much weight when spread across multiple joists. Are you located in the US ?

Yes, I am in the US. North Alabama.

Insulate from earth? To keep from losing heat to the ground below? what do I use for that? Wouldn't both earth and concrete be batteries for heat generated above. would straw work? Something else?

Yvonne; Yes , to keep from trying to heat the earth (can't do it ) you must have some form of insulator below your rmh. Using 4" hard foam is common when pouring concrete. Or a thick cob (perlite , clay and straw ) layer would also work and be cheaper. You want your heat to collect in the mass not under the floor.

If you had dry sandy soil under your floor, you might be able to use that as some mass, but damp soil or any groundwater will wick the heat away faster than you can put it in.

thomas rubino wrote:Yvonne; Yes , to keep from trying to heat the earth (can't do it ) you must have some form of insulator below your rmh. Using 4" hard foam is common when pouring concrete. Or a thick cob (perlite , clay and straw ) layer would also work and be cheaper. You want your heat to collect in the mass not under the floor.

That's a bit misleading. (I know of places where the ground is used as thermal mass, and I'm not referring to ground source heat pumps.) The problem in this scenario is that there would be air flowing all around the supporting pier convecting the heat away.

For a house with lofted wooden floor, where one wants to heat the house and not the space and dirt underneath, building the RMH on an insulated foundation is indeed a very sound approach.

For my compact 8" indoor RMH, in a similarly constructed house as Yvonnes, I cut an opening in the wooden floor. The bare ground in the crawlspace was prepared and a steel reinforced concrete pad poured. Then a "box" constructed on top of that concrete pad consisting of 4 stacked courses of concrete block, reinforced with steel "ladder wire" embedded in the mortar of each course. The "box" then filled with clean #57 stone, capped with a mortared in place Hardi concrete panel, then a final concrete pad poured on top, to approximately floor level. The trapped air in the stone fill serves to insulate the entire unit from the ground and crawlspace air.

Thank you all for your comments. It really helps.

Hi Yvonne
Both your climate and housing style are similar to mine and many others in the south of Australia. I will be interested to hear how you go with your heating plans.

Erica spoke about some issues in "warm" climates https://permies.com/t/41595/rocket-stoves/Rocket-mass-heaters-Texas-warm

A lot (most) of the discussion about mass heaters is from people in cold climates (understandably) so I'm interested in what would be appropriate for more moderate climes

Current thinking around here is to reduce the drafts through doors, windows etc and to insulate ceiling walls and floors in that order of priority. This also helps in the summer.

Free standing wood stoves are the most common in my part of the world but their low efficiency and bad output make them less desirable that a mass heater

Alan

I'm trying to get a sense of your weather. Is it similar distance from the equator that determines similar climate? A lot of my friends from northern parts of the U.S. think Alabama is hot like Florida. But we're temperate with a definite winter and summer. I'm at about 33° 59' 22" latitude.

It's true here that, in the main, cooling a home in the summer (it sometimes feels like we have 6 months of summer) is more of a concern than heating it in winter, but heating is absolutely necessary here.

My house was not built to profit from solar gain. It almost faces due south, but the windows on the front are shield from the sun, winter and summer by a covered porch. So heat builds up on the slab concrete front porch, but none of that heat is available to the house's interior. This past winter was colder than average and I felt it as I had nothing but space heaters. You don't need long, consistently bitter winters to want to be warm when it's cold outside. House is set up for propane, but I would rather not be dependent on that.

Plus, the house sits on 19 acres. Most of that is steep slopes, unsuited to neat garden beds, but there is plenty of deadfall that could be harvested for firewood. Additionally, I'm in an area where people put out fallen branches and logs like trash. No shortage of fuel.

I have to some work to stabilize the floor, so reinforcing for mass heater at the same time is probably a good idea. I'll let you know how my project goes.

Hi Yvonne
Your climate seems very similar, I'm 37 degrees south. We have 4 seasons and definitely need heating in winter we get down to 40 F frequently. Summer temperatures are often in the 90s F.

With all that firewood a wood fired solution to heating is a no brainer.

I wonder how important the mass is in our climates as there are many times in the year when we only need heat at night particularly if the house is well insulated. Perhaps a double barrel would be sufficient rather rebuilding your floor to take the weight of the mass. https://permies.com/t/44001/rocket-stoves/Double-twin-heat-riser-heater

A setup with just barrels is not going to store heat for any significant time, no more than a conventional woodstove. You would be best served by a thin masonry/cob wrapper to the duct or barrel/bell. That will let the heat start radiating within a couple of hoursof starting the fire, and last for maybe 6-12 hours depending on details. You would not want to run an RMH for hours on end, as they can't be throttled back and would bake you out on moderately cold nights. You would most likely burn it for one or two hours in early evening, then once it's fully out seal it up for the night.

That leads to the question how much mass is enough? The weight issue can be critical for those of us with existing floors.

I guess I would want a system that was responsive but would still keep the chill off overnight. I noticed Peter used bricks in a barrel as a mass storage device https://permies.com/t/40007/rocket-stoves/Results-batch-box-thingy-Innovators

Yes, the bricks in a barrel is a good idea. You could alter the mass if you found you wanted to without fuss or mess.