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rocket mass heaters

I have a theory (about my RMH)

Mark Bowers

Posts: 22

Location: Just south of the center of Vermont

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posted 4 years ago

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A few months back I posted about my RMH having too much draw.

It still does.

It is difficult to light, but more importantly, I believe that this excessive draw is blasting too much heat through the mass too quickly and wasting a LOT of heat up my chimney. This is also causing us to burn about triple the amount of wood I thought we would (and planned for). We are having to run fires longer and more frequently in order to get the mass up to a reasonable temperature. On a day like today--snowy, little to no sun (and no solar heat gain), outside temps in the 20s during the day and single digits at night--I started the fire at 5:15 am and it ran until 9 am. Many armloads of wood were used. Wood is a mixture of spruce, maple, and birch. It's pretty dry and is cured on top of the RMH mass prior to burning. The fire was restarted at midday and run for an hour or 2. It will be run again this evening from 5 pm to 9pm. This morning it raised the inside temp from 45 to 51. Midday it should rise from high 40s to mid 50s, and this evening I should be able to get the temperature up to the 60s.

My theory is that we can capture more heat by doing the following (later, once the ground thaws and we can access clay, etc.):
1) increase mass by extending bench to where it goes vertical
2) add another 2" or so of mass to bench so that it is not boiling lava hot, but more temperate
3) install more mass throughout house (more clay plaster on walls, radiant floor, etc.)
4) wrap fluid-filled copper coil around some of vertical chimney to heat radiant floor slab in bathroom

...and of course, most importantly, finish construction of home--install final roof materials, complete interior and exterior plastering, check all material transitions (floor to walls, walls to windows, etc.) and augment/upgrade as necessary.

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Michael Cox

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Location: Kent, UK - Zone 8

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posted 4 years ago

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You could test your theory by using a temperature probe in the exhaust where it exits the building and compare it to other, known to be well functioning, RMHs?

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Glenn Herbert

Rocket Scientist

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Location: Upstate NY, zone 5

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posted 4 years ago

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An immediate way to mitigate the excessive draft would be to mostly cover the feed tube with bricks or a piece of cement board after loading, to a point where you still hear the rocket but not too loudly. Trial and error will get you to a good place. Of course covering the feed completely after the fire is out will also be important to avoid sucking heat up the chimney all night.

I think any of these mass-extending or augmenting ideas is likely to be helpful in extracting more heat after you reduce the flow to what the fire needs. Too much draft is likely to be diluting and cooling the fire in the core, reducing the combustion efficiency, so I think you may always need to damp the feed when running. A damper in the stovepipe, to be adjusted to reasonable flow and fastened there, may be a good idea. I would set this to the generous side of flow, and adjust daily at the feed.

Mark Bowers

Posts: 22

Location: Just south of the center of Vermont

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posted 4 years ago

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I typically restrict the feed to 3-4" after the fire's going. I keep it like that unless feeding the fire. We completely close the opening with 2 bricks when we're finished running it. We've experienced what happens when we leave it open--far too cold...

I will be setting up some thermometers soon and we'll be able to track actual temps for further research.

Satamax Antone

gardener

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Location: Southern alps, on the French side of the french /italian border 5000ft elevation

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posted 4 years ago

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Use bells!

Well, if you use a bell, it's like a siphon in reverse, it traps the heat. Shame you have a window and a wooden wall where the rising pipe goes up. As that would have been a good spot for experimenting.

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Mark Bowers

Posts: 22

Location: Just south of the center of Vermont

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posted 4 years ago

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Yeah, the RMH and house were designed prior to my reading Ernie and Erica's RMH book. I had read Ianto's book and was planning on sending the chimney out the wall between the windows into an attached greenhouse. Upon further reflection and research, I decided that our home was (unfortunately) more conventional in design and needed to follow Erica and Ernie's advice. Hence the less than ideal corner.

Glenn Herbert

Rocket Scientist

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Location: Upstate NY, zone 5

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posted 4 years ago

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Examining your photo, I think you could add a tall thin bell at the end of the bench where the existing stovepipe rises. Is the cooking stove in its final position, or could it be moved a foot or so away from the bench? You mention "extending" the bench - how far is it from the wall now? A bell with inside dimensions 8-10" x say 30" x 6' high, and 4 or 5" walls, would give significant added mass and absorption and radiating surface, and be only 20" thick from the wood wall.

The existing bench connection to the stovepipe can simply be removed and feed into the bell cavity, and the stovepipe leading to the chimney could be rerouted to come from the bottom of the bell next to the kitchen stove, or as a plunger tube dropping directly down inside the bell to around 6" from the bottom. In case of a plunger, you would need to swell out the cavity so there was adequate circulation space around it, I think at least 4" clear full height. I would put the plunger tight against the exterior side.

I would make the exterior wall of the bell with a solid brick wall and a layer of rockwool board and an airspace so the exterior house wall doesn't get dangerously hot. With the bell at the end of the duct circuit I don't think there is likely to be hazard there. How hot does the current stovepipe get?

Mark Bowers

Posts: 22

Location: Just south of the center of Vermont

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posted 4 years ago

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The chimney pipe is a little too hot to touch for longer than a quick swipe. No burns yet.

Glenn Herbert

Rocket Scientist

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Location: Upstate NY, zone 5

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Okay, that sounds like there is enough heat remaining to be worth building a bell at the end of the run, and not so much that a bell close to the wall would be hazardous.

Daniel Ray

gardener

Posts: 501

Location: Victor, Montana; Zone 5b

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posted 4 years ago

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That does seem a little too hot if you cannot keep your hand on the stovepipe. My cleanouts on my bench are to hot too hold my hand on, but I can keep my hand on the single wall that goes up to the ceiling.

Another option not mentioned is to extend the initial bell that houses your heat riser. It isn't ideal to increase the height of the barrel as you don't want it too close to your ceiling is, but you could get much more ISA with adding a quarter or half barrel on top, clamping them, then adding mass by cobbing up the sides of the barrel with a little bit of refractory wool or paper to protect the cob from cracking. Either that or you could take the current barrel off and just raise that with bricks or more cob.

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Mark Bowers

Posts: 22

Location: Just south of the center of Vermont

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posted 1 year ago

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2 years later and I can attest that a lot of our heat issues stemmed from heat leaking out our floor and western wall. We have since built a small addition on the west side of the house that is super tight and well-insulated. We have also skirted our pier foundation and will be berming it later in 2024 (currently it is bermed with snow and the difference is extraordinary). We obviously were leaking a lot of heat.

Most days we now only run 2 fires--first thing in the morning and in the evening. Starting house temp is typically around 50 with it rising into the mid-60's within a few hours. Mass temperature remains warm all day at 90+ degrees. Evening fire is run for a few hours and shut down right before bedtime. Temp typically goes from 60-70 degrees.

We still have a super cold corner of the house. We haven't yet finished the construction there so insulation is sparse at best. 2023 was supposed to be the year we completed that, but then it rained all of July and August. 2024 has to be the year it's done.

I just recently purchased a digital infrared thermometer and initial scans are revealing additional weaknesses in our building envelope.

Andy Ze

Posts: 69

posted 1 year ago

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My first thought upon looking at your original image is that having the thermal mass attached to the wall isn't a good thing. An air gap between your thermal mass and the wall would probably help a lot. (Air doesn't conduct much heat.)

When you insulated the outer wall, it accomplished the same thing, but it probably took more work than leaving an air gap would have.

Mark Bowers

Posts: 22

Location: Just south of the center of Vermont

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posted 1 year ago

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We did leave a 6"+ gap in between the plastered wall and the bench seat back (it's not super-obvious from previously posted pictures). Our RMH is built on a brick foundation which has air channels and we get a decent breeze drawing under the RMH towards the rear of the bench and then up that wall. Both windows are non-operational, but the one closest to the barrel is the least drafty. The other one near the chimney needs to be rebuilt and re-cased.

This picture shows the beginnings of the air gap behind the RMH. We made tubes out of hardware cloth and then cobbed around them.
RMH build showing air gap

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