Rocket Mass Heater and PAHS

While performing some high-brainpower work today (painting a concrete building), a whole bunch of ideas I've been reading about here (and there) came together ... how to stack functions (and solutions) with PAHS and the rocket mass heater.  Attached are two sketches adapted from one of my Sketch-up self-education projects ( ) showing the front/top of a notional PAHS home built with concrete or blocks with a rocket mass heater inside against the back, curved wall.  Sketch two shows the backside of the house and the rocket heater exhaust path within the insulated PAHS soil, with the addition of a "bypass" route through an insulated water heater.  The bypass would be switchable using a vent.

This seems to give a way to add heat to the PAHS mass at your own discretion/need or when using the rocket heater for cooking, etc while also allowing hot water using the bypass.

Any merit to this?  I've been wondering how to put in cleanouts.  Would the vent/switch thing work or would it be a soot collector?  Would the PAHS mass get too hot and drive you out of the house?  Could you reduce the absolute PAHS mass based on the regular heat input due to cooking tasks?    Ideas/improvements?   

Bill

Cool drawing!  How did you draw that?

First off, have you seen the thread around here about heating water with wood?  I know that Ernie and Erica say they won't teach anybody how to do it (although they will come to your home and do it).  Seeing the mythbusters thing with the exploding hot water heater should give you an idea of why.

You don't wanna have any bench inside?

As for exhaust:  I keep thinking that if you do this right, the exhaust tends to want to go downhill rather than rise - like in a chimney. 

I think I would want to plan for tubing in the house (bench?) followed by maybe something under the floor and then a bee-line for the outdoors via the path you suggested.  Maybe even go further downhill first.

Hi Paul,

Yes, I've been reading as many of the rocket threads as I can.  I definitely agree about the potential dangers of heating water ... all this stuff is just some concept exploration for now.  I've ordered the book, but will also be on the lookout for one of the Mass Heater classes. 

I've realized that the first pics have too long an exhaust run and the actual "house" is way too big.  I'm working on something much smaller using PSP, PAHS, and the Rocket heater.  With PAHS, the insulated mass of soil is actually inside of the house envelope, just on a larger scale than the bench. 

Good data point on the exhaust wanting to go downhill.  I'm also struggling a little with cleanouts.

These pics are made with Google Sketch-up.  I'm using the free version (for now), but they also have a "Pro" release with many more features and tools.  Here's a rough model of a Rocket Mass Heater set into the floor.  This reduces the height of the unit and I've added a 1/2" thick steel plate on top for cooking.  I'm thinking that the plate will function similar to the cooktop on my wood burning cookstove with different heating zones, etc.  I am imagining that the area directly above the insulated "chimney" will get very hot!

Maybe we should rename this thread "how I learned to use Sketch-up"?     I'm spending way too much time messing around with this when the weather is still so nice, lol.

Bill

I think the flat thing on the top would do the opposite of what you seek.  I think it will act as a heat sink and thus, be too cold to be useful for any cooking.

I think that by the end of the day I might be tinkering with sketchup too!  So far, the best feature appears to be that the word "ketchup" is right in the name! 

We had a mass heater class a few weeks ago - where were you?

9anda1f wrote:
Hi Paul,

Yes, I've been reading as many of the rocket threads as I can.  I definitely agree about the potential dangers of heating water ... all this stuff is just some concept exploration for now.  I've ordered the book, but will also be on the lookout for one of the Mass Heater classes. 

I've realized that the first pics have too long an exhaust run and the actual "house" is way too big.  I'm working on something much smaller using PSP, PAHS, and the Rocket heater.  With PAHS, the insulated mass of soil is actually inside of the house envelope, just on a larger scale than the bench. 

Good data point on the exhaust wanting to go downhill.  I'm also struggling a little with cleanouts.

These pics are made with Google Sketch-up.  I'm using the free version (for now), but they also have a "Pro" release with many more features and tools.  Here's a rough model of a Rocket Mass Heater set into the floor.  This reduces the height of the unit and I've added a 1/2" thick steel plate on top for cooking.  I'm thinking that the plate will function similar to the cooktop on my wood burning cookstove with different heating zones, etc.  I am imagining that the area directly above the insulated "chimney" will get very hot!

Maybe we should rename this thread "how I learned to use Sketch-up"?      I'm spending way too much time messing around with this when the weather is still so nice, lol.

Bill

Hi, Bill,

Thanks for the tip on Google's modeling program.

Since rocket heaters don't use a lot of fuel, the top of the barrel itself is generally a very nice temperature for cooking.  Ernie used to fry things on one at Cob Cottage Company, just by oiling the top of the barrel like a griddle.  Helps keep it from rusting, too.  I'm inclined to agree with Paul that putting a big slab of metal over it would result in a warm, but not hot, slab of metal.  We put a 3/4 inch stone cap on top of ours, and it will just barely simmer water.

Your insulated dirt wall reminds me of Ernie's favored building scheme, Cob-Bale.  Strawbale insulation, with an extra-thick cob or plaster inside (4-8") to hold passive solar heat.  With cob's heat capacity and slow conductivity, you wouldn't need more than about 12" max. to absorb and retain your day's solar heat.  And it's self-supporting; and depending where you are, it could be exactly the same as dirt, so don't mind me rambling.

As far as size and heat capacity - there are some tools architects use for this, and they rely on a pretty thorough understanding of the place the building is located.  Lattitude, average days of sun in different seasons, stuff like that.  You can build eaves to protect the building from excess heat in summer, with an angle that allows winter light in.  Your current design looks like it's trying to maximize heat, and there are very few places where that's a year-round ideal.

Cleanouts: there should be one near every sharp turn, but especially any 180 degree turns, and at the bottom of any vertical section that turns sharply (where ash might collect at the bottom of a vertical pipe).  We use the T-joints that are a standard ducting option; you could probably model them by plugging in another cylinder or pipe at right angles to something.  You can also use a T in place of a 90-degree elbow, with the extra leg sticking out in line with either of the sections it connects to.

What are PAS and PAHS?  Got any links I should look at?

Thanks,
Erica

http://www.ErnieAndErica.info

Hi Erica!

PSP is Mike Oehler's Post, Shoring, Polyethylene technique for underground housing
http://www.undergroundhousing.com/

PAHS is Passive Annual Heat Storage, a technique for storing heat in an insulated soil volume surrounding an underground house, designed by John Hait and the good folks at the Rocky Mountain Research Center
http://www.earthshelters.com/

I've been thinking about ways to combine the two, and the Rocket Mass Heater just seems like a natural complement to both ideas.  The initial thought was to put the heat from the RMH into the insulated soil around the underground structure instead of into the "butt warmer".  However, putting that heat anywhere within the insulated volume is effectively the same thing.

Where I live, it's cold in the winter and there are few trees (yet) for renewable firewood.  I've made up my mind to find a solution that will allow me to focus my main wintertime energies somewhere other than trying to keep the huge farmhouse I currently live in warm.  I partition off most of the house and warm a small area with an old wood cookstove, but it has no flywheel effect due to it's low mass, and though the firebox is small, I put a lot of wood through it last winter. 

It's also quite warm here in the summer and underground/earth sheltering will keep it cool inside.  So, one of my (many) goals is to build a PSP-PAHS-RMH home across the orchard from this house. 

Thanks for the real-world data on cooking.  My musings led me to expect otherwise and actual experience is helpful and welcome on this journey.   

Bill

Bill,

In another thread here I've been exploring ideas of adding levels of insulation to a rocket mass heater for cooking.  For an oven, think about having a big insulation thing over the top of the barrel. 

I think the thread is called something like "cotton vs. wool on the stove top"

9anda1f wrote:
Hi Erica!

PSP is Mike Oehler's Post, Shoring, Polyethylene technique for underground housing
http://www.undergroundhousing.com/

PAHS is Passive Annual Heat Storage, a technique for storing heat in an insulated soil volume surrounding an underground house, designed by John Hait and the good folks at the Rocky Mountain Research Center
http://www.earthshelters.com/

I've been thinking about ways to combine the two, and the Rocket Mass Heater just seems like a natural complement to both ideas.  The initial thought was to put the heat from the RMH into the insulated soil around the underground structure instead of into the "butt warmer".  However, putting that heat anywhere within the insulated volume is effectively the same thing.

It's also quite warm here in the summer and underground/earth sheltering will keep it cool inside.  So, one of my (many) goals is to build a PSP-PAHS-RMH home across the orchard from this house. 

Thanks for the real-world data on cooking.  My musings led me to expect otherwise and actual experience is helpful and welcome on this journey.   

Bill

You're welcome. I love being able to provide real-world data when I can -- so much of my efforts are speculation.

Thanks for sharing those links about underground houses and Passive Annual Heat Storage.  I'll definitely vouch for the comfort that thermal mass provides for year-round comfort: a thermal mass heated in winter, and allowed to cool somewhat in anticipation of summer, maintains a comfortable temperature indoors in spite of outdoor weather swings of 20-40 degrees beyond comfort.

Where the heat goes may make no difference on the actual temperature of the house... Yet one of the design elements of a rocket mass heater is to heat people, not houses.  Let people curl up on it, warm themselves efficiently and thoroughly by conduction. 
As you've discovered in your farmhouse, heating the whole house by 2 degrees is a lot less valuable for your own comfort, than heating yourself (or one small room) by 10 or 20 or 30 degrees.

Bodies in contact with the heat also gives the advantage that someone who is elderly, tired, has thrown out their back, etc. can get right into the warm spot and soak up that heat.  Someone else who is just coming in from the cold for a quick lunch break, and being very physically active, and may be feeling too warm already, can hang back in a cooler corner and not get overheated.

Just something to think about.


Re: Paul's musing on insulation for cooking on a rocket barrel:

Just be careful not to insulate the whole barrel. 
The temperature difference betwee heat riser and barrel is a critical part of the draft that pumps exhaust through the heat-exchanger.

  Some friends did this to a stove we helped them build, thinking it would improve draft (by making the barrel hotter); in fact, it did the reverse.  Only the combustion chamber and heat riser should be insulated completely; partial insulation to protect combustible framing or exterior walls is OK too.

If you want to focus the heat at the top, play with the gap between heat riser and barrel, and you can create a red-hot cooking spot on the top of the barrel.

And as the book sez, you can add an "oven" of aluminum foil crumpled to trap insulative air, on top of the hot surface.  Kinda like those fancy domes you see over a covered dish in room service cartoons.  That helps with baking or re-warming of oven-type foods.

Yours,
-Erica

Now there's an idea.  A cob oven on top of a rocket stove barrel.

Nifty!

Wouldn't get as hot as a traditional cob oven, but it would do for baking and such.

Ever since learning about all of the rocket stuff, I've been kinda against the cob oven idea.  They are so smoky and they seem to waste a lot of fuel.

paul wheaton wrote:
Ever since learning about all of the rocket stuff, I've been kinda against the cob oven idea.  They are so smoky and they seem to waste a lot of fuel.

They do use a lot of fuel, and smoke.

You can reduce the smoke by building in a chimney - Ernie has plans for one with two doors, where the chimney is above the "entryway" between the oven door and the outer door.

You can build one as an empty space inside/above a masonry heater, or a rocket mass heater, so the oven is just picking up surplus heat from an already-efficient system.  It's a tradeoff of ideal temperatures & conditions for artisan baking, vs. more efficiency and the ability to use stored heat elsewhere in the system.

The other thing to bear in mind is that a well-designed oven has thermal mass inside; it heats, and cools, slowly.  One firing can accomodate everything from crackers and pizza (very hot oven, fast bake) to breads to cakes to a slow-roast at the end of the heat cycle.  The cooling curve is gradual, and a well-prepared cook can use every part of it.

If you're firing up a high thermal-mass oven to bake 1 or 2 loaves of bread, then yes, you're wasting a lot of fuel (and time).

-Erica