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Rocket stove driving underfloor heating?

 
pollinator
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Burra Maluca wrote:

Kathleen Sanderson wrote:I didn't see anyone addressing the fact that he said the house is in zone 9a, which should only need minimal heat.  It should have far more of the year in need of cooling, in fact.  It's fine to play with something labor-intensive and failure-prone like under-floor heating if you really want to mess with it 'just because,' but in your climate, honestly, I wouldn't.  

If I was building a house in that climate, and planning to use my heat source for cooking (presumably year-round), I think I would build a sun room -- a screened porch in hot weather, but close in the screened windows with glass for the cooler parts of the year.  Attach that to the house, and put the heater/cook stove in it.  With such a small house, all you would need to do to heat the house is open the door between the two spaces when you need heat, but in summer when you absolutely don't need extra heat in the house, close the door and keep the cooking heat out in the screened porch.  In fact, unless there's a lot of cloudy weather during the coldest parts of the year, the sun room would probably provide most if not all of the heat necessary for such a small house.



I've been living in this area for 15 years already, and the climate isn't just 9a, it's also mediterranean, which means 'cool, wet winters'.  We generally get one or two six-week sessions of solid rain at some point during the winter, where we will absolutely need heat both to keep us warm and the building dry.  I've been dreaming of a rocket mass heater for so long simply because it's a perfect solution for this climate.  It's likely that a single burn of around an hour a day during wet or frosty weather will be all we need so long as there is some way to store the heat and give it out gradually over the next 24 hours. And as Austin has been dreaming of under-floor heating for years and years, we'd really like to try to combine our dreams into one functional reality.  

I totally agree about cooling in the summer, though I've never had air-conditioning and am fairly well adapted by now and have got pretty good at managing air flow to keep the temperature indoors relatively stable.  We will have gas cooking stove, designed to be swapped around so that during the heat of summer it will be in a screened porch, as you suggested, but inside the main house during the cooler months.  During the wettest, coldest periods we'll cook on or in the rocket mass heater as much as possible.  



My family is from the Oregon Coast, and I lived there for a few years when I was in my teens, so I'm very familiar with a Mediterranean climate!  Though the Oregon Coast seldom gets very hot.  But it certainly does rain a lot in the winter.  I agree that a rocket mass heater should be a good heat source for you.  Suggest if you are going to try the under-floor heating, make a way to disconnect it easily when you don't want it, or if it doesn't work as well as you'd like.  Your plans sound very viable!  I think one of the biggest advantages of a mild to warm climate is being able to do much of your living outdoors, or almost outdoors.
 
gardener
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Burrita.

I forgot one thing. If you go batch, don't go over 200mm. Or it will be way too big.  Better even, 150/160mm.  If you want to cook on it. You need a sturdy 2 hours of burning i would think. And a 200, would heat 70/80m² in that time.
 
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Tim Skufca wrote:we have a ground-source heat-pump which heats our hydronic floors from a well in our in-town Missoula well. In-floor heat is very comfortable. In the summer heat we circulate well temperature water through the floors to keep the space comfortable, which is very important if the Missoula air quality gets bad from forest fires. Closing the windows and running 55 degree water through the floors is an effective way to hold off those awful smokey periods.

ALSO: what is not talked about with Rocket Mass Heaters/Stoves is the necessity to be there at all times. There is no way to take off for a few days, or else everything freezes. I have the mind-set of embracing technology and install solar panels that power the electrically sourced hot water.



Again, in a 9a climate, you don't have the same freezing issues as in a 4 or 5 climate. Indoor plumbing for a 4b climate is a whole nother topic; but in my experience, a masonry heater beats any other wood heat for days you can leave the house untended before freeze-crunch. In those settings, you learn to invest in good neighbors, vacation-style drain systems, and/or heat tape.

In this case,
Climate sounds like it needs more cooling than heating. Shaded collonades, courtyard with fountain, or grape arbors are good. Run air through these spaces, draw it up into the main house by solar chimneys or wind scoops aiding natural convection.
If you have the option to run well temp water thru floor, why not? But it is more complicated than traditional methods. Air flow aids evaporation, which is a huge asset for cooling, unlike heating.

For underfloor: if you like digging, and you haven't hit bedrock yet, you can run the RMH pipes under part of the floor. Insulate wall perimeter, keep heater pipes at least 12 inches (30 cm) away from any combustible insulation.

Give yourself a margin of safety from the maximum lengths of pipe runs in our book, because you are in a warmish climate, and old and work-worn enough that you may want to run the heater when it is actually still warmer outside than in. I would take about 25% to 30,% off those maximum lengths, or add a bypass for chimney priming.
Use a vertical chimney, to about a meter above roof height, and insulated where it is exposed outdoors.

Since there is boiler experience in the house, I don't think running a pump and pipe system is beyond your capacity. You need to decide whether the heat exhange pipes will be kept potable, or non potable.
If potable, you can run well water thru them to a sink or other uses for cooling. But the heat exchanger run must not have dead ends, and must be careful with all heated water sources to avoid warm stagnation.
If non potable, you can run any old water thru, recirculate, or collect excess for gardens, until algae gunks up the works.
I like Tim's systems with an open pot or tank for the heater, which is non potable and easily cleaned out, and a coil in the tank thru which you run pressurized clean water.  This offers pressurized warm water with built in protection from boilover; reduces the stagnation problems substantially; and gives the option of running a shower or washing-up station off the same system, if wanted.
You could do the same for a solar collector tank, run separate heating coils to a tank and a potable coil for heat transfer. Many systems run water thru solar first to warm it, then use fueled heat to reach final target temps.

Again, pressurized usually goes with potable, because both require clean, alga and scuzz-free pipes. Which means precautions against warm stagnation in the lines. More options means longer pipes and longer stay times. If any loop is being shut off for weeks or months depending on weather, or for repairs and tinkering, it needs to be flushed and possibly decontaminated when re-activated.

However, I don't know if you'll produce enough extra heat off a tank on top of the rocket to heat the rest of the floor anything like the area just over the RMH ducts.  Might want the hydronic heated areas to be shallower, with copper conductors under tile, and better insulation.

 If you can live with hot, warm, tepid, and cool zones, like an old fashioned wood cookstove, then this sounds DIY doable for an old steam engineer. Cooling this way may produce floor temps that are not entirely comfortable while the air is still too warm, so a zone for chilling might be better than the whole floor anyway.

It will turn the entire space into a project, and one which may invite tweaking and improvements over time. If you tend to let projects take their own time, you may want a spare barn for overflow while tinkering,  since this is the primary utility room.

I would also not be inclined to pour solid concrete over everything, at least not without careful attention to cleanout access points. Discrete tiles or pavers may serve better, since you can bet that anything with functioning parts will eventually need inspection, cleaning, repair, or replacement. Just removing a bit of sidewalk that was mistakenly laid over a septic tank is a major hassle, which I would hate to deal with indoors.

This will probably be a sort of personal system, due to unusual features. I would not expect the next owner to want to operate and maintain it. So invest what effort and value in it you feel is worthwhile to please yourselves.

If you want zone control, where you can direct and dial the heating and cooling, or do that programmable timing stuff, I'd look for commercial products and an experienced local installer.

Please let me know if you'd like to hire us for a customized plan set or tech support.  Burra gets special rates. 😘

Yours,
Erica W
 
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“One mistake a lot of people make is in adding antifreeze. Not only is it toxic and expensive to buy, it automatically makes your system 10% less effecient.”

It is not necessarily a mistake, depending on where you live. In the far north where it may get to -40, a system that was shut down for whatever reason (power outage, etc) for an extended period would be subject to freezing, especially if you had a garage floor involved that may be a less well insulated space. There are two main types of glycol common to heating and cooling use: Propylene glycol (PG), and ethylene glycol (EG). Propylene glycol is considered non-toxic.
The other consideration is if you are using just water, you may need to add a conditioning agent from time to time if you have ‘hard’ water. Reason being, you are not constantly running fresh water through the system, so the minerals never get flushed out. The pex will be fine but metal components are subject to corrosion. Or you could just flush the system annually.

One other ‘plus’ of a heated floor- you get a warmer space with less heat input. Because you are introducing heat at the lowest point of the living area, a lower setting is perceived as being warmer than the same setting if you had radiators or forced air. And since your body is warmed from the feet up, you may be quite comfortable in a house with the air temperature at 60-65 degrees instead of 70.

 
pollinator
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First step is the floor so let us talk about aircrete. Using foam instead of sand and gravel with cement. This would make an insulating layer for the first layer of your floor. then you put down the piping and cover it with concrete.  The aircrete can also be very useful in certain portions of the Walker Stove.Here is a source for the formulas and foam generator and method of building the firebox.
 
Erica Wisner
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Sorry, just caught that the Walker stove has already been chosen.

Disregard the bit about pipes under the floor, in that case.  There's not enough spare heat and "push" from the Walker style fireboxes to run much in the way of additional heating channels, unless you have an unusually tunable chimney.

The classic Franklin stove has a water tank in the body, which makes the stove terribly inefficient due to cooling the firebox.
I would prefer to see the water heating elements well outside the firebox area - the logical place on a Walker stove or most other rocket stoves is the cooktop-type area.
This raises the problem of just how much heat output do you want for cooking, vs. heating the floor.

As Satamax points out, you will probably want to cook for specific times, and this may or may not match the amount of heat you want to deliver to the floor.  
I like Kathleen's and Burra's screened summer kitchen idea.  As long as you have the option to cook outdoors, you can use the indoor stove more strictly according to your indoor comfort goals.

I might sic your engineer on calculating the total BTUs needed to heat up your various targets - get an IR thermometer, check the surface temperatures that work best for you to cook, and the temps you want to raise the floor mass to for comfort, and do some heat loss calcs for your coldest days using the BuildItSolar.com or similar home heat calculators.  You can convert that total heat (BTU or joules) to figure out what weight of wood you'd need to burn to achieve this, and what proportion of your heat needs to be available to cook, vs. heat, vs. keep the chimney working properly.  

It's all well and good to pick a compact stove for a small space.  But if it can't burn wood fast enough to deliver your total output, or if you want space for 3 pots but you also want to send 60% of the heat output elsewhere, you might need to pick your stove to suit the job rather than pile more jobs onto a cute stove.  
A dedicated boiler for a hydronic system is usually a reasonably large tank, with a lot of insulation, which makes it a somewhat ugly beastie that most people don't want in their living rooms.  Even if it doesn't explode, it creaks and groans and collects cobwebs.
If you want a cute and wood-sipping stove, get used to the idea that the heat will be uneven; there's only so much you can do with circulating a fraction of the heat from hotter to colder parts of the room, if the stove is originally designed to collect heat in a specific place for its own thermal performance.  

Walker stoves are designed to put heat out the top fast, and slowly out the sides, so they are great when installed roughly in the center of a space (as a kitchen island for example).
I've seen Lasse Holmes put a small bench on to capture waste heat from the exhaust on its way to the chimney - but you have to be careful how much heat you capture in this location, as the chimney needs heat to function (and these stoves definitely need a decent chimney to burn clean and draft well).

Yours,
Erica W
 
Kathleen Sanderson
pollinator
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Good information on the Walker stoves -- I have the plans for both of those stoves.  Just not sure about cutting a hole in the floor of my old house in order to put in the foundation one of them would need.  

For me, uneven heat is a feature, rather than a bug.  If I'm cold, I can move to a warm spot; if I'm too warm, I can move to a cooler spot.  I *really* like being able to keep the bedrooms/sleeping areas cool to down-right chilly.  

 
Satamax Antone
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Well. I wanted to telle Austin another thing.

There is no insulation under m'y workshop heater. But after a few days of heating, the floor around it gets hot. Pretty much like an underfloor heating.

Insulating under it could proove even better. In a passive underfloor heating..

Or even madder.  Lay heatpipes in that floor.  To spread the heat.

That would be an engineer's project !
 
Satamax Antone
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This heatpipe idea could be interesting to heat upstairs for some people. When convection is not enough.
 
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Lots of good info in this thread now

Kinda thinking that it'll be a non-pressure, pump-circulated system which uses RMH/Walker type stove to heat non-potable water which circulates through the UFH.  It will also have an insulated tank, also part of the circulation, so it can store heat in a mass of water.  When the tank's warm and the stove is out, the same pump will circulate the heat out of that tank and into the floor.  Cooking on the wood stove will happen if the stove's hot when we want to cook.  I doubt we'll light the stove *just* too cook if the heat isn't required generally (see below).

The same circuit could also feed heat into an indirect domestic hot water tank - although we might not bother with that; in this place we have an on-demand gas heater for the shower, which is actually a pretty efficient way to work if you don't require big amounts of hot water fast, as you only heat the water you're using right now.  The same on-demand can also feed taps in the house; not *quite* so efficient as you have pipe runs to consider, but judicious placement should keep those tolerably short.  On-demand with a long pipe run to the tap isn't clever, of course.  

As for cooking, I will have a gas hob, which I plan to be movable: it'll be inside until the hot weather, and then move outside into a mozzie-screened "conservatory" (won't be glazed though) so the cooking heat stays out of the house.  This is something we've been messing with here this summer - I've been looking at what works and what helps and what doesn't.   One thing the new place will have is an air-flow system, taking air into the house low on the north side (with option to install some kind of evaporative-water cooling on the inlet) and exhaust air into the roof space, with insulation at first-floor ceiling level.  The first floor itself will have an opening for my superladder (pics after I build it ) and won't be insulated so enough heat will get upstairs to the bedrooms all by itself.  
 
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Scott Weinberg wrote:  If you don't trust a relief valve, then put in two of them.
The odds of a double failure at the very same time, is well.................. pretty darn hard to calculate.




In areas that they tend to gum up in a couple of years, it would seem that in five years of use, the odds of both of them being gummed up seems to be about 100%.


 
paul wheaton
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Burra Maluca wrote:I totally agree about cooling in the summer, though I've never had air-conditioning and am fairly well adapted by now and have got pretty good at managing air flow to keep the temperature indoors relatively stable.  



With our rocket mass heater, we leave the wood feed open on a hot day.  

We open the windows at night.   When the temperature outside is colder than inside, the air is pulled into the rmh.  In the morning, once the temperature outside is warmer, the air stops moving.   The mass feels cool.

The rocket mass heater cools the house.

 
paul wheaton
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Tim Skufca wrote:what is not talked about with Rocket Mass Heaters/Stoves is the necessity to be there at all times. There is no way to take off for a few days, or else everything freezes. I have the mind-set of embracing technology and install solar panels that power the electrically sourced hot water.



The house I am in came with a propane heater.  We set it to 50.   If we were to ever leave the house, the propane heater would come on.

Of course, with community, you can leave the house and there are still people to keep the house warm.
 
Satamax Antone
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Austin, a good way to cool air; is a Canadian / provençal well.

I have, i would say a Canadian version, with a plastic pipe. Running 20 or so meters under my land. May be not deep enough, but i didn't lay it myself.

The best ever, i think, is the provençal type, made either with clay pipes or tiles, so you mix earth cooling properties with evaporative cooling.
 
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[b wrote:paul wheaton[/b]]

Scott Weinberg wrote:  If you don't trust a relief valve, then put in two of them.
The odds of a double failure at the very same time, is well.................. pretty darn hard to calculate.




In areas that they tend to gum up in a couple of years, it would seem that in five years of use, the odds of both of them being gummed up seems to be about 100%.


yes, I had completely forgot those areas of the world where there are NO protected water heating systems. With everyone sitting on time bombs... to bad for them....

]

 
Julie Reed
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I tried to find a statistic for how many water heaters explode every year but I couldn’t. I’m not quite sure what areas have problems with T&P valves gunking up so frequently, but it sounds like a filtration problem mostly. Why would you send gunky water into any part of your plumbing system? Common sense says you filter your water. Common sense also says that you have some sort of annual (or perhaps semi-annual) maintenance list for your dwelling. Things like chimney and gutter cleaning, smoke detector batteries, water heater flush and safety valve testing... kind of like changing the oil in your vehicle so IT doesn’t ‘explode’.
And if you do have some esoteric water issue that filtration won’t solve, then change the pressure relief valves every year. $15 and 10 minutes of your time. Big deal.
 
Hans Quistorff
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Walker stoves are designed to put heat out the top fast, and slowly out the sides, so they are great when installed roughly in the center of a space (as a kitchen island for example).
I've seen Lasse Holmes put a small bench on to capture waste heat from the exhaust on its way to the chimney - but you have to be careful how much heat you capture in this location, as the chimney needs heat to function (and these stoves definitely need a decent chimney to burn clean and draft well). --Erica W


Aircrete method for making inexpensive high drafting chimney.  
 
master pollinator
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Julie Reed wrote:“One mistake a lot of people make is in adding antifreeze. Not only is it toxic and expensive to buy, it automatically makes your system 10% less effecient.”

It is not necessarily a mistake, depending on where you live. In the far north where it may get to -40, a system that was shut down for whatever reason (power outage, etc) for an extended period would be subject to freezing, especially if you had a garage floor involved that may be a less well insulated space. There are two main types of glycol common to heating and cooling use: Propylene glycol (PG), and ethylene glycol (EG). Propylene glycol is considered non-toxic.
The other consideration is if you are using just water, you may need to add a conditioning agent from time to time if you have ‘hard’ water. Reason being, you are not constantly running fresh water through the system, so the minerals never get flushed out. The pex will be fine but metal components are subject to corrosion. Or you could just flush the system annually.

One other ‘plus’ of a heated floor- you get a warmer space with less heat input. Because you are introducing heat at the lowest point of the living area, a lower setting is perceived as being warmer than the same setting if you had radiators or forced air. And since your body is warmed from the feet up, you may be quite comfortable in a house with the air temperature at 60-65 degrees instead of 70.




But freezing may, or may not be an issue even if the air temperature is cold.

With most systems, people can shut their boilers off, and just run my circulator pumps. Flowing water does not freeze.

But admittedly it takes electricity to keep those pumps moving, but that is not an issue really because an inverter system, or a backup generator is an option. Just do the math on antifreeze, not only do you save not having to buy the expensive stuff, but it is 10% less efficient. In about a month's time, I will have saved enough money to buy a small generator that could power my circulating pumps...or a few deep cycle batteries. Even if you use wood as a heating fuel, why have to gather 10% more of it every year just for antifreeze?
 
Julie Reed
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“Flowing water does not freeze.”

Actually that’s a fallacy. At a certain point, if it stays cold enough, it will (I saw it happen to a boiler in northern NY in less than 24 hrs). But yes, for a couple days the circulation would probably save you.

“Just do the math on antifreeze, not only do you save not having to buy the expensive stuff, but it is 10% less efficient. In about a month's time, I will have saved enough money to buy a small generator that could power my circulating pumps...or a few deep cycle batteries. Even if you use wood as a heating fuel, why have to gather 10% more of it every year just for antifreeze?”

If a 10% loss of efficiency over 1 month equaled the value of $1000 for a decent generator or decent batteries/inverter system, that would be a very large system. That would mean your cost to heat was $10k per month.
To me, a 10% efficiency loss, coupled with the $500 it costs (one time) to add enough for a 2000 sf house (typically the largest home most people would have), is more than a fair trade off to not have to worry about maintaining batteries or a generator, or having to hope the generator starts, or having to fuel the generator for a week...
And since infloor heat is probably the most efficient way to warm a building (aside from passive solar), you’ve already gained far more than 10% over any other systems anyway. So it’s a net balance at minimum.

Aside from calling it toxic, you are mostly correct in saying it’s unneeded, but it really depends on your location and potential for extended power outages. To those of us north of, say, latitude 50, it’s cheap peace of mind.
 
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Julie Reed wrote:

Aside from calling it toxic, you are mostly correct in saying it’s unneeded, but it really depends on your location and potential for extended power outages. To those of us north of, say, latitude 50, it’s cheap peace of mind.



Besides that, it's possible to buy antifreeze that's less toxic.  Additionally, if your system was designed to use antifreeze from the start, the anti-corrosive effects of modern anti-freeze can save you money on the quality of your components.

Anti-freeze is a mixed bag.
 
Travis Johnson
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Julie Reed wrote:“Flowing water does not freeze.”

Actually that’s a fallacy. At a certain point, if it stays cold enough, it will (I saw it happen to a boiler in northern NY in less than 24 hrs). But yes, for a couple days the circulation would probably save you.

“Just do the math on antifreeze, not only do you save not having to buy the expensive stuff, but it is 10% less efficient. In about a month's time, I will have saved enough money to buy a small generator that could power my circulating pumps...or a few deep cycle batteries. Even if you use wood as a heating fuel, why have to gather 10% more of it every year just for antifreeze?”

If a 10% loss of efficiency over 1 month equaled the value of $1000 for a decent generator or decent batteries/inverter system, that would be a very large system. That would mean your cost to heat was $10k per month.
To me, a 10% efficiency loss, coupled with the $500 it costs (one time) to add enough for a 2000 sf house (typically the largest home most people would have), is more than a fair trade off to not have to worry about maintaining batteries or a generator, or having to hope the generator starts, or having to fuel the generator for a week...
And since infloor heat is probably the most efficient way to warm a building (aside from passive solar), you’ve already gained far more than 10% over any other systems anyway. So it’s a net balance at minimum.

Aside from calling it toxic, you are mostly correct in saying it’s unneeded, but it really depends on your location and potential for extended power outages. To those of us north of, say, latitude 50, it’s cheap peace of mind.



You would never need to spend $1000 on a back-up Generator. My boiler system only uses (1) 15 amp circuit to operate. That would mean you would only need a TINY generator to power the system. Here in Maine at least, you can buy a small generator for $150 that would power the sytem, and that is online at Walmart. Generators have really come down in price.

And while I admit this only works on radiant floor heating systems with concrete floors (or other floors that have mass like brick, rammed earth, etc), by just shutting off the boiler, and circulating the system, you actually add heat to the floor. That is because while you lose heat on the outer edges of your slab, in the center of your slab you have constant constant ground heat. Here in Maine that temperature is 57 degrees 24/7/365. By circulating the floor, you are taking that warm 57 degree heat, and pumping it throughout the slab. Since the square footage of warmed geothermal heat, is a lot bigger than the narrow zone where you are losing heat, you are not just flowing water, you are flowing warmed water.

But that is one of the reasons I really like radiant floor heat, it has built in geothermal heating. Since you already have the water at 57 degrees, you only need to bring 20-40 degrees to bring your house up to temperature. That saves btu's, no matter what you use to heat the water.
 
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There's another possibly-relevant point to the anti-freeze argument - at the same time as lowering the freezing point it also raises the boiling point.  I think that's part of why they use it in solar water heating systems.  The solar system we had in Wales was pressurised (in the solar loop) *and* had antifreeze, and so far as I know it never boiled.

That was of course a closed system; it circulated through the panels on the roof and through one coil in a 2-coil domestic HW tank.  And, so far as I know, it had a PRV but that never was invoked in the UK.

 
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Austin: 'Now, I did wonder about running the rocket flues under/through the floor, ... '

There's a name for that in Korea, "Ondol', and a long tradition of use, and there is a thread here at permies about it:

https://permies.com/t/40500/Ondol-ancient-original-form-heating
 
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For a house that will not get into the negative digits I would suggest running your exhaust through the floor to warm the floor. The only time I have had trouble with this was when I did it in a tiny house that didn't have an insulated floor and it worked well most of the time but when it got really cold out and no one had been there to feed the fire for a few days the floor mass took all the heat out of the exhaust instead of just most of it so the fire burned fine for about 20 minutes then vapor-locked when the pressure from the "cold plug" of super cooled 40 degree F exhaust steam was too cold and heavy for the chimney to lift it out of the floor even though the heat riser was pushing with all its might. In cold climates especially you can have too much mass. I love when I can get warm feet without the hassle of heating and distributing water.
--Mud
 
Austin Shackles
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Hans Quistorff wrote:

Walker stoves are designed to put heat out the top fast, and slowly out the sides, so they are great when installed roughly in the center of a space (as a kitchen island for example).
I've seen Lasse Holmes put a small bench on to capture waste heat from the exhaust on its way to the chimney - but you have to be careful how much heat you capture in this location, as the chimney needs heat to function (and these stoves definitely need a decent chimney to burn clean and draft well). --Erica W


Aircrete method for making inexpensive high drafting chimney.  



I imagine you didn't mean to post 2 videos the same?  Looks like there are 2 parts about how to make the aircrete double-wall flue, but you've got part 1 twice
 
Austin Shackles
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Erica Wisner wrote:
Walker stoves are designed to put heat out the top fast, and slowly out the sides, so they are great when installed roughly in the center of a space (as a kitchen island for example).
I've seen Lasse Holmes put a small bench on to capture waste heat from the exhaust on its way to the chimney - but you have to be careful how much heat you capture in this location, as the chimney needs heat to function (and these stoves definitely need a decent chimney to burn clean and draft well).

Yours,
Erica W



Just had another thought about this.  I can still make a bench to capture heat from the stove, by building a water tank into said bench.  It may be that the Walker stove as such is not really what we need, now, as plans morph and change.  Perhaps more of a RMH than a stove, although the ability to cook on it occasionally would still be nice.  If I build a bench with a tank in it, that can be as insulated or otherwise as I choose to make it and I can circulate the water through the floor and feed it from solar capture on sunny days - and it's hidden from view but still accessible at need.  Plus Burra can lounge on it with her laptop.
 
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Austin, check this one.

https://permies.com/t/38889/a/46625/batchetage2.skp?download_attachment=true


Well, i wouldn't use the vertical batch this one spots. As i haven't finished development on it.

But a horizontal batch, with a top load (for aging people! ;D )

And replace the big bell at the left, by a bench.

I can promise this one would fit your bill. You could put your hot water storage, in the big bell. The bench would be too small for housing a water tank of any volume, and still have the right CSA.
 
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Don't yet see any mention of various ancient Asian systems for underfloor heating.

Going back probably 1000's of years, Japan and I think also Korea and maybe elsewhere in Asia had "under-floor" heating that consisted basically of a charcoal fire that warmed air under a raised floor.
One good reason for sitting on the floor or a few relatively thin cushions on the floor, instead of up on chairs (Ianto Evans' famous "Arse-value," instead of "R-value").

Some locations like an eating table had a partially sunken floor, and that sunken area around and under the table where one's feet and legs go was warmed by a charcoal fire.
Or just the area under a low table on a flat floor was warmed that way.

Combine that with clothing designs (robes of various types) that let that warmth filter up to the rest of the body inside the clothing, while insulating from cold air outside the garment.

A charcoal fire is well suited to this - slow and steady and even heat output and little or no flames coming off the top.

The output of a downdraft rocket stove is already successfully run through thermal mass benches - cob, masonry, etc.

So run it a foot or two lower, through under-floor channels, and it would be similar to the long-standing Asian charcoal-fired version.

If you have low thermal mass floor you get quick heat but need to keep the source fired up as it doesn't hold heat. And the heat source needs to be even, to be comfortable.

If you have high thermal mass floor, then (as with hydronic) it will take a while to heat up but will also absorb and slowly release more heat. So the heat source can be more intense, or intermittent, and that is moderated by the thermal mass.

For floor heating you want warm not hot, and steady/even not fluctuating, so that's a primary design factor - how to get that from rocket style burner in combo with floor construction.

But you can have a potentially much simpler system, compared to hydronic.
Hydronic heating has a huge overhead in highly manufactured materials. Valves, tubing, fittings, connectors, metal & plastic; controllers for all that, unless you are really going to do all the valves etc. manually. Also pumps, because in most cases your floor is not going to be above your heat source so thermo-siphon is not an option so you have to have an active (pumped) system.
 
Austin Shackles
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Good points about underfloor - and the Romans also had hypocausts where the fire was under the building and, I believe, the flues went up through the walls.  The point about using a water tank and conventional style underfloor piping is that the water tank can be used to even out the rocket's heat output.  

As for the mass/bench etc. - a useful sized bench, say about 2m long by 80cm wide by 40cm high, mebbe, if we translate the dimensions into dm (decimeter) then we get a volume of 20x8x4 = 640 dm^3 which is 640 litres.  Now, you obviously wouldn't fill the WHOLE bench with a water tank, but you could fit 500l in there with some insulation, or make the bench a tad bigger.  Probably use a fire-tube style thing like a steam boiler, where hot gas flues run through a water tank.  But no, it WILL NOT BE PRESSURISED!!  Build the tank so the sum CSA of the fire tubes is a bit more than the nominal rocket size, it shouldn't restrict the exhaust too much.  

Might also want a bypass to avoid cooling the flue too much.  That could either be an adjustable bypass which redirects the flue so it doesn't run through the tank at all, or maybe have one tube that doesn't run through the tank so a proportion of the rocket exhaust isn't cooled.

Being a welder fabricator, I don't have a problem making that sort of tank.
 
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Austin, for a bench to work as a bell, You need volume, so gases have more time to exchange heat with the surrounding parts, would it be water tank or mass. Peter recommends a minimum distance of X5 times the CSA between intake and exit of a bell.

That is if you use the bench as a bell.

If you use it as a flue, i think you'll encounter a dew point problem. I mean, the length of flues is limited by the adequation of the power of the rocket (given usually by it's size) the draft capabilities of the chimney. Which gives you the length of flue you can fit in a system. Or the bell's isa.  Flues restrict the flow in two ways, by friction, and by cooling. Why you have startup problems on a cold rocket, condensation. The stack effect doesn't work, because the hot gases heat the surrounding mater, and try to re evaporate the condensing liquids in the stream of gases, and what is condensed on the surfaces. Then there isn't enough energy to do both tasks. And and drive the stack effect at the same time. Then your rocket stalls.
I would be leery to fit a water tank in the flue of a rocket. And i don't know the heat absorption of such a tank, but you should reduce your flue length accordingly. Not of equal surface, but you will have to adjust for the greater heat absorption of the water in the tank.

I wouldn't aim for just the CSA around the water tank, but more like 1.5 to 3 times CSA, due to the increased surface area of the tube around, and the tank inside.

Which leads me to think, a vertical bell system would be cleverer. If you want to store water.
 
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Austin Shackles wrote:Build the tank so the sum CSA of the fire tubes is a bit more than the nominal rocket size, it shouldn't restrict the exhaust too much.  



It doesn't work Austin.

You need far more than just a smidge over CSA.

Your tubes need to be at the very least 7 cm, so the boundary layer doesn't effect the center stream of the gases running through it. That means you have a square cm in the center which isn't affected by the drag. And i would guess for a rocket to work nicely. You'd need 60 to 80% of the CSA as a free flowing stream of gases.

But by then, you've increased tremendously the friction, due to soo many tubes. I guess, it would be over the maximum iSA of a bell.  I think the right compromise can be calculated.

But you will run into inequalities, unevenness in the draft through the several tubes. I don't say it can't be done. But that won't be easy toi calculate, and do all tre trial stages.

The rocket is run between normal chimney speeds, with all the excess air cooling the fire, and the exhaust gases. And a condensation boiler. And the tipping point is very hard to avoid sometimes.


http://donkey32.proboards.com/thread/1425/flow-visualization
 
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Not done detailed calculations as yet.  
But consider if I had a 6" x 6" square* rocket.  The CSA of that is 36 sq in, so the volume of the flue is 36 cu in per inch length whereas the wall area is 24 sq in per inch length.  If I fed that into a manifold from which come 4 no. 3" x 3" tubes which run through my water tank, the CSA in total is the same but the wall area per in doubles to 48 sq in, making more drag.  So obviously the answer is not to fit 4 such tubes, but 5 or 6 or even more, so the increase in volume offsets the increase in drag until the flow rate of the whole set is the same as the original big flue.

I'll work it out properly before I build it


* 'cos it makes the math easier.
 
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Austin Shackles wrote:wonder if I can use it to heat a water tank and use that warm water to supply more-or-less conventional underfloor heating.  The advantage I see in this is that I could also heat that tank from a solar pane



WOOD-WATER-SOLAR MASS HEATERS

* A thesis analyzing adding water thermal-mass to a traditional Chinese mass-heater called a Kang. There is also a solar panel added. Open access to the PDF file. "An Evaluation and Model of the Chinese Kang System to Improve Domestic Comfort in Northeast Rural China. By Andrew Porter Yates, University of Colorado at Boulder."

https://scholar.colorado.edu/cven_gradetds/338/


*  "Rocket stove water heater redux" . . . "Our rocket stove water heater has been doing its thing for nearly 3 years now, so we decided to take it apart and do a full examination of how it had fared."

https://www.milkwood.net/2011/10/28/rocket-stove-water-heater-redux/
 
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Antigone.

I would have loved to see the metal heat riser in the rocket water heater.   If you are getting creosote build up, your combustion is incomplete. I'm not saying this lightly, or jokingly. There is something wrong. Either the heat riser and insulation is too short.  You might also have way too much primary air. In a properly built rocket, combustibles are burned.

There is one thing, to my liking, the water jacket is too close to the heat riser. Hence it's condensing the unburned gases.  



Austin, to further push for a real mass heater. Have you ever been near one?  I love the heat from those. Ok, the floor is not evenly hot all around. But it can be for several meters around it, if built with insulation underneath.
 
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Austin Shackles wrote:There's another possibly-relevant point to the anti-freeze argument - at the same time as lowering the freezing point it also raises the boiling point.  I think that's part of why they use it in solar water heating systems.  The solar system we had in Wales was pressurised (in the solar loop) *and* had antifreeze, and so far as I know it never boiled.

That was of course a closed system; it circulated through the panels on the roof and through one coil in a 2-coil domestic HW tank.  And, so far as I know, it had a PRV but that never was invoked in the UK.



Antifreeze will raise the boiling point of water somewhat because it makes the fluid more dense, but using pressure is how you really raise the boiling point of water. For every 1 pound of increased pressure, it raises the boiling point of water by 3 degrees. So assuming your old system was the typical 12 psi, your boiling point was raised from 212 degrees, to 248 degrees.
 
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Where we are the ground NEVER freezes so underground heating should never have that problem, no antifreeze required, unlike in Wales (bbrrrrr!). Just waiting for ours to be connected as the nights are dropping to 10°C. I don't  like to get up unless the bedroom is 16°!
 
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Satamax Antone wrote:Austin, to further push for a real mass heater. Have you ever been near one?  I love the heat from those. Ok, the floor is not evenly hot all around. But it can be for several meters around it, if built with insulation underneath.



No he hasn't, and I'd love for him to see one.  

Is that an invite?
 
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