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Building an RMH with hot water storage for use with a hydronic baseboard system?  RSS feed

 
Posts: 23
Location: Underhill, Vermont
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I would like to build a RMH in my home and store the heat in water, and then connect to my baseboard system for whole house heating - has anyone done anything like this?
 
gardener
Posts: 1278
Location: latitude 47 N.W. montana zone 6A
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Hi Jeff; Have you built any rmh before? Do you have a copy of ianto evans RMH book ?. Heating water is a subject with alot of interest. It really should not be your first build. But if you tried this, my novices opinion is that you would be burning your stove way more than normal and the room where your rocket is located would get very hot. Water is a very good mass, but heating it safely is a big issue. Only a VERY experienced rmh builder with hot water expertise would attempt a whole house system, it might be a little more than a rmh was designed for.
 
Jeff Thorpe
Posts: 23
Location: Underhill, Vermont
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I haven't built a RMH before, I don't have that book, but I do have Paul's DVD set (which I haven't watched entirely yet). I've been researching RMH's for a few years now, and am competent in plumbing systems. My current thinking is to have a large tank to put the heat into, either instead of all the cob as mass, or as a remote tank located above the RMH with a coil around the barrel and heating the tank via thermosiphon action. If you use a pressure relief valve, and perhaps distilled water, is there any real concern about a build up of pressure in the coiling?

 
Posts: 19
Location: Emerald, Australia
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Hello Jeff, I was thinking along those same lines. Have you done any work on this project as yet? My main concern is matching the size of the RMH to the volume of water, especially if the water is to be pumped around the house to hydronic radiators. The volume could soon become quite large.
I put a brief outline of this topic up on Elance to see if there were any thermodynamic engineers out there with this sort of knowledge or background. It did generate some interest with several qualified people wanting to take the project on, but none knew anything about RMH. One guy that had some type of degree in thermodynamics said he would do some research on RMH and get back to me. I'm happy to pay anyone that can help with design criteria.
 
Jeff Thorpe
Posts: 23
Location: Underhill, Vermont
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Eric,

I haven't started my project yet - I'm still in the research phase. I'm leaning away from putting a coil around the barrel, and in fact leaning away from a hot radiating barrel at all and am going to build something that stores more into mass:

http://www.dragonheaters.com/6-rocket-masonry-heater-castle-build-kit/

Given the potential dangers involved in heating water, Erica and Ernie have shied away from discussing this subject in these forums, which is why threads like these don't go far.

I found another forum where it's talk of RMH all the time (including heating water):
http://donkey32.proboards.com/thread/1232/water-heat-storage


I'm thinking about putting a (probably unpressurized) storage tank inside the rocket mass heater. Thoughts range from a 30 or 55 gallon barrel with a copper coil submerged in it, to a 40 gallon water heater (with the jacket and insulation peeled off), all the way up to the insane and ridiculous idea of using an old 250 gallon oil tank.

My goal with in designing this heater would be to burn for a few hours in the evening and have enough heat stored to heat my 1800' house overnight. Once or twice a year it will get to -30F here, but usually winter nights are around 0F in February. So how much heat do I need? That is the question.

Running some quick heat loss calculations on the house, I come up with 14,000 BTU heat loss per hour. If I want to go for 10 hours overnight, I'll need to be able to store 140,000 BTUs.

Theoretically we will only be able to extract 80 BTUs per pound of water (water boils at 212 obviously, but the radiators require water in the 130-140 range. 212-130=82. I'm going to use 70 in my calcs, because water doesn't like to stay at 212.

140,000/70=2000 pounds of water. water is 8.3 pounds per gallon 2000/8.3 = 240 gallons.

So it looks like I really may need a 250 gallon tank. Not sure if that's practical, and how long it would take to actually heat it up. It may be necessary/simpler to use a 250 gallon tank remotely, and put a smaller tank in the RMH, and feed the bigger tank with a pump or thermosiphon.



 
Eric Kendall
Posts: 19
Location: Emerald, Australia
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Hello Jeff, Thanks for the detailed reply and also the links. I've got to learn more about these heat loss calculations. It seems like a good starting point. 250 gallons, that's about a cubic metre and about a ton in weight. Thanks also for those figures on BTUs. I'll have a go at some crude calculations also. Have you seen this thread?
http://www.permies.com/t/28282/rocket-stoves/rocket-stove-water-heater-designs#221538
There is a lot of discussion on water heating there. I'll post any future develpoments as they come about.
 
Posts: 33
Location: Glasgow, KY zone 6b
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Jeff, I've been trying to figure out how to do pretty much the same thing. I posted it in the wrong category, but look at my post to see if my link may help you.
 
Posts: 7
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Jeff Thorpe wrote:I would like to build a RMH in my home and store the heat in water, and then connect to my baseboard system for whole house heating - has anyone done anything like this?



I am on the same kind of project, researching, and experimenting with the idea. I have built some crude wood fired boilers, some rocket stoves, and am currently building a large rocket stove intended to heat the house as a boiler. Have done some interesting firing experiments with cordwood as well as wood chips, quite successful but so far the stove is not hooked up to anything, just running out in the driveway.

{and yes, I am competent to build a wood fired boiler that won't explode or burn down my house. An important consideration}

Buffer tank sizing is a little complicated, but not overwhelming. I went through that in this post on Hearth.com, read it and then come back here. The short answer is you are sizing the buffer tank to absorb the full output of one firing of the wood stove (if it is batch fired) and then sizing the system so that so-many-firings of the wood boiler will provide so-many-btus daily for a house heating load. I ended up with about 450 gallons needed to allow 2 firings per day. That's typical.

The question I am pondering right now is how to make a good heat exchanger to absorb heat out of the flue gas from a big rocket stove. There are about three good ways to go:

Coils of copper pipe (water tube)
Steel or stainless steel tubes inside some sort of flue box (water tube)
steel or stainless steel tubes inside some kind of water tank (fire tube)

Copper melts at low temperatures and corrodes easily, but is a great heat transfer medium and is easy to work with. Copper does not get along with flue gases or dissimilar metals. I am imagining using copper coils inside a layer of castable refractory cement (to separate it from the corrosive flue gasses and provide a heat transfer medium) adjacent to a flue. Haven't thought this all the way through.

Water tubes inside some kind of flue box have been done by several hackers. There is a company that sells stainless steel tubes for this purpose. Simple but expensive answer would be to buy 3 of their coils, install them in a 55 gallon barrel, and plop that on top of my rocket stove. The rest is plumbing.

A fire tube heat exchanger is also interesting. If one was a good welder one could weld tubes into a tank watertight, fill it with water, add a circulating pump (and safety controls and temp/pressure relief!) and pump hot water to your buffer tank. A 30 gallon barrel, with fire tubes inside, inside a 55 gallon barrel, arranged so the flue gases went through the fire tube pipes, might make a pretty good heat exchanger.

All these heat exchangers are heavy and require independant structural support. Fire tube is the heaviest.

Water treatment is an important consideration often missed by us hackers. All of this stuff (flue gases, water) is corrosive. These guys sell all the chemicals and test kits you'll need to keep your pipes from rusting out in a year.

Open vs. Closed system? Open systems seem simpler at first, have less (but not zero) potential for a steam explosion. But they evaporate water, requiring makeup water, they introduce oxygen, which accelerates corrosion and thus use more chemicals. Pumps in open systems may have higher head requirements and may need to be self-priming. Closed systems solve about half these problems and introduce several more - need expansion tanks, perhaps more overtemp/overpressure relief valves, and so on.

What I am finding out is, the hydronic end of this will be more expensive than the wood stove itself. Pumps are $200 a pop, copper or stainless is outgrageous, aquastats and overtemp/overpressure valves are needed, expansion tanks aren't cheap either.

 
Jeff Thorpe
Posts: 23
Location: Underhill, Vermont
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Larry,

Thanks for the reply, I'll check out your links later. After doing the calcs I realized that water in an open system really isn't capable of storing a vast amount of heat like I thought it would. I've been wondering a similar thing as one of your ideas - if it wouldn't be easier to bury some coils in firebrick or cob? Storing the heat in solid form instead of liquid - the bricks can be heated up to 800 degrees or whatever, then just pumping water through the coils should extract that heat fairly well. Obviously preventing a steam kaboom would be important, and you'd have to protect the copper from both flue gases and cement.

I haven't thought about this much, but another idea has crept into my mind - everybody is afraid of steam, but what if we just acknowledged the power of steam and built a steam system? Instead of trying to prevent steam, using steam to our advantage? People have been building steam systems for 150 years, and you can readily get old steam radiators pretty cheap....?
 
Larry Lile
Posts: 7
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Jeff Thorpe wrote:Larry,

Thanks for the reply, I'll check out your links later. After doing the calcs I realized that water in an open system really isn't capable of storing a vast amount of heat like I thought it would. I've been wondering a similar thing as one of your ideas - if it wouldn't be easier to bury some coils in firebrick or cob? Storing the heat in solid form instead of liquid - the bricks can be heated up to 800 degrees or whatever, then just pumping water through the coils should extract that heat fairly well. Obviously preventing a steam kaboom would be important, and you'd have to protect the copper from both flue gases and cement.

I haven't thought about this much, but another idea has crept into my mind - everybody is afraid of steam, but what if we just acknowledged the power of steam and built a steam system? Instead of trying to prevent steam, using steam to our advantage? People have been building steam systems for 150 years, and you can readily get old steam radiators pretty cheap....?



Well, there are two schools of thought on this. Solid materials don't leak, corrode metals, turn into steam, or go kaboom. Many people have built high-mass stoves using cob or brick successfully. Others have simply built a conventional wood stove into their house and surrounded it with a nice arch of brick - I know some beautiful and successful homes built on this principle. But the physics of heat are in favor of water. Water has a high capacity to store heat per a given weight or volume. You'll need 4-5 times as much volume cob or brick to store the same amount of heat as water. Your brick isn't going to get to be 800 degrees, not after the first inch anyway, it will be warm, maybe hot to the touch near the heat source, so mostly it will be about the same range of temperatures as a water heat storage system. You'd control your water system so it doesn't get more than 180F, dropping to maybe 110F. Water *IS* capable of storing a lot of heat, and that is exactly why many people use it to communicate energy from an outdoor boiler to a house. Water is also a lot more portable than bricks, being easy to pump from point A to B.

This is a cat that can be skinned several ways, and thar ain't no one right way to do it. (Well, there are wrong ways. KABOOM! )

My great-grandfather died in a steam explosion at a homemade steam-powered sawmill. That's plenty of information for me about steam engines. A steam engine is a HUGE technological leap over a wood boiler, with high pressures, moving parts, tolerances, etc. Plenty of guys idle away many hours with this pleasant hobby, the ones that are not in the burn ward anyway, but I would not recommend trying it.

 
Jared Blankenship
Posts: 33
Location: Glasgow, KY zone 6b
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Has anyone completed any of the projects mentioned here yet?
 
Larry Lile
Posts: 7
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Jared Blankenship wrote:Has anyone completed any of the projects mentioned here yet?



Still working on it.

I am building an outdoor stove out of firebrick and lightweight concrete block, will put it in a small well insulated steel shed (with steel studs! Can't burn!) Completed the foundation before it started freezing this winter, masonry is on hold until thaw.
The stove will look a lot like a rocket stove, with a secondary preheated air supply. Although it will have a steel door, inside it will be all firebrick lined. It will accept 3 foot logs to reduce woodcutting effort. I can ut plenty of wood off my place that doesn't need to be split, just cut it to length. By the numbers one charge of wood should heat the tank.
I have a 450 gallon insulated wood tank, lined with EPDM roofing for a water storage tank.
Welded up a watertight firetube heat exchanger. This is a 20 gallon drum with firetubes inside. The drum is filled with water. the assembly goes inside a 55 gallpon drum, with 6" tube on either end, and baffles that make the heat go through the firetubes. Tested airtight to 30 PSI.
Water side is all closed loop. The tank is open, but that water isn't pumped,does not contact any steel, all the pumped water is inside closed heat exchangers in the tank. After studying open and closed loop, I concluded that open loop systems create a lot more corrosion and require more expensive pumps.
.
There will be several safety controls. Of course, PT relief valves. There will be one near the heat exchanger at a high spot, and one near the tank. Another I am considering is a spring-loaded diverter that closes a flue damper if there is no power to the pumps. A damper motor keeps it open to the heat exchanger, but if power fails or temperature goes to high the damper closes off heat to the flue heat exchanger. This would help prevent no-flow problems during power outages. I have several ways I could put this on backup power, so the pumps run when the power is off, and I will probably do that as well.
The loop to the stove has *no* inline valves, no strainers, nothing that could potentially clog up or be forgotten and block flow.

I weighed the pros and cons of putting this stove inside, and decided that, although I will engineer everything possible to prevent a steam explosion, the remote possibility of such a catastrophe inside the house would be a deadly disaster, but outside in a shed it would just be a mess. The big stove goes outside.

This is a complex project, and it'll take a year to complete. However, I've built stuff this complex before, and been successful after a few tweaks, so I'm not worried. My wife, however, is about to call the guys with the loonie truck.
 
Posts: 15
Location: durham nc
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a wood fired boiler would probably fit this function better l
hey yall have a 350 gal thermal secondary storage and a closed thermal siphon loop under city pressure inside the barrel of the rs.

the heat exchanger is 2 42" x3/4" hard copper pipes located next to the riser verticle it rises through the bung on the barrele with a refratory sliver and mudd gasket (this has been replced 3 times this winter, the gasket) probably becuase i keep drying wood up there
the bottom of the heat exchanger is inline with my clean out in the flu gas path the entire time.

the hot end of the thermal siphon is all copper or black steel fittings all the way to the top of the hot water heater tank i would ditch your abs plastic stub out to your pressure release valve tee, in with all metal with the pressure release valve pointed away from people

a rs is not likely to boil 40 gal water as if your taking direct heat away from the rs you wont have a fiesty burning rs, have built several homemade lout door boiler hydronic systems their is no boom! just a fizzle when the pressure release valve ever so slighty opens.
besides most homes are plummed with pex (plastic) yes you can boil water in these pipes under city pressure- sketchy i have an expansion tank on the hot water coming out of the tank . have not boiled water with the rs yet

1 hour takes the chill out 2 hours hot showers 4 hours multiple scalding bathes or steam sauna the whole bathroom, also when you heat water this way you have no thermalcline inside your hot water heater, more hot water

the cold water in the copper pipes causes dew point which quiqly gets coated with black scaly creosote looking stuff probably some surface oxidation then quickly coated, this stuff acts like a insulator i scrape away when i open the clean out i have stress tested the whole system running it on a shop vac for an hour feeding cedar framing drop cuts wvo ect i think the most dangerous aspect of the rs is the thin walled barrel which i have never gotten cherry hot with the vac

 
Larry Lile
Posts: 7
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Thanks, Derek - So if I understend you correctly you have a watertube copper heat exchanger inside the flue? Didn't quite catch your drift. Sounds like it is something of a creosote problem, but you are handling it by frequent cleanings. Is that right?
 
derek dihoff
Posts: 15
Location: durham nc
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so inside the barrel next to the spiil way of the heat riser where the flu gases downdraft i have two copper pipes with a upper and lower mini manafold that tees back to a single pipe, top pipe,to top of water heater always rising steeply, the draincock on the hot water heater is teed off and always falls slightly to the bottom of the heat exchanger bottom pipe that pops out of the cob. ( i have a slight constriction point on the bottom cold side so the heat can have a clearer choice which way to go) i use 2 guage copper grounding wire twisted around the manifold then drilled part way into the refractory bricks riser opposit sides like tongs i have them loose for thermal exspansion cycles - i wonder how well that copper is holding without liquid in it , also is breaking the mud seal on top of barrel every few weeks just pour some near by liquid maybe an old beer and re mud the seal

the creosote thing insnt so much an issue it probably helps sheild corriosion i peaked once in my main 1st cleanout because i thought it wasnt seal as good as i wanted, i did notice less buildup flakes like in the boiler systems more just surface black coating i only had a few hand fulls of looked like glossy magizine ash in there.

it seems like the rs takes a long time to get hot water hot especially starting with a cold core compared to the water jacketed boiler systems, but once its there with daily burns we usually have hot water or scalding hot water
i figure all of these fire on water sytems will fail ive only run a homemade heat echanger sodder/copper for 3 years straight on a single one and it still looked functional, hopefully the copper will pin hole first get to the cut off valve and the benefit will out weigh that muddy suck when it comes
 
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