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Rocket Hot Water - The SAFE Way

Posts: 1288
Location: Okanogan Highlands, Washington
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I get a lot of questions about whether a rocket mass heater can be used to heat water.

There are three usual variations:
1) can it make my domestic (potable, or near-potable e.g. for aquaponics) hot water
2) can it heat non-potable hot water for heating other things (boiler for radiant floors)
3) can I use water as the thermal mass so that I can fit into a small / portable space (milk cans in my bench, tank instead of masonry bench, etc).

It turns out that the purpose is not the important question.

The important thing is whether you are able to be happy with an amount of hot water you can heat, safely, within your skill set.

Fancy and Dangerous: Pressurized
If you can only be happy with a boiler-thingy with piped, pressurized hot water, you need expertise.
Cheapest way to get it is to buy certified, off-the-shelf options.
Other ways are to hire a boiler mechanic, get qualified yourself (solar hot water installers and trained boiler mechanics can probably do this OK), or make a LOT of pies for someone who knows this stuff. Dead Men Steam School (www.heaterhelp.org or .com) does a wonderful job of answering technical questions from boiler repair people, and a few minutes on that site will let you know whether this is your passion, or over your head.

Functional and Manageable: Non-pressurized on-demand or tank
If you are willing to do non-pressurized tank-style or batch heating with, e.g, something like Geoff Lawton's on-farm water heater (a coil within a large tank over the heat), or a Mexican water-jacket hot water heater, you may be able to do something off a rocket firebox design.

Basic and Easy: Non-pressurized pot or kettle
If you are willing to use open kitchenware, non-pressurized, like a teakettle or stockpot, anything all 4 of your grandparents could handle safely, you are very much in safe territory.
Minor modifications and improvements can be made, if they don't result in pressurization (like an insulative hat on the lid, or a silicone-caulked tap in the side for drawing hot water off a pot like it was a cooler).

Before I go into detail (or annoy the nice Internet Cafe people), I'd like to invite interested folks to post links to threads, and particularly images and videos.
REmember, we are looking for SAFE, wood-fired hot water.
Safe does not have to mean 'a baby could run it' - it just means that the person running it has achieved an acceptable level of safety that is compatible with their apparent skill, and the skills and knowledge of the people who will be entrusted with operation and maintenance.

If you want to post something as a counterexample, feel free to label it FREAK SHOW OF FLAMING DEATH, or " " " STEAMING DEATH, etc, for the benefit of the curious.
No offense is meant - I imagine some people will post their own proud efforts in this category, along with lessons learned, and the stronger the warnings the better.

This thread is intended to become article-fodder, so please post only material you are willing to share (e.g. no patent-pending stuff).

Erica W
Posts: 126
Location: Springfield, mo
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Erica, Thanks so much for starting this thread. I saw a video of Paul Wheaton's where he said one of the top questions he gets about RMH's was how to heat water for aquaponics. I'm amazed that it hasn't been worked out to a simple set of options of basic designs yet. Although I guess everybody who builds a RMH does so with their own personal twist. Such appears to be the case with heating water as well.

Since I'm a highly technical guy I enjoy delving into all the possibilities of this subject. Consider this, all electricity plants (with the exception of hydroelectric) produce electricity by first converting heat from either burning coal, natural gas or nuclear reaction into steam which powers turbines that turn generators.
First basic point I think everyone should understand is that what we're talking about here is using the highly efficient nature of a rocket heater and using the heat energy produced by burning wood into heating all manner of other substances like air, cob, and water. As such we are limited by the amount of energy contained in the fuel used and the rate and efficiency at which it can be converted to heat. How we want to divide that heat output between space heating and water heating for consumption or simply water heating (like an aquaponics fish tank or thermal mass) will determine the methods used.

So if for example we have an 8" RMH and it might be capable of producing for purposes of this discussion say 50K BTU's/hr (a bit of a guess). If we are going to split that heat output up between radiant heat off the barrel, heat absorbed by a thermal mass, AND heating water, we need to understand the limitations that presents. Mainly the amount of BTU's /hr you'll need to heat the space and store for later radiation will limit the amount of heat that can be put into the water. Further the point of heat extraction for the water can affect the space heating output.

As Erica has said expressing ideas at 3-5am may get a bit foggy, so I'll let this simmer for a while and add to it later.
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Hi, i am in the plumbing and heating trade for 30 years master plumber/oil burner/ propane n.g tech. I currently heat my home, 2400 sqft plus full basement in western Maine with a 11000 btu oil boiler and more importantly a old Shenandoah wood furnace. The wood furnace was recovered from a boiler swap out i did 20 years ago, it originaly had a hot water coil inside the fire box but had long since expired. I set this unit up in the center of my basement and it is connected to a 45' tall interior chimney. I have since found an old cast iron water jacket that came from a similar stove and have it installed in the fire box. Along with this is 100' of 1/2" soft copper tubing coiled around inside of the duct plenum directly above the stove but not in the fire box. All of this piping is tied together, routed through the cast iron jacket and tied to my oil boiler. I have all the proper safety equiptment installed, and it is set up to run like this.
I start a fire, a thermal switch located just above the copper coils turns on power to a standard heat system pump when the temp reaches 200 this pump simply circulates water from the wood stove to the boiler, at this point the water in the stove portion is only heated slightly. As the fire heats the water that is now constantly moving, a aqustat senses the temp rise. When the temp reaches 180 the aquastat turns on the pump on my boiler that heats the main floor of the house, as the water cools from circulation the aquastat turns the pump off, and waits for the next rise in temp. This system has worked well for me for years, the only drawback is power failer normaly means no more fire untill the power comes back on. My main problem with this system is now in my older age firewood is getting heavy and i burn about 5 cord of wood.
I am interested in the opinions of those of you who use a rocket mass heater for any purpose, im not really interested in a heated bench since it will be in the basement, i am interested in using water as my thermal mass, but i want to be able to obtain 180 + degrees water. I have thought about simply coiling copper tubing through the cob mass maybe wrapped around the exhaust piping, or imbedding a small tank within the mass. There is also the barrel where a coil could just sit on top or be wrapped around it(i have seen this crudly made) in the end i fear after all the work i may end up right where i am now or slightly worse off, if i cant reach the temps needed.
Here is my question.....if you heat with a rocket mass heater how would you attempt to heat water to these temps if you where not concerned with safety? (sorry but i will take care of the safety aspect)
Posts: 133
Location: Kooskia, ID
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For those who haven't seen it, here's a video of the one Geoff Lawton has that Erica mentioned.
Posts: 3268
Location: Southern alps, on the French side of the french /italian border 5000ft elevation
Posts: 24
Location: BG
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Hello all.
Not sure if this helps but I posted this today in the wood burning forum - not sure how to paste it direct here either - sorry.


all the best
Posts: 110
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Looking at Mr. Lawton's outstanding design, one thought that comes to mind is to put an overflow catchment on the side for the mass tank, much like the overflow tank on an automotive radiator. Suction would refill the tank, at least partially, after the system cooled. It wouldn't be proof against having to top off, as there would still be some boil-off, but it would help.


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