Rocket stove hot water heater, the easy way

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Rocket stove hot water heater, the easy way

Postby Nirky » Tue Mar 10, 2015 10:05 pm

Has anyone simply taken a gas hot water heater, put it 3 feet off the ground, taken out the gas hardware at the bottom and substituted a small rocket stove? Build a cinder block enclosure if necessary.
Seems to me like the easiest & cheapest way to get free hot water. Anyone done this?
I don't think I'll ever get over Macho Grande, those wounds run pretty deep.
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Re: Rocket stove hot water heater, the easy way

Postby Nirky » Wed Mar 11, 2015 6:34 pm

95 views, and yet no replies. Nobody has tried this yet?

Here's what a gas water heater looks like inside:

Image

Remove the burner & all the gas controls and lines.
The flue baffle slows down the exhaust gasses to transfer more heat, but creates a bit of a restriction for our rockets.
So I think they would not burn as clean or as well. But hey, it's free heat!
I don't think I'll ever get over Macho Grande, those wounds run pretty deep.
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Re: Rocket stove hot water heater, the easy way

Postby pa_friendly_guy » Wed Mar 11, 2015 10:10 pm

I have never tried this idea, but I think it will work. The inner vent would allow the heat to get to the water as well as the bottom area that would apply a lot of heat to the water. To heat water for your home or for a remote cabin this unit should work well. I would make two comments, 1st it would take awhile for that many gallons of water to get up to temperature, 2nd you would need to take a lot of precautions so that the unit would not over heat the water and explode like a bomb. This could be done with pop off valves, but I think you should not let the unit unattended while it is in operation. This unit would need to be located outside because of the exhaust. The unit I like for heating hot water uses the barrel of a normal rocket mass heater and wraps copper pipe tightly around the out side of the barrel. It acts as an instantaneous water heater and heats the water as it is used. You can have it located inside the house so freezing is not such an issue. You still need a pop off valve to avoid explosions but you have a lot less water in the unit and there fore much less danger from the unit over heating and becoming a bomb inside your home.
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Re: Rocket stove hot water heater, the easy way

Postby 4seasons » Wed Mar 11, 2015 10:19 pm

I have never built a hot water system so I was waiting on someone who had to reply. Could it work? Sure, but unless several layers of safety are added, you have basically built a steam bomb. A gas water heater has controls that shut the gas down when a certain water temp is reached. With a rocket stove there is no shut down before you boil the water, steam pressure builds, and boom. There are ways to keep this from happening but since I am no steam engineer I will not try to explain them.

The safer (and simpler) way to heat water with wood is to use an open system. Basically you set an open water tank on top of your heater. Something like setting a second barrel, on top of your heat riser barrel, full of water open to atmosphere and topped off regularly. Then you run your pressurized lines thru the open water tank. This way the water in the tank is allowed to boil (212F) but because of the open system pressure never builds. Then the pressurized lines in the tank are never exposed to more than 212F and therefore there is little danger of creating a steam explosion. Of course you still might want to set up a pop-off valve and you have to be aware that typical water heaters only give you 100F-150F water while this type system can get you over 200F water.
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Re: Rocket stove hot water heater, the easy way

Postby hpmer » Wed Mar 11, 2015 11:30 pm

A guy in Australia did that very thing. I think the problem was the exhaust pipe is only 3" or 4" which means it's hard to build a good working stove, as anything less than 6" or so is quite tricky to get working well.

I'll see if I can find a link to the video I saw. It was on a farm if I recall correctly. Might be worth a look.
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Re: Rocket stove hot water heater, the easy way

Postby Monsanto » Thu Mar 12, 2015 8:22 pm

I am at the initial design stage of using an RMH as a means to heat water.
My thoughts were to fabricate a cylindrical water jacket (inner and outer metal tubes with 3cm gap between them) with two ports to thermo-siphon water to a thermal store. This would be about 1000 litres capacity. The water jacket would have a steel plate welded across its top and be in place of the normal 55gal oil drum. This part of the system would be open vented.
In addition a pumped circuit could be added to heat the primary coil of an indirect hot water cylinder.
Another coil inside the thermal store could provide pumped hot water for either central heating radiators or underfloor heating.
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Re: Rocket stove hot water heater, the easy way

Postby Nirky » Sun Mar 22, 2015 6:19 pm

pa_friendly_guy
I think it would work, too. I agree the water would take a while to get up to desired temp, but in gas form these heaters take 35-40 minutes to come up to temp. A 4" rh would take at least that long. One would just have to plan on when to take a shower/bath or do the dinner dishes. More work than we are used to, but it's free heat!

4seasons
Water heater mfgs are also aware of the "splat" factor, so all newer gas water heaters come with pressure relief valves. Finding a newer gas water heater is essential.

hpmer
Too bad we have no info on the Australian guy who did the conversion, but I think it's pretty straightforward. My first rocket was a 4", and it drafted very well. Perhaps the Aussie made the mistake of leaving in the baffle?

Monsanto
Open system is definitely one way to do it, but for me, I like to take baths, especially in winter. So I'd need a shot of 40 gallons of hot water. To get that from an open system you'd need a bunch of copper piping for the heat exchanger, yes? That's a bunch of money right there, and the scope is bigger than I intend. Also, a residential water heater has insulation already there in one almost-complete package. I need to try it before I'd move a larger, much more expensive open system.

I'd love to see your system though, please post pictures or links here if you get it going!
I don't think I'll ever get over Macho Grande, those wounds run pretty deep.
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Re: Rocket stove hot water heater, the easy way

Postby hpmer » Sun Apr 26, 2015 1:38 pm

Here's the link to the stove in Australia I mentioned earlier:

http://www.milkwood.net/2011/08/03/our- ... -years-on/
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Re: Rocket stove hot water heater, the easy way

Postby Nirky » Wed Apr 29, 2015 11:12 pm

Thanks for the link, hpmer. They of course did it different than I was describing, using the tank as just storage and not directly heated with a rocket underneath. Like engineer775 demonstrated: simple hot water

Since nobody has shown it done the simple way, that might mean there is an inherent problem with just putting a rocket where the gas burner was? I still think it's worth getting a hold of a used & broken gas heater and trying. I'll post it here if I can find one.
I don't think I'll ever get over Macho Grande, those wounds run pretty deep.
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Re: Rocket stove hot water heater, the easy way

Postby hpmer » Thu Apr 30, 2015 12:24 pm

Yeah, that wasn't the link to the project that had the problem, but was a good solution that seems to work well for them.

I think the issues with what you described are several. First the flue pipe I think was only 3" which makes for not a lot of heat, and it is a straight shot up through so doesn't transfer as much of the heat as it would if it had baffles in it. Also, that's a lot of weight of potentially very hot water suspended above the stove, and thus requiring significant bracing to insure safety. The heat exchanger model seems to solve many of those issues.

Donkey, on his board, posted other solutions which seemed very promising.

But certainly worth a try if you find a heater and think you can do it safely. Keep us posted.
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