Rocket Stove Hot Tub Heater?

[hankyknot]

I bought the Rocket Mass Heater book when I first heard about rocket stoves while researching Yurts. I am now looking at ways to use a Rocket Stove to heat a hot tub and have come up with the following idea which I will try to explain as best I can.

Picture a rocket stove next to a cedar hot tub. The feed tube would also be next to the hot tub within reach of occupants. Instead of the feeding out throught a day bed though it would feed through a "storage pad" under the hot tub. The tub would actually be supported on a sub floor supported by those concrete deck piers people use. The space under the floor would hold the pipes with cavities being filled by urbanite and other stuff to store as much heat as possible.

Inside the heat riser would be a coil made of 1" stainless pipe that would carry the water into and out of the rocket stove and using a thermosyphon process in turn heat the water in the hot-tub.

While typical rocket mass heaters are a linear set up I cant help but wonder if it would be possible to "rotate" the exhaust so it exits nearer the feed tube. Instead of having the feed and exhaust at 6 and 12 (when viewed relative to a clock face) the feed could be at six and the exhaust at 9.

A small towel heater could be placed on the rocket stove to keep them nice and warm.

Is this feesible? or does it have a be a linear setup?

[canyon]

You can route the exhaust any direction you want. I hope you are talking about the ss pipe coil being on the outside of the heat riser as that is preferable. You want to harvest the heat after the heat riser not from it. Also, for thermosyphon you need both adequate height differential and pipe diameter to keep enough flow to avoid steam/vapor lock. I'm not sure if you have either from the brief description. Are you talking about stainless pipe or tubing? How are you going to bend the coil (or do you have one already?) I think a rocket fired hot tub is a great idea but unfortunately is a few notches down on my priority list so I don't have any real experience on this yet... But that doesn't stop the interest! Please keep us posted!

[hankyknot]

I was planning to put the coil inside the heat riser assuming that it would harvest most heat there. After all the stoves primary purpose is to heat the water in the hot tub. If however it would work better with the coil between the riser insulation and the heat exchange barrel then that is where it will go. I suppose there is the possibility of the pipe getting so hot that it will affect the connection to the tub (the source of the tub has yet to be determined but I am considering a large plastic stock-tank type tub that would then be clad in cedar for aesthetic reasons).

I bow to everyones greater experience in this field.

As for the pipe diameter, I was looking to use either 1" or 1.5" for the coil and would presumably carry this size through to the connections with the tub. Are there any recommendations for this pipe size or is it purely a case of bigger (above a certain size) is better (up to a limit)?

The heatpad on which the tub would sit is more of a "why waste it" idea than anything else, after all if the pad can provide a little bit of thermal mass and slow down the cooling or speed up the heating its not really costing me anything and just saving me time.

I would really like to see any examples of what someone else has done along this line and as always welcome any advice or criticism, after all I'm here to learn.

Thanks in advance

[canyon]

The purpose of the insulation around the heat riser is to achieve the highest temps possible within for combustion. If you put the coil in there you would be lowering the temp in the heat riser(as well as prematurely burning away your expensive coil).

Are you looking to maintain this hot tub warm and spike it up for use or will you be firing it from cold once a week or what? What is your region/climate?

Thermosyphon is a beautiful thing but does need the right conditions to work. Bigger is generally better in the pipe size department as well as the higher the better in the height differential department. That is where the down flow design after the heat riser helps. Have you checked out Donkey's cool sketch elsewhere on this forum?

If you included a small pump you could save a bit of money on pipe size and get a lot more hot water quicker. You could even set copper tubing (or stainless of course) in the first (hottest) part of your mass underneath the tub and harvest warm water from there once up its up to temp. You need quite a bit of surface area for heat exchange. I've been thinking for a while that those older Paloma on demand propane hot water heaters (that some people are now ditching because the diaphragm froze and or they want the newer versions) would come in handy for cheap or even free heat exchangers that when stripped down you could set in the exhaust flow. I picked one up recently for $15 with this in mind.

[Donkey]

Hey... a legit post.. man.. The spam posts are start'in to bug me..

Personally, I'd avoid the complexity of pumps, coils and the like..
It seems that a loop of (somehow) waterproofed pipe going from the top of the heat riser, directly into the tub, round and out to chimney could make for a simpler, perhaps more efficient setup. It would have to be sized properly. Too long will have condensation problems, to short will waste heat, etc..
Perhaps a radiator of sorts could be made that would clip to the side of the tub. A steel box with a divider down the center and attachments for in from heat riser, out to chimney.

[rocket_richard]

I wonder about having mass under the tub?

I'm worried the mass will slow everything down in terms of heating and cooling. It depends on how you plan to use the tub. If you plan on heating the water only when you use it, then the mass will rob heat from the water as you're trying to heat it, increasing the time it takes to get your water up to temperature. The only benefit to mass would be to slow cooling. But the rate of heat loss is so high you'd need to be using it frequently to get a benefit from the mass.

I would skip the mass, and add lots of insulation, under, around, and on top when not in use. The water itself holds more BTU's of heat for it's mass than any other substance, so just trying to keep the heat in the water itself will go a long way towards reducing heating time before each use.

Finally I'd say heat the water directly through some type of tube heat exchanger using thermosiphon.

Good luck, I'd love to see some pics if you get something built.

[Donkey]

Yeah... There's plenty thermal mass in the water. Probably no need for more.

If you plan on using a thermosiphon, you must do as canyon said earlier, and provide at least 1 1/2 to 2 feet of height difference between the top of the coil and the bottom of the hot tub. This is to prevent vapor lock and reverse heat cycling when the fire goes cold.
It will help to have your tub on a hill or some kind of slope.. If you imagine for a minuet -- two to three feet of rocket stove (height) then a foot or so of coil/heat exchanger space, then 2 more feet of insurance space THEN the bottom of the tub-- that puts your tub up in the air a bit.. Now, you can locate the coil around the outside of the heat riser, between it and the barrel which lowers things a little, but only a little.

Use larger gauge pipes, 3/4 inch or bigger.. With the kinds of heat available in a rocket stove, smaller diameter pipes will heat too fast and can flash to steam inside.. I know of this happening in at least one case, with a half inch coil of copper. It's VERY dangerous and can lead to explosion.

Seems to me that simply sticking the stove exhaust pipe directly into the water would be simpler AND more efficient for heat exchange. No flash worries, etc. The only negative issue I can think of right off the top would be condensation inside the pipes and that can be handled with a little experimentation on the length of run inside the water.

[ekw]

In addition to steam explosion problem, even with a large pipe you'd risk scalding people with super-hot water coming directly out of the stove core. If you do a thermosiphon, wouldn't it make sense to go to an overhead hot-water storage tank (that's large enough to absorb your stove's heat)? Then you could feed the stove by hand from the tub as you describe, and turn on more hot water when you need it. But maybe I'm thinking in terms of a bathtub, not a big hot tub.

I think the exhaust would be safer and more practical to use. You can build the stove beside the tub as you suggest. If you can get waterproof, heatproof plumbing that would carry the gases through the water itself, maybe under a "suspended floor" in a big hot tub, it could deliver a lot of heat to the tub at reasonable temperatures, away from tender skin.

Either way, it's going to take a lot of heat. Keeping it covered and insulated between uses would preserve some of the heat for a while.

This might be why traditional peoples combine saunas and cold dips in most regions. A little steam goes a long way to warm up a closed room, but it takes a lot of fire to heat a whole tub of water. "Hot springs" are great, but volcanic activity does the work and we just wander downstream until we find a temperature we like.

Hot running water: what a marvelous invention! Good luck re-creating it.