Pocket Rocket to heat a hot tub

[grahamh]

Hi there, I'm a newbie, but have bought the book.

Planning to build a Rocket Mass heater for my office as a long time project, but in the mean time would like to build a pocket rocket type design to heat a hot tub.

My current thinking is as follows: To buy a second hand stainless steel beer keg and modify to a pocket rocket. The aim will be to keep the design water-tight so that most of the keg can be immersed in the water o f the hot-tub to heat the water directly. It will be a home-made version of the snorkel www.snorkel.com/hot-tub-info/snorkel-and-scuba-stoves.php

Do any of you, with much more experience than me, have any comments on this plan?

Apart from getting hold of the beer keg, my biggest problem is what to use for the chimney and air inlet.

[Donkey]

Well.. I imagine that it WILL work.
I've used snorkel stoves and while they do work (better than a coil thermosyphon system) they have a couple drawbacks.
They take up half the tub, not the worst problem ever but there it is.. They can't get hot enough to burn the fuel cleanly so they tend to be a smokey mess. Otherwise they're great!

The only difficulty I see for you is that the stove will want to float. You'll need to weight it down or tie it down somehow.
Oh.. you should also build some kind of divider to keep people from accidentally touching the stove, it's gonna be hot and will burn on contact.

[grahamh]

I think I am going to be OK for space as I am being given an 8' diameter plastic pool.

I hadn't thought about the running temperature though, I wonder if there is anything I can do to help?

Thanks for the feedback.

Graham

[Donkey]

If it's plastic, then you gotta be EXTRA careful that the stove doesn't come in contact with the tub too.

I've had an idea for a rocket stove hot tub heater, though I haven't messed with it yet.
Your idea is simpler in many ways and WILL work.
The basic design REQUIRES that the fire is in contact with the water as much as possible. You can't really fix that without doing something completely different.

My idea places the rocket stove outside and beside the tub and running the heat through some kind of heat exchanger in the water. This way, the fire can get hot enough to burn all the fuel and at least most of the heat can be exchanged into the mass of the water afterwards. There will likely be condensation issues and I'm sure other issues that will crop up at the appropriate time.
All in the life of an experimenter.

[endrunner]

What about the idea of doing a traditional rocket mass heater with the feed tube outside the tub and the barrel inside the tub. This would allow the burn to be insulated and facilitate complete combustion, Then the heat is tranfered to the water.

[tubbing]

Mar 4, 2010 18:34:36 GMT -8 Donkey said:

Well.. I imagine that it WILL work.
I've used snorkel stoves and while they do work (better than a coil thermosyphon system) they have a couple drawbacks.
They take up half the tub, not the worst problem ever but there it is.. They can't get hot enough to burn the fuel cleanly so they tend to be a smokey mess. Otherwise they're great!

As an employee of the Snorkel Stove Co., I'd like to address the above comment. The stoves do not take up half the tub, certainly not the average size tub. We sell two sizes of stoves. The larger takes up the equivalent space of one person, the smaller stove somewhat less. In a standard 6' diameter tub, the occupant space is about 4-5 people with a stove and 5-6 without. Of course, if you put the larger stove in a very small, 4' diameter, tub then Donkey's assertion would be correct.

With regard to them being a smokey mess, I'm a little surprised. All that is necessary to maintain a non smokey fire - with our stove or any other - is to make sure there is a sufficient air to fuel ratio under normal burning. When adding wood, as with any fire, if you toss a bunch of new logs on the fire all at once you will get smoke until they reach combustion temperature. If you add them slowly there is little to no smoke produced.

It is true that our stoves (surrounded by cooling water) do not reach a sufficiently high temperature to gasify and then burn all the combustible compounds, achieving a complete combustion of all materials. That said, very few stoves do and that is not a precondition for a non smokey fire. That can be achieved by some basic attention to fire management.

[Donkey]

I don't mean to say that snorkel stoves are bad.. Quite the opposite! So far, of all the wood fired hot tubs going, they've been the best. I love 'em! I like a good hot, Japanese style tub and it's now illegal to manufacture an electric or propane tub that CAN get up to the temps that I like. I don't know about the rest of you-all, but I just can't abide a tub that maxes out at 105.. With the snorkel, I can get enough heat to really scratch my itch, so you guys/gals keep up the good work!

I have to disagree about how much space they take up. Perhaps the stove itself only takes up enough room for one passenger, but once the wooden fence is added (keeps people from touching the fire-box) the space the thing takes up is FAR greater. That fence is pretty important, it's too easy to burn yourself without it, so if you own one of these, DON'T go without the thing. I have a good friend with a snorkel stove hot tub, his tub is a six footer (rough estimate) and I'd say a little more than a third is taken up by stove, fence, etc.
As to proper fire management, what tubbing says is true. Proper fire management is key with ANY wood burner. However, the basic elements to high efficiency burn is two fold. The right amount of oxygen for the fuel, no less, no more, and a VERY HOT FIRE. If any one of these two elements goes missing, you will be unable to burn the fuel completely. Any time you sap heat from a fire's combustion zone, you cool it drastically. Even when you can't see the smoke, if that fire can't get hot enough, you've got inefficiencies and creosote.

What Snorkel stoves do have is a VERY efficient method of getting heat to the water, which is lacking in every other system I've seen. Usually, we see coil type systems or something similar, where a water coil is placed in the fire and the water will (often) thermosyphon into the tub. These systems are the worst of all in that they cool off the fire, reducing efficiency of the burn AND the coil has too little surface area for high efficiency of heat transfer. I think (correct me if I'm wrong) that that was the main reason for the Snorkel stove, to fix at least one of the major drawbacks of wood fired systems, and they've accomplished this admirably well. I see some room for improvement in burn efficiency though.

So.. Tubbing, welcome to the board. Excuse my highly opinionated self AND my big mouth. I hope you stick around, I'm sure your input will be valuable.

[tubbing]

Hi Donkey,

I like the board and your opinions (even if I disagree with the one about the space the stoves take up and couldn't agree more about the necessity of having a very hot fire for efficient and more complete combustion as well as to reduce smoke and creosote. I should have mentioned that, but guess I was thinking more of the situation where people will overfeed a moderate sized fire (which of course cools the fire as you mention) with no hope of getting sufficient air to it.

At the risk of straying off topic, I'm guessing your friend has a 5' tub with one of our large, Snorkel model, stoves in it. That's the only way I can see how you would have the perception that it takes up that much room. Most people getting a 5' tub would opt for our smaller stove, the Scuba, which was designed for smaller tubs and takes up less space, although at some loss of heating rate.

I was allowing for the fence in my comment about the stove occupying the space of one person and I think that that is a demonstrably reasonable estimate. However, when you get into smaller and smaller diameter tubs, the fixed length fence, which is effectively the chord of a circle, cuts off more and more of the arc of the circle. For example, the fence on the Snorkel Stove is 4' long and in a 4' tub would bisect it and probably not leave any room for anyone.

Back to stoves and fires, you are right that one of the main advantages of the Snorkel Stove is that the entire stove body is a heat transfer surface. Also, it being inside the tub eliminates the need for plumbing. The advantage there is that there are no pipes to freeze and burst (which can happen rather quickly in sub-freezing weather). A problem common to all though is the cooling effect of the water on the fire. Your point that our stoves have room for improvement with respect to efficiency is well taken. We are working on that. There are a number of approaches that work. Integrating them into a cost effective product that people can and will buy is the challenge.

[Donkey]

You may be right on the 5' tub.. I'm sure my buddy got the larger stove, he ain't one to wait an extra second for his hot soak.

Would it be possible to place a rocket stove (or something like it) INSIDE a snorkel stove's metal body and force exhaust down, through and around before going out the chimney?
The rocket elbow is highly insulated (as you probably know by now) and does allow for complete burn.
I imagine that an eight inch stove would be a little too small for heating up all that water in a reasonable time. It would be overly fiddly with the small gauge feed tube too. Perhaps a ten or twelve inch model?? Also, given the need for the height of the heat riser, it would stick out of the top of the tub a distance, which would make using the original stove body impractical.

It's hard to imagine improving on the snorkel stove without a COMPLETE re-design, which as you say would, almost inevitably, lead to more expensive stoves. Really, it ain't broke and needs no fix'in.

[woodman]

I purchased one of these years ago: Wood-fired Water bath Canner/Cooker

www.lehmans.com/store/Kitchen___Canning_and_Preserving___Canners_and_Cookers___Wood__x2D_fired_Water_bath_Canner_Cooker___32185?partnerid=googlebase

It is all stainless and am planing on installing 11/2'' pipe one at the top and one at the bottom of the upper water tank. This not being a sealed unit I will have to make sure it is sitting level with the hot tube water level. I am hoping thermosyphon will work on this set up.

[Donkey]

What a COOL gizmo!

It looks a bit under powered for heating a hot tub..
Worth a try..

[woodman]

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Well I cut holes in this 1/8'' tank to install 1 1/2'' thru-hull fittings.
I had left over hose from a swiming pool vac that fit perfect.

[woodman]

Update pics. to give you the idea.
I also have an idea to shrink the amount of water in the tank so it would heat up faster. The outside dia. of a plastic 55gal drum is about 2'' smaller, so if I cut it to fit inside with 4'' of space off the bottom that should do the trick. Just bolting it around the top lip to keep it in-place. I also have a stainless lid that came with this but it just lays on top. It would be nice to have a sealed lid for it.

[canyon]

I would definitely try it without the plastic barrel to cut volume in the heater first. You might need that volume to buffer cuz' the thermosiphon might happen slower than you think. It would be good to see what kind of temps you are looking at as the plastic won't hold up to higher temps and will offgas stuff you don't want to breathe.

[Donkey]

It'd be nice to get the heater farther below the hot tub.
The further down the heat source, the better the thermosyphon will work.

Looks like yer place is kinda flat tho'.. Also, the heater would need to be sealed, which would add complexity to the system.
You'll at least need to pay attention to the relative levels of the pipe outlets. The upper outflow of the heater tank should be below or at the same level as the inflow of the hot-tub. Conversely, the lower outflow of the hot tub should be above or at the same level as the lower inflow of the heater tank.

You gotta make sure that the water level stays above the pipes.
I can see several people climbing in and displacing out a bunch of water. When everyone gets out again, the level can easily fall below the pipes, particularly (it seems) at the heater tank.