Should we buy this 1.25" (31mm) stainless steel coil to heat an outdoor tub?

Hi all,

My husband just found somebody selling this water heating coil: list price is $625 (gack) and they're asking $500 (plus they'll throw in the propane tank they bough). They hooked it up to too large a tub, galvanized, uninsulated, and it's not working for them.



It is stainless steel and large enough to function as a thermosiphon, apparently. I looked around and though it's easy to find 1/2" stainless steel coils, this large diameter stuff is not so easy to find. . .

It comes with a propane burner, but I'm wondering if the heat from a rocket stove could be applied.

Get some copper tubing.
Get a piece of 6 inch PVC
Fill copper tubing with sand.
Bend copper tubing around PVC
Remove sand from tubing
Cut ends of copper tubing to desired lengths
Add fitting to copper tubing.
So long as copper tubing does not run dry I can not see how a rocket stove can melt it.

Or look for a Stainless Steel Immersion Wort Chiller
$59.99
https://www.midwestsupplies.com/stainless-steel-immersion-wort-chiller.html?utm_medium=feeds&utm_source=google&gclid=CjwKEAjw2MOhBRCq-Nr87_j-lDASJAAl4FNh7WWBgx3RXxa3YR8ELcldKNRFYvrTewzQiLxbtJtfaxoCmOjw_wcB

The special thing about this coil is the massive diameter of the tube, allowing the water circulation to work via thermosiphon, without a pump.

Of course, a pump isn't all that expensive. The original purpose of this item is to let you set up a hot tub when you are camping, somewhere where you don't have electricity to run a pump. Perhaps the only time it's worth spending big bucks for the giant spiral in stainless steel is when you want to take this thing off grid.

I'd love to hear from somebody who has gotten a thermosiphon to work, though.

Seems awful pricey. A quick search of my usual metal suppliers turned up this.

1-1/4" OD X .065" wall X 1.120" ID 304 Stainless Round Tube
BRUSHED - 20 ft. - $108.00

http://www.metalsdepot.com/products/stainless2.phtml?page=stainless%20round%20tube&LimAcc=%20&aident=#p1715

Almost certainly not the best price.

I have nothing on the thermosiphon working or not....

Julia,

I am not an engineer, so confess I may not complete understand all the physics behind a 'thermosiphon'. However, the large bore of the material actually works against heat transfer. The large circumference of the tube means that more water can travel through the core of the tube without touching the stainless steel, which is not the best thermo transmitter to start. Large water columns tend to laminate (read self insulate.) You want the smallest diameter that is practical. As to thermosiphon effect, even a coffee percolator siphons (to an open top) with heat. There is nothing magic about stainless steel in regards to thermosiphon.

Alex had a good post. To it I will add, $500 will get you a LOT of copper tube. As long as the copper has water to carry away the heat, it would be difficult to get it melting point. Although you might have a steam engine on your hands if you not careful. Make sure the water stays above both the inlet and outlet ports.

Copper tubing in sizes that can be bent by the ordinary person is going to have a small enough volume that an intense heat source could make it boil in one spot faster than the heat can escape... popularly known around here as the "boom squish" effect. The benefit of large diameter piping is the reduced susceptibility to instantaneous overload, as well as the greatly reduced drag allowing the water to flow faster and more easily without pumping. The benefit of stainless steel would likely be strength, offsetting the lower heat transfer capacity versus copper. If you did get a flash to steam, it would withstand the sudden pressure better than copper.

OK, so the stainless steel coil is overpriced. Agreed. However, now I'd like to hear how to expose the copper coil (or wort chilling coils - those seem like a decent middle ground) to the heat of a rocket stove in a safe way.

I have no need for a pressurized system: I'm hoping to heat water in an insulated tub to maybe 110 degree Fahrenheit, with something that is more efficient than the "snorkel" wood stoves that are out there. This is an outdoors application.

If you really wanted a steady state heat on the tubes you could take a metal pipe that could fit inside the coil and take a larger pipe that can fit outside the tube and weld them together using a piece of metal that looks like a washer forming a ceiled pocket for the coil fits into. Fill the coil pocket with BB's, iron bird shot, etc. You now have a way of preventing hot spots.

You can also rap the SS coils with pure copper wire. It takes time to do this but you can get several layers of copper wire on the SS. You lose some heat transfer but you also greatly decrease the formation of hot spots. You are increasing the mass and copper is a very good conductor. Since it is better than the SS it will spread the heat outside the SS.

Julia Winter wrote:OK, so the stainless steel coil is overpriced. Agreed. However, now I'd like to hear how to expose the copper coil (or wort chilling coils - those seem like a decent middle ground) to the heat of a rocket stove in a safe way.

I have no need for a pressurized system: I'm hoping to heat water in an insulated tub to maybe 110 degree Fahrenheit, with something that is more efficient than the "snorkel" wood stoves that are out there. This is an outdoors application.

Julia,

I wanted to give this some thought before I replied. Sorry for the delay. Copper melts at over 1900F. You are going to be hard pressed to melt copper without direct contact to the flame or chimney. The joints are a different issue. Flux will melt around 800, so keep fixtures/transitions away from direct heat. Water is steam at 212F obviously, if you are regulating combustion to keep water around 100F, you should not have a problem. Don't plumb in any valves or restrictors.

To answer your question: I would use a rocket mass stove concept. Just as you are heating the mass to radiate over time, the pipe can run into the thermal mass and absorb the heat you require without direct contact with the chimney or getting to hot. Also, since a rocket stove burns quickly and cleanly with smaller masses of fuel, if it gets too hot you can alway cut off air or pull fuel from the burn box, so temps in the pipe stay down below steam level. I figure if you can sit on a cob bench warmed from the heat of a chimney without burning one's self; one can plumb the pipe as to not over heat. I assure you, your bum has a much lower melting point than copper.

(As Alex suggested) A bucket of sand with a chimney up the center and the wort cooler suspended in the sand a few inches from the chimney will work fine. when the water gets to the temp you want, kill the combustion and let the sand maintain the water temp. If both ends of the pipe are free flowing you will get a siphon effect. Fine tuning for the right temperature range will be trial and error, I am afraid.

To make your own copper pipe without purchasing a wort chiller, a conduit bender is an option. Chose the radius you want and bend. Advance the pipe a few inches and bend the same angle. Slight lateral pressure while bending will get you a coil instead of concentric circles. Refrigerator tubing is small diameter and thin walled, so fairly cheap. You could do a small scale model for not a lot of money if you wanted to be sure the water would not over heat.

thanks! I will keep this in mind.

For more money you could buy a complete thermosiphoning wood burning hot tub heater:
https://islandhottub.com/wood-fired-heater/

To thermosiphon effectively, the hot feed line needs to slope/flow up. So if you want to use a rocket stove, geometric considerations require the hottub to be elevated and/or the stove lowered. You could plumb the lines through the wall of a (probably aluminum) cookpot (with stainless steel bulkhead fittings with gasket inside the pot) on a rocket stove. It helps if the cold return slopes/flows down, but it is less critical. Bigger pipes have less friction and therefore flow better.

if your handy you can probaby bend tubing yourself, at hardware stores there used to be steel coils at of wire for bending tubing without crushing it they are very inexpensive. if there is a scrapyard that is friendly you might find stainless tube for pennies by the pound instead of hundreds, just a thought from a frugal individual.

I was looking at the exact same thing!  I've found other less expensive stainless coils, but the inlet/outlet tubes are not positioned the same to enable easy thermosiphoning.  I wish others would replicate and market this exact design at a more reasonable price.  Presently I'm trying to copy it myself using what I can find.  Thanks for the post.  

Maybe you can consider  wood fired type coil .It`s cheaper if you want to enjoy bath outside.My friend ever brought it and DIY a very good hot tub,perfect project.

I guess my main question would be, who is going to get in and out of that tub in the cold, while dripping wet, and rebuild that tiny fire under that coil every 15 minutes?  :-)

That’s a huge coil require large amount of firewood consumption but it’s a interesting design. For camping where a fire is desired and constant source of hot water. Wow that’s glamping!!
Ive got a huge coil of 1” copper tubing and might try it just for grins. Unfortunately I don’t have a good supply of firewood like I used to have.
One drawback of the open fire is the fire bans and burn hazard even if there is no fire ban. A small enclosed controlled flame could be used year-round without anyone knowing or being the worse for it.