Help with wood fired hot tub!

Hello all!
I was recently gifted a wood stove, so i decided to use it to heat a DIY hot tub. After some research, i put it together and i am not impressed with the output! I am trying to heat about 100 gallons, and the water temp is around 50 deg right now. The best i could get was 84 deg of output, which barely heated the tub. i had spurts of energy that spiked the temp to over 100, not sure why! I am using 25 ft of 1/2 in copper tubing wrapped around the stove pipe with a small pump to circulate. Since the stove was hand made by a family member, i do not want to cut or drill into it, so putting the coil inside is not an option. Water flow is pretty high, so. I put a valve on the output side, and that seemed to help. I also shielded the coil, and then filled it with sand. i am thinking that i need to go down to 3/8 copper, and extend it to 50 ft.
Any help would be greatly appreciated!
Thanks!

Does your pump introduce bubbles into the water?  You could be getting some water restriction from that which could cause the spikes of hotter water to spurt out.  

Shielding should help a bunch, I'm not so sure about the sand helping.  Going longer with the pipe will help but be sure you don't get anywhere near the boiling point or you could have steam and interesting expulsions of very hot water.

Could you rig up a way to put the coil inside the stovepipe?  Then you'd have to be very careful to always have water flowing to prevent steam creation.

I wonder if snaking the pipe around the back of the stove would produce a higher temperature. Could you test the temp of the surface of the back & sides of the stove compared to the stove pipe?

the pump you are using looks identical to the one I use for my system, it is very sensitive to air bubbles and if there is no filter on the intake it also can clog very easily. (I found what looked like a miniature hairball in mine once) It is easy to check, four screws remove the outer pump shell from the motor and debris will be easy to spot and remove, also easy to put back together.

The idea of steam bubbles suddenly forcing their way through sounds about right for the temp spikes.

I would shorten the supply lines as much as possible, returning hotter water to the coil for reheating and saving more heat in the output of the coil for the tub, insulating them might help also.

If you could lower the coil somehow you could almost do a thermo siphon without the pump

I think you are having trouble because you are not taping into the most efficient part of the stove.

With the copper tubing wrapped around the stove pipe, I think you are experiencing fluctuations in heat depending upon the draft of the stove. For instance when there is low pressure system in the forecast, the draft is not as good, the stack temperature is lower and your hot tub does not get as warm. Yet when there is a high pressure system, there is better draft through the stove and chimney, the temperature goes up. Because water is 600 times more dense then air, there would be a long delay between when the hot tub gets hot and what the weather is doing so you would not necessarily make a correlation between draft and hot tub water temperature.

I would thus move the copper tubing, and add a lot more of it.

I would put this down by the stove itself and here is why. With the tubings current location you are just getting some heat transfer, but the air is being forced up the chimney in a fairly quick draft. If it was located down along the firebox you would be getting more radiant heat transfer and thus getting a lot higher temperatures.

You aren't touching the stovepipe in the top pic, heat transfer will suck. Don't use smaller tubing, you'll get more steam bubbles.  Take the pipe you have, wrap it tight around a slightly smaller pipe, then force it over the bigger stovepipe so as much of it is in contact with the stovepipe as possible.
Alternately, cut a notch at the top and bottom of a section of stovepipe and put the coil inside the stovepipe. You'll need plenty of flow to prevent any steam bubbles.
The heat source should be below the hot tub as well so you always have the hottest water going up, not trying force it down after it's hot.

Thanks for all the replies! I am thinking at this point that if i cannot get more heat out of the system in its current configuration, i wil probably try and put it into the stove pipe. Wrapping it around the back of the stove seems like it would work a lot better, but bending the tubing to fit the small stove would probably be a challenge!
When i shielded the tubing i tried to tighten it up as much as possible to the stove pipe, but some of it isn't touching the pipe because of the oval shape of the connection of the pipe to the stove. I also compressed the tubing down significantly, then added the shield. I was hoping that the sand would help transfer some more heat, but it didn't appear to help much.
Is it possible that i have too much water flow? I would think that the pump is pushing 2-3 gal/min.
Would it help to open up the coil so it is longer-meaning it runs along more of the stove pipe?
I am definitely going to shorten the lines and insulate them. I also plan to insulate the tub.
Thanks again everyone!

I noticed in the second picture most of the black was gone  (high temp paint?)

I use vegetable oil on my stove periodically to stop the rust, fumes aren't as deadly and it only smokes a short while--just as easy as wiping the stove with a paper towel, then use the paper to start your next fire.  the residue is basically carbon left on the stove, maybe not quite as black as stove black, but a lot easier

Try using fiberglass insulation without the paper on top of the tubing but under the shield, that will lessen the losses, keep temps higher around the copper. Sand is more an insulator in this application.

If you can't add to the coil and your shield is already cut, leaving it spaced out may be slightly better, but it probably doesn't matter much.

sliding it closer to the stove is a good idea too, or as close as you can get it.

I've experimented a bit on heating tubs of water, sometimes successfully!
My water heater is a propane BBQ so things are a little different but worth discussing I think.
The first thing I learned was that insulating the tub of water helped more than adding copper tubing. My tub was built using 2x4 house construction methods with the corners braced with steel angle brackets. With a little forethought I bet ratchet straps would work too. The tub was lined on the inside with 2" construction Styrofoam and when I wasn't in the tub I had a layer of the same Styrofoam floating on top. The water was contained by a single layer of common tarpaulin (tuck the corners down as you fill or you risk tearing)

Having the copper tubing touch the steel of the stove or pipe will help greatly. Would you be able to solder the copper to the steel in a few places?

Neal Giffin wrote:Is it possible that i have too much water flow? I would think that the pump is pushing 2-3 gal/min.

I don't think too much flow is a problem as long as the output temps are higher than your desired hot tub temp.  The real thing you care about is the energy transferred to the water.  Cutting the flow rate in half will probably increase the temperature of the water by double.  But it all adds up to the same amount of heat added to the tub.  And if the temp gets too close to boiling you end up with steam problems.  I'd probably turn down the flow until the temp coming out is in the 120-140 range.  Granted you aren't there yet but once you get the heat cranking better, that's what I'd aim for.

Neal Giffin wrote:Would it help to open up the coil so it is longer-meaning it runs along more of the stove pipe?

I don't think that would help much. Unless if by stretching it lengthwise it causes the coil to constrict on the stove pipe better.

chad duncan wrote:Would you be able to solder the copper to the steel in a few places?

My guess is that it would be hard to get the solder to stick to the stove pipe.  If you could maintain water flow through the tubing it possibly would "cool" the solder enough to keep it from melting due to the very hot stovepipe.  But if you ever lost water flow and the pipe heated up the solder would all melt

I have 120 feet of 1/2 in copper around a rocket stove barrel, using a similar pump with the thermostat set at 150 degrees, it pumps into a hot water heater tank about 30 gallons capacity.

takes about an hour to get a nice long hot shower, pump turns on and off 5 or 6 times during the heating,

I'd really like to find out how well your system works once you get it optimized.

Also what changes you end up making.

good luck

Sand is indeed a very good insulator, and not a very good conductor — which is what you want. I suspect the entirety of your troubles do stem from lack of direct contact between the copper pipe and the chimney. The more direct contact you can maintain over a longer distance, the more heat you will transfer. I really do have to stress how important the direct contact bit is. A small air gap between the stovepipe and copper pipe will result in orders of magnitude less heat loss — meaning 100x less efficient or more. I've attached an example picture of how tight you should be aiming for this.

Have you considered wrapping it around the top part of the stove between the handles and the top of the door? It would give you a larger circle to wrap around (easier to keep tighter) and get you closer to the radiant heat of the fire, which is going to be much more efficient than the stovepipe. I don't know how many wraps you would get out of your length, but two or three tight circles would do wonders right there since it's the hottest part of the stove.

After reading some of your replies, i made some changes, and it is working much better! I have increased my output temp 15 degrees. I filled up the tub with 60 degree water, and it was at 92 after three hours, that was without the shield and insulation.  I replaced the 1/2in copper with 50ft of 3/8, i read on my home brewing forum that the 3/8 has more surface area and would better transfer the heat. The smaller tubing was more pliable, so i was able to tightly wind the tubing along the stack, and closer to the body of the stove. I will report back when i get it finished! Thanks for the help y'all.

Pic of stove

Update: It is finished, and i am very happy with the results. Just to give you an idea of the efficiency...the last run i started off with 52 degree water, i ran the water from the faucet through the stove, and into the tub. It took about two hours to fill the tub, then i turned on the pump. About 5 hours later, i had about 120 gal of 101 degree water. It really helped when i addled the smaller tubing, insulation, and enclosure. I hope it works this good when it gets cold!
What should i use on the stove to inhibit rust?? I put some wd40 on it, but it didn't work to well.
Thanks y'all
Neal

To coat it I'd use the black spray paint they sell for stoves (or pink, blue, etc).  I used it on a rusty smoker I got and it turned out ok.  But I also used an angle grinder with a wire cup brush on it to take off much of the rust before painting it.  And I'm keeping it under cover now so it's not out in the elements.

There aren't too many things you can put on steel to both handle stove temps and prevent rust if it's kept outdoors.  Could you cover it with a tarp when you're not using it?

DIY: Stove black. Grind some charcoal very finely and mix with equal portion of graphite lubricant powder. Mix this in with an oil binder, we use bacon fat because it smells good as thew stove heats up for the first time after treatment. I use the end of a small piece of 2x4 wood to rub it into the surface. I do it hot sometimes too. It needs be done at least once a year, more is better.
On the subject of water flow through the coils, more is better. Yes the output seems cooler but the beast heat transfer happens when there is the highest temperature differential. This allows for maximum extraction of heat from the flue.

Another, DIRECT FIRED, method that I have been very happy with is my Biochar Kannibal-kooker.
https://www.edhat.com/news/biochar-and-the-kannibal-kooker
It takes a bit over an hour of feeding to get hot and provides a nice hot soak after working to make 5-10 gallons of biochar. Obviously the fire is completely quenched, to save the biochar (ash no bueno), BEFORE getting into the tub. WARNING: While firing, the metal tub surfaces are dangerously hot, especially above the waterline.

That article didn't have much detail about how to make the Kannibal Kooker.  Do you have more info?

Hi Mike,
I started making biochar with an oil drum cut open like a butterfly to facilitate introduction of brush while precluding excess oxygen (pyrolysis).  There are a few small, (sand covered) holes in the bottom so that quench water can drain but oxygen cannot enter during pyrolysis. Realizing/enduring the heat produced by the hour long production of biochar, I thought, "There must be a use for this obnoxious, face searing heat!" And so I set the barrel into a trench beneath an enameled steel tub. The shallow end of the tub is over a dug out flue that goes to a barrel modified as a chimney. This provides the draw for the flame. This is a prototype and rains have caused problems with the fill in which it is dug. Translation: Extremely happy with proof of concept, however I need to build proper masonry retaining wall for the surrounding earth.
While it is an ecological honour to make biochar, it is absolutely sublime to rest ones weary body in a hot soak.
Josh

Mike, This is a vid that i made about the barrel pyrolytic converter. It is in Spanish, and I hope that you can understand the ideas,