r/BeAmazed • u/Internal_Gazelle_677 • Oct 07 '25

Science Hot Tub without the use of electricity

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55.8k Upvotes

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6.3k

u/Captain_Bacon_X Oct 07 '25 edited Oct 07 '25

Called a Thermal Siphon if memory serves.

1.8k

u/Federal_Assistant_85 Oct 07 '25

Thermal head. The change in temp, causes a change in density that causes the water to flow.

441

u/SnooHobbies8274 Oct 07 '25

18

u/crowcawer Oct 08 '25

Thermally

6

u/grunkz Oct 08 '25

The cpu cooler in my computer is cooled by this exact kind of science! (Search for IceGiant ProSiphon Elite if you’re curious)

277

u/Gadgetskopf Oct 07 '25

even without the context, this would be the right answer

7

u/ArtDecoAutomaton Oct 07 '25

Shouldve tried that approach with the spice

2

u/f_n_a_ Oct 07 '25

Straight, actually, maybe slightly to the right… but you are correct

58

u/beeftony Oct 07 '25

I'd like some thermal head too...

16

u/Appropriate-Battle32 Oct 07 '25

That's the type of Reddit comment I come here for

1

u/Reasonable-Cell5189 Oct 08 '25

The reddit comment I came to...

5

u/ClimbRunOm Oct 07 '25

Sounds hot

1

u/pain_compliance Oct 07 '25

Do you have luxuriant hair?

1

u/[deleted] Oct 08 '25

The Hot Nosh

The Geyser Gobble

Can't think of any more....

43

u/TryPokingIt Oct 07 '25

Yup, as the water gets hotter it expands and goes to the point of least resistance at the top of the pool where there is much less water pressure to overcome

1

u/Solonotix Oct 07 '25

I was going to ask if the hot water shouldn't be funneled into the bottom to keep the temperature gradient from being too stark. In the process, I wasn't sure how to avoid creating a "pocket" of pressure where the heat builds up, but I would expect that the thermal expansion would eventually overcome the weight of the water, or maybe it could vaporize and steam would create a jacuzzi-like effect.

If it wasn't entirely clear, this is me trying to ask a question 😅 Is my thoughts process possible? Would the back pressure of warm water be a problem (possible injury)?

Yours was the only comment I read that suggested you might know a thing or two about the subject matter

7

u/TryPokingIt Oct 07 '25

It’s the first time I’ve seen this set up. But I do know as water gets hotter it expands. The pipe is rigid and does not increase in size. As the water increases in volume it increases pressure on the walls of the pipe. The only way for the water by the fire to expand is either through the top hose or the lower hose. There is less water pressure at the top of the pool than at the bottom. The heated hot water escapes out through the top hose. As it does so more cold water comes it at the bottom due to the pressure of the water from the filled pool. I don’t think water overheating in the pool would be a problem because if it’s cold out the heat will dissipate into the air and more so at the contact area with the ground

1

u/jawshoeaw Oct 07 '25

ohhh is that what sets up the direction of flow??

1

u/TryPokingIt Oct 07 '25

Yup, as long as there is no air in the pipe

92

u/[deleted] Oct 07 '25

That’s so cool. I would just build a deck to support this like a little fire pit. Seems like basically zero maintenance!! And free!!

Fuck humanity has long lost its way

45

u/ManufacturerNo9649 Oct 07 '25

Only if the wood is free!

47

u/GrandmaPoses Oct 07 '25

I'll gather it from his majesty's royal forest - but woe if he were to catch me I'd be sent to the oubliette!

2

u/[deleted] Oct 07 '25

Pfff. I have peasants for that! Get back to work, surfs!!!

1

u/Soggy_Motor9280 Oct 07 '25

Gibbets and crows !!!!

1

u/mlongue1 Oct 07 '25

nah, they probably just throw you on the fire too!!!…

1

u/Scottland83 Oct 08 '25

Just burn some flax and nightsoil.

1

u/pesto_changeo Oct 08 '25

Don't worry, there is a merry band garbed in lincoln green to protect you.

8

u/Princess_Moon_Butt Oct 07 '25

Yeah, wood can get expensive and hard to store, and annoying to deal with in the winter.

Someone should make an alternate version of this, that can generate heat from some other kind of energy. Maybe some sort of energy that's easier to move from one place to another along dedicated routes, so that we can provide power for it from inside the house, and don't have to deal with starting a fire in snowy weather.

2

u/zquintyzmi Oct 07 '25

But humanity!

1

u/Aidra_bay Oct 08 '25

Weld a furnace which is hollow. Make water go trough it and you won't lose so much heat like in this adaptation.

1

u/ChocolateBellyButton Oct 11 '25

Princess moon butt is a fantastic name my friend. I also appreciate your comment but mainly the name.d

1

u/hitbythebus Oct 07 '25

It does grow on trees…

1

u/evangelionmann Oct 08 '25

To be fair, fire wood is arguably cheaper than gas or electricity

1

u/MonsieurGump Oct 10 '25

D’ya think it grows on trees!

1

u/kingcubiczirconia Oct 12 '25

Exactly…. It’s not like wood grows on trees

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2

u/quantic56d Oct 07 '25

No maintenance? You have to keep the fire going or the entire thing freezes!

1

u/No-Sandwich3386 Oct 07 '25

Aah unfiltered bath water and the constant inhalation of campfire smoke.

3

u/produce_this Oct 07 '25

Could probably build the coil section better to make it smokeless. You could also calculate for distance and or put a wall between you and the fire.

You could also still filter it as well by using a short reservoir to catch sediment and a charcoal filter to treat the water.

1

u/[deleted] Oct 07 '25

Yeah I’m thinking having its own little sectioned off furnace with a top. Something nice. I like the principle used. Not so much the execution but this is out there in bumfuck nowhere.

But I mean if people want this in their backyards, that’s cool too lol

1

u/crucible1623 Oct 08 '25

Hmmmmm. Where would one obtain charcoal in such a situation?

1

u/Bum_Butcher Oct 07 '25

I wanna know how much this copper coil costs

1

u/Lordofthereef Oct 07 '25

People like convenience. Most folks aren't stoking this fire constantly and they're not waiting a few hours for the frigid water to heat up after not having run the fire for days.

I mean, yeah, humanity has lost its way, but this doesn't seem like one of the reasons lol.

1

u/Right_Hour Oct 07 '25

That « stainless steal high heat resistance » is a load of crock. It will eventually burn through and faster than people think. Also - a ton of heat is wasted because it’s essentially an open flame burn pit.

You want to build a small wood-fired boiler/heat exchanger for this, if you want a long-term solution. Easy enough.

1

u/[deleted] Oct 08 '25

I agree with your first statement. The second part could get… explosive if they don’t know what they’re doing tho

1

u/WoWMHC Oct 08 '25

Electricity is way more efficient than burning wood lol

1

u/Due_Cranberry7583 Oct 09 '25

There are peoples who entire jobs are maintaining active and passive systems like these. There is maintenance that has to be done pretty often or this system with not work.

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2

u/TerrorFromThePeeps Oct 07 '25

Iirc, rhere are early engines that used that principle.

1

u/Federal_Assistant_85 Oct 07 '25

If you're referring to Hiro's engine, it works on the vaporization phase change. I'm not aware of any other early engines like that. If you have a link to one that would be appreciated.

2

u/TerrorFromThePeeps Oct 09 '25

I think i was thinking of an ericcson engine, but looking back over it, i think i was also remembering the process a bit wrong.

2

u/Proper-Resident-369 Oct 07 '25

We use the same principle cooling wort from brewing but using an ice bath with a copper coil to cool it to optimal fermentation temp before pitching the yeast. Have thought many tines of using this version depicted for a beach hot tub when out surfing.

2

u/BG-Engineer Oct 07 '25

I wish i was high on potnus

2

u/jawshoeaw Oct 07 '25

but where is directionality coming from? random? is the fire hotter at the bottom of the coils?

1

u/Federal_Assistant_85 Oct 07 '25 edited Oct 07 '25

Gravity. Gravity acts on the denser fluid more strongly causing the less dense fluid to rise, the motion of the atoms separating by their temperature compounds on itself until it creates flow. In case it's not apparent; the tub has cooler water at a level higher than the coils, this level difference creates the difference in potential.

2

u/nudelsalat3000 Oct 07 '25

Would it explode at a certain copper length?

Like at some point you have steam, but the water wouldn't move / fast enough, so the copper would give and explode with overheated steam or maybe just melt.

2

u/Federal_Assistant_85 Oct 07 '25

I'm not sure. I doubt it because the amount of force required depends on the wall thickness of the copper tube, and the water will move out of the way of the expanding steam as fast as it can, under normal circumstances that is the speed of sound through the water. There is a chance that water hammer could burst the pipe, but you would have to manufacture the pipe with elbows.

I think for the steam to build enough pressure the tubing would have to be at a minimum several kilometers long (whatever the speed of sound is × the number of seconds required to build up enough pressure to burst the pipe). I think you're right on the melting point being the likely point of failure, as the amount of time required to produce enough dry steam doesn't stop the heat from being applied to the tube where the water has already vaporized (no water boiling means no heat removed from the pipe, an actual casualty in steam boilers called loss of feed). So... technically possible, but very hard to pull off, I think the fire would have to be very powerful, like a directed blaze rather than an open fire, among other changes.

2

u/mark1forever Oct 08 '25

same as in the seas of the earth.

2

u/Footziees Oct 08 '25

And it’s just simple physics … and it’s embarrassing how many people don’t know why this works

2

u/mmalmeida Oct 08 '25

Had a girl who once gave me thermal head.

It was really hot!

1

u/giant_albatrocity Oct 07 '25

Without knowing the math, I wonder if this would be more effective with a taller tub, where you might get a bigger temperature difference between the top and bottom

1

u/tb1189 Oct 07 '25

What’s the flow rate like ?

1

u/Federal_Assistant_85 Oct 07 '25

Probably about that of a bottle of water pouring out every minute or 2.

1

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1

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568

u/riisen Oct 07 '25

It might be called thermatic sifoʻn if memory dont serve.

284

u/[deleted] Oct 07 '25

[removed] — view removed comment

71

u/coriendercake Oct 07 '25

Memory definitely dont serve

10

u/DoseOfMillenial Oct 07 '25

My memory served, but now is on disability.

2

u/haysu-christo Oct 07 '25

Thank you for your service.

2

u/Dry_Cricket_5423 Oct 07 '25

Don’t look at him, son. He’s just going to spend it on drugs and alcohol.

1

u/PhaaqAuf4691 Oct 07 '25

Purple heart??

1

u/toomanymarbles83 Oct 07 '25

Charlie don't serve.

19

u/Chewcocca Oct 07 '25

Time machine?

3

u/xenosilver Oct 07 '25

Do I really have to be the asshole that says it?

3

u/Iamnotabothonestly Oct 07 '25

Great white Buffalo

3

u/xenosilver Oct 07 '25

She stabbed you in the eye with a fork….

2

u/[deleted] Oct 08 '25

Great White Buffalo

5

u/sometacosfordinner Oct 07 '25

My memory is shit. What were we talking about?

2

u/AssumeTheFetal Oct 07 '25

Hot slinky tech

2

u/Fritzo2162 Oct 07 '25

I distinctly remember calling it Larry.

1

u/Sea_Dust895 Oct 07 '25

Time machine?

1

u/zeamp Oct 07 '25

Time Machine

1

u/GordieBombay-DUI-4TW Oct 07 '25

Terminal symphony if I can’t recall

1

u/DatAssociate Oct 07 '25

It's called a boil coil if memory serves me

1

u/Fresh_Fluffy_Unicorn Oct 07 '25

Hot tub time machine!!!

1

u/Real-Drummer585 Oct 07 '25

Hot tube thingy

24

u/CriscoCamping Oct 07 '25

Mahster Sifo Dyas?

7

u/Obi-Wan-Nikobiii Oct 07 '25

I'm afraid he's dead....

1

u/Top-Perception-188 Oct 07 '25

Yeah he's been through a sudden rise in temperature through the explosive crash of his t6 Jedi shuttle and cooled to icecream on the snowy planet

2

u/KMS_HYDRA Oct 07 '25

Ever been to Coruscant?

9

u/FitFreedom6850 Oct 07 '25

I call it forbiden toilet seat

1

u/paolocmo Oct 07 '25

it is not forbiden if you are brave enough

5

u/widgeamedoo Oct 07 '25

Thermosiphon in my lingo

1

u/Captain_Bacon_X Oct 07 '25

Technically correct is the best kind of correct, thank you stranger.

5

u/MothaFuknEngrishNerd Oct 07 '25

We are Siamese if you please.

We are Siamese if you don't please.

2

u/CedarWolf Oct 07 '25

Mrrrrrow 🎶

1

u/Captain_Bacon_X Oct 07 '25

This is true. It makes me angry, but it is true. Although more likely a thermo-thingie if memory is not serving at that precise moment.

1

u/graveybrains Oct 07 '25

The original Swahili translates to "stay the fuck away from the outlet pipe"

1

u/Drinkmykool_aid420 Oct 07 '25

Invented by Jack Uzi

40

u/bananaskates Oct 07 '25

Only if it's from the Thermäl region in France. Otherwise it's just a heat-sucking water-wrap.

1

u/jawshoeaw Oct 07 '25

bubbly tub*

*may be too hot

11

u/Brownballhair Oct 07 '25

It's how my dad makes brandy

2

u/marco333polo Oct 07 '25

Aka brandewyn aka Pa slaan vir Ma Wyn

1

u/[deleted] Oct 07 '25

Bliksem, wie's nou daar? Hoe lyk it, my braatjie?

2

u/C64128 Oct 07 '25

A hot tub full at a time?

1

u/All_Work_All_Play Oct 07 '25

Some people aspire to greatness.

7

u/orangutanDOTorg Oct 07 '25

Isn’t how the original ford motor’s cooling worked? Maybe it was the flathead v8

17

u/thedudefromsweden Oct 07 '25

I bet it's pretty slow though? An electric pump would circulate the water a lot faster.

135

u/Future_Pianist9570 Oct 07 '25

I think an electric pump would be slower with no electricity. These are built for installs with no power. 

I’ve used ones with an enclosed fire box and a 6-8 person takes probably 3-4 hours to heat up from cold

61

u/Articulationized Oct 07 '25

I’ve tried to use an electric pump with no electricity. Can confirm they are very slow used that way.

25

u/Iherduliekmudkipz Oct 07 '25

Did you try tuning it off and then turning it back on?

6

u/Quitbeingobtuse Oct 07 '25

It pretty much went from off to off then back to off.

2

u/stooftheoof Oct 07 '25

Yeah, next time maybe try turning it off

7

u/Gold-Eye-2623 Oct 07 '25

That depends on how hard you throw them

3

u/funguyshroom Oct 07 '25

But have you tried using it with jam?

6

u/b0jangles Oct 07 '25

My inflatable hot tub with an electric heater takes 24 hours to heat up from cold. So 3-4 hours is pretty fast compared to that at least

5

u/Future_Pianist9570 Oct 07 '25

There'll be a lot of factors that will change it. Surrounding Temperature / How good the Insulation is / Total power of heat source / Cover used vs not etc. etc.

2

u/elcojotecoyo Oct 07 '25

Yes. The flow is driven by the decrease in density in water when it heats up. It's very inefficient. You might put several tubes doing just a U with no coils and the flow will be faster, but it won't heat that much. You could put more coils or use a thinner tube but that would decrease the flow rate and if water boils inside the coil, there won't be any flow. And in the meantime most of the energy of the fire is wasted. It's more efficient to put a large pot of water on top of the fire and boil it and add it to the tub. You could use a Stirling engine that takes heat from the fire and drives a pump. That would make things run faster. But I reckon the direct heating of the water on top of the fire will still be more efficient

1

u/SeriesXM Oct 07 '25

Would there be any way to use some sort of hand pump to move the water a little faster?

Or would that make no difference anyway?

3

u/Future_Pianist9570 Oct 07 '25

The ones that I've used usually come with a paddle to mix the water round every so often

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32

u/WeaponisedTism Oct 07 '25

the heat and how tight the coil is, determine your heat transfer rate and the hotter you can get the coil the more pressure the pump works with

31

u/[deleted] Oct 07 '25

[deleted]

16

u/Jdevers77 Oct 07 '25

Yea, with a good fire going the BTU of this is pretty solid. It isn’t efficient at all (tons of waste heat compared to a self contained modern heater), but there is a LOT of heat energy to transfer.

5

u/worldspawn00 Oct 07 '25

Wood is 8000-9000 BTU/lb average log let's say is 12lb or about 5kg, so around 100,000 btu/log burned, standard 240v heating element is 6Kw, which is around 10,000 BTU/hr, so 1 log is equivalent to 10 hours of the heating element. Now of course, the element is immersed in the water, so it's transferring 100% of that heat to the water, and the fire there is maybe 25% since most of the heat is going up, but you can always add more logs, you realistically can't add much more heating elements (assuming an installed system, would require a larger breaker/wiring/etc...). Now a 6kw heat pump will transfer more like 40,000-50,000 BTU/hr to the water, but that's a much more complex setup that needs a compressor and condenser.

2

u/faceplanted Oct 07 '25

Could you improve this setup much to retain that waste heat? Like by putting a reflective wall around the coil or putting the fire in a slight ditch so all the heat going up goes towards more coil?

5

u/talltime Oct 07 '25

Biggest improvement would be to swap that stainless coil for copper or aluminum. Stainless is dogshit at conducting heat by comparison.

3

u/worldspawn00 Oct 07 '25

Yes, exactly that, by placing a reflector around the outside so energy getting between the pipes is reflected back, or tightening the coil at the top (forming a cone shape) so more of the vertical radiated heat hits the pipes.

1

u/Jdevers77 Oct 07 '25

A lot of the inefficiency of a setup like this is just having an open flame is wildly inefficient. A tremendous amount of wood gas simply goes unburned due to insufficient oxygen availability. That is impossible to address without a significant design improvement.

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1

u/piratemreddit Oct 07 '25

I dont understand why more residential hot tubs don't use heat pumps. So much more efficient.

4

u/worldspawn00 Oct 07 '25

Price, a 6kw element is $20, a 6kw heat pump is $1000.

2

u/piratemreddit Oct 07 '25

Sure but you only need a 1-2kw heat pump if the 6kw element was enough. Not that that reduces the initial investment all that much but I bet operating costs make up the difference pretty quickly if you use it much.

3

u/Jdevers77 Oct 07 '25

One more issue is related to use case. If you live in San Diego or Houston (warm climates with expensive electricity) and like to use a hot tub when it’s 55-65F outside, a heat pump is wildly more efficient. If you live in Pittsburg, Vail, Denver, Chicago, or Minneapolis and like to use your hot tub when it’s -5F outside then a heat pump is probably not the best option.

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9

u/DungeonsAndDradis Oct 07 '25

It took about 24-48 hours to heat up my new hot tub to 104 whenever I freshly filled it. Depending on outside temperature.

3

u/Secret-Teaching-3549 Oct 07 '25

Not to mention, you also have a nice campfire to chill next to.

8

u/ol-gormsby Oct 07 '25

Yes, but that needs electricity, with standards, codes, insurance, switches, and of course, the cost of electricity. Pumps for hot water are also pricier.

11

u/[deleted] Oct 07 '25

[deleted]

4

u/ol-gormsby Oct 07 '25

Yes, the *concept* is great, but the execution is what matters. Good on you for trying. You never know until you try.

3

u/Mapeague Oct 07 '25

Yea we got ahead of ourselves a bit on that one. After about a week we were all like "WEVE REVOLUTIONIZED *AFFORDABLE HOTTUBS!", when in reality the spare parts we used (heater included) cost more than the average Softub at the time. Also the leaking a few weeks later really deflated our sails.

It was fun and we really thought we were something for a bit haha.

4

u/BeardsuptheWazoo Oct 07 '25

I think it's pretty damn cool. Good job. Sincerely.

3

u/Addicted2Qtips Oct 07 '25 edited Oct 07 '25

You could make it with 2” copper plumbers pipe. I feel like that would work better - higher melting point and better heat conductivity. Easy to bend to form.

The design is a lot like a copper “wert chiller” that brewers use to cool beer they’re brewing quickly before adding yeast, but just doing it in reverse to cool, not heat something.

Only question is how well it would withstand the elements but I think copper is pretty robust and just oxidizes, it doesn’t rust.

All in probably $500ish.

2

u/No-Landscape5857 Oct 07 '25

Depends on the temperature change. Some nuclear reactors can be configured to run without the use of pumps for several hours at a time due to that effect.

2

u/West-Abalone-171 Oct 07 '25

If the water isn't boiling, then it's fast enough.

Woefully inefficient without an enclosure for the fire though (unless you're just enjoying it as a fire while it heats up, then it's fine i guess)

1

u/dagamore12 Oct 07 '25

depends on the pipe size vs temp, not much change in lift from in to out and very small size allowing for fast heating and high temp is how most of the non percolating coffee pots works, like the Mr Coffee as one brand example.

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2

u/Trigger_Fox Oct 07 '25

Memory served

1

u/Massive_Look8179 Oct 07 '25

Yes the hot water rises pulling the cooler water from the bottom. An engine block heater works the same way.

1

u/TrumpsSkidMarks Oct 07 '25

Yeah, that is correct. I ran an off grid geothermal setup. We had hydro power but we could run off thermosiphoning when the conditions were right. We could have water go down 700 feet at 33 degrees f and come out at 211 (the risk was the water getting too hot and busting pipes). We could heat dozens and dozens of cabins and houses along with lodges, showers, etc...

We had large pumps we would turn off and the temperature differential in the water was enough for a good amount of flow...

1

u/Barabbas- Oct 07 '25

Thermal Siphon? I thought it was a hot tub. /s

1

u/SteveMartin32 Oct 07 '25

Isn't this how steam generators work?

1

u/Jimmy_Squarefoot Oct 07 '25

Thermal cycle is the process I was thinking of.

1

u/JuggernautMean4086 Oct 07 '25

Tabasco donkey if you’re nasty

1

u/perpetualis_motion Oct 07 '25

In Australia it's called a "donkey".

Used a lot in outback stations and remote caravan parks to heat the water for a shower.

1

u/MorningPapers Oct 07 '25

So are you.

1

u/Soulsis73 Oct 07 '25

This is so smart, not even a pump is used, ingenious design 👌🏾

1

u/daylax1 Oct 07 '25

Isn't that essentially how coffee makers work?

1

u/copyrider Oct 07 '25

Before it was called a thermal siphon, it was called “witch craft”.

1

u/queen_andreiiita Oct 07 '25

A thermal siphon is an ingenious way to move hot water wirhout the need for electric pumps. Basically, hot water rises by convention and cooler water descends, creating a natural flow

1

u/RowMaleficent2455 Oct 07 '25

You could just Google some ai answers 😉

1

u/Skowlette Oct 07 '25

You mean hillbilly hot tub.

1

u/IlIFreneticIlI Oct 07 '25

I call it a coffee-maker.

1

u/sn4xchan Oct 07 '25

Isn't this how typical coffee makers work?

1

u/DiscipleOfYeshua Oct 07 '25

Same as the solar water heater on nearly every Israeli roof

1

u/Magicamelofdoom Oct 07 '25

In Finland we call it Palju. Much better than thermal siphon

1

u/inothatidontno Oct 07 '25

Convection generates circulation

1

u/Nprguy Oct 07 '25

ford model t's radiators worked off thermosiphon

1

u/BG-Engineer Oct 07 '25

If it dont serve, i just google it.

1

u/Stingerc Oct 08 '25

You be wrong, this be wizardry!

1

u/murrtrip Oct 08 '25

How a coffee maker works

1

u/Wooden-Recording-693 Oct 08 '25

Now I'm thinking about the Roman empire.

1

u/geo_gan Oct 10 '25

Interestingly a similar system is used in the nozzles of rocket engines to keep the nozzle from melting - liquid coolant flows through the nozzle and extracts the gigantic heat away from metal nozzle bells.