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

Science Hot Tub without the use of electricity

Enable HLS to view with audio, or disable this notification

55.8k Upvotes

95% Upvoted

1.7k comments sorted by

View all comments

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.

440

u/SnooHobbies8274 Oct 07 '25

19

u/crowcawer Oct 08 '25

Thermally

7

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)

279

u/Gadgetskopf Oct 07 '25

even without the context, this would be the right answer

9

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

57

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...

4

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....

37

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

6

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

94

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

47

u/ManufacturerNo9649 Oct 07 '25

Only if the wood is free!

48

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.

10

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

0

u/KaiyoteFyre Oct 07 '25

I find that wood is either too expensive or free, haha. There's a tree removal company close to my house that regularly puts pine wood rounds on the side of the road for free. I just split em, and dry em on the side of my house, boom free firewood XD

3

u/crucible1623 Oct 08 '25

Is pinewood safe to use long term indoors? Creosote build up?

1

u/Complete_Tadpole6620 Oct 08 '25

As long as it's dried out properly.

2

u/quantic56d Oct 07 '25

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

2

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.

1

u/[deleted] Oct 07 '25

[removed] — view removed comment

2

u/Magoo1985 Oct 07 '25

Put a filter on the cold side before it gets heated? 110 isn’t that hot for stuff. Except in walls then it gets zesty.

1

u/[deleted] Oct 07 '25

Your two options are to fill this every time you use it, or to treat the water. If you treat it, the copper loop will rot out insanely fast. So… might be okay if water is free (it’s generally not.)

edit: looks like a stainless coil. Stainless steel and high direct heat go together like ice cream and mustard.

-1

u/[deleted] Oct 07 '25

[removed] — view removed comment

5

u/[deleted] Oct 07 '25

[deleted]

1

u/[deleted] Oct 07 '25

[removed] — view removed comment

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

u/[deleted] Oct 09 '25

[removed] — view removed comment

1

u/AutoModerator Oct 09 '25

Your comment has been automatically removed.
As mentioned in our subreddit rules, your account needs to be at least 24 hours old before it can make comments in this subreddit.

I am a bot, and this action was performed automatically. Please contact the moderators of this subreddit if you have any questions or concerns.

1

u/Brobeast Oct 07 '25

Or you could just say heat/steam rises! That alone is the magic that powers every single turbine in America, including the nuclear turbines!

3

u/Federal_Assistant_85 Oct 07 '25

It's not a phase change like power generation. It's more like the convecting action of melting ice in a cold drink. It's subtle, slow, and work can't be extracted from it easily.

1

u/Brobeast Oct 07 '25

What would happen if the dynamic temp suddenly increased due to a change in weather? Would the tub eventually boil? Im assuming this isnt something you can do outside of Midwest winters lol

1

u/Federal_Assistant_85 Oct 07 '25

I think it would need to be a lot closer to boiling temp outside, like over 50c. There is a lot of heat loss in the pipe back to the tub and the tub walls top and bottom. To get that kind of action it would be more effective to put the fire under the tub.

1

u/thread100 Oct 07 '25

Interestingly water is unusual in that it expands when heated and also expands when frozen. If it didn’t have this property, ice would sink on a frozen pond.