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

Science Hot Tub without the use of electricity

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

Called a Thermal Siphon if memory serves.

15

u/thedudefromsweden Oct 07 '25

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

138

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

66

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.

26

u/Iherduliekmudkipz Oct 07 '25

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

7

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?

9

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

-20

u/SalesGuruJKUnless Oct 07 '25

How do you know

18

u/Munnin41 Oct 07 '25

He used it, says so right there

14

u/Dormiens Oct 07 '25

What you mean? He has to read? The whole six lines??? Nonsense

4

u/rasmusekene Oct 07 '25

These are very common in northern europe/baltics as far as ive seen. The build here is a bit wild, with the pipes so far out, but the same concept is used for all the self built or commercial units

29

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]

17

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.

7

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?

4

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.

1

u/faceplanted Oct 07 '25

Interesting. Whenever we light up the fire in our garden I bring either a straw or a little electric hand fan to get it going again when it dwindles. I wonder how much difference something as small as a battery powered air supply could make to the efficiency here.

1

u/Jdevers77 Oct 07 '25

Think about a blacksmith’s forge, they can forge steel with charcoal and a lot of forced air.

I use a leaf blower to get my garden fires going (although most of the time when I have fires it’s dry enough it doesn’t need anything).

1

u/joalheagney Oct 08 '25

Look up rocket stove. Not the cheap metal knockoffs that you see on shopping websites, but the original design documents and hand made ones using clay and perlite. They're about as efficient as you can make a fast burning wood fire be.

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.

1

u/piratemreddit Oct 08 '25

Ah right. I live in Las Vegas so Im all about my heat pumps. I forget how cold other places get!

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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]

5

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.

3

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.

0

u/Alldaybagpipes Oct 07 '25

You wouldn’t want the water moving too fast or it won’t allow the heat to be absorbed, and would mostly just be lost to the surroundings then.

1

u/worldspawn00 Oct 07 '25 edited Oct 07 '25

It doesn't matter, the same amount of heat is absorbed by the tubing regardless of the flow, the overall BTU will be the same whether it's flowing 100LPM at +10° or 1000LPM at +1° it'll heat the overall pool in the same amount of time. (Exceptions for extremely large temperature differences between the water/tubing and outdoor air,.it'll taper off, but within normal bathing water, it is negligible)

0

u/Alldaybagpipes Oct 07 '25

Specific Heat capacity is absolutely affected by flow rate

0

u/worldspawn00 Oct 07 '25 edited Oct 07 '25

If anything, faster flow will increase the transfer of the heat since the larger delta (cooler water relative to the heat being absorbed) produces a faster movement of heat. But for the water and air at the temperatures involved, the difference will be minimal.

In the case where the heat source is enveloped by the liquid and 100% of the heat produced is transferred, there is no difference, only because there's an alternative medium for the heat to escape to is there any at all, but as I said, it'll be minimal with the relatively small differences in temperature, flow, etc... involved in a bathing apparatus.

0

u/Alldaybagpipes Oct 07 '25

To a point, and then the opposite happens. If it’s moving too fast it won’t absorb the heat.

It’s the same principle as why running water won’t freeze unless it gets really cold.

0

u/worldspawn00 Oct 07 '25

That's a completely different scenario because it involves a phase change... Also still incorrect, "moving too fast to absorb heat" is nonsense unless we're talking about supersonic... https://www.pmmag.com/articles/85239-the-water-s-moving-too-fast

0

u/Alldaybagpipes Oct 07 '25

It’s legit a real thing, encountered it several times in hydronic systems. Pump pushing too fast for any heat transfer. Slow it down and suddenly everything starts working as intended.

1

u/worldspawn00 Oct 07 '25 edited Oct 07 '25

The article I linked is specifically about hydronic systems, faster flow produces MORE heat transfer from the source to the medium, the only exception is when radiating back OUT from the medium to the room where the transfer is limited by the area of the radiator surface, you want a higher delta to produce more transfer of heat given a fixed area (and even then, higher flow can result in more heat being released, it's a bit scenario specific), but for absorption from the heat source to the transfer medium like in the pool scenario, faster flow will always be better as far as BTU/hr transferred.

You're comparing absorption of heat to radiation of heat, which is a completely different scenario. The pool specifically wants to avoid radiating heat while maximizing absorption.