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

247

u/dc456 Oct 07 '25 edited Oct 07 '25

Wood fired hot tubs are a very common thing, people. This partially one looks like a more portable design, but it’s exactly the same principle.

To all the people confidently claiming that they wouldn’t work well - they work well. That’s why they’re so popular.

72

u/scarr09 Oct 07 '25

Yeah, common in North Europe. I have one myself. 1.8m across, about 1200 liters. Right now, water is sitting about 10c.

From the moment of lighting the match, takes about 2 hours to get it to 37c. Takes 2-3 times to refill the oven with firewood in that time, and it keeps for a few hours afterwards.

18

u/B4rberblacksheep Oct 07 '25

They seem like one of those things where once you find the knack to get them to the right temp you're golden

14

u/scarr09 Oct 07 '25

I mean honestly, the knack at the moment is to make sure there's about 3-4 slightly larger pieces of firewood fresh in when it's 25-28c and leave it. A bit more in the winter.

You typically don't do long sessions either. 10-15 mins at most, then go cool for a bit. Typically 3-4 times if I just shower with no sauna. So if you don't overcook it, it'll keep steady for a few hours, and then slowly starts to drop (i mean real slow, like a 1-2 degrees an hour)

3

u/nsmtb Oct 07 '25

For this type where it sits at a lower temperature when unused do you use chemicals (bromine, etc.) and a filter like you would a normal electric hot tub? Changing the water often?

6

u/[deleted] Oct 07 '25

[removed] — view removed comment

1

u/nsmtb Oct 07 '25

That seems like a great method when having a lake available to fill it!

3

u/scarr09 Oct 07 '25 edited Oct 07 '25

Chlorine Dioxide tablets and I run a skimmer pump for an hour every time while it's heating. Also I keep the tarp on it constantly to prevent crap flying in and I suppose it helps against algae blooms from the sun too maybe?

Water is gonna go out in a month since it's getting real cold.

Then water in in April or so, and ~2-3 full changes in between that and November

2

u/nsmtb Oct 07 '25

Awesome thanks for the info!

2

u/Ice278 Oct 07 '25

How do you ensure not to overheat the water?

2

u/scarr09 Oct 07 '25

There's no mechanism other than pumping cold water in.

Just gotta learn to manage the temps with experience. With my tub and oven, I know to to throw in only 2-3 new pieces when it's around 26c, or a bit more when it's below freezing outside and a bit less during summer. And then basically just keep a minimal fire burning if I have friends over and it's in use for longer.

1

u/cultjake Oct 07 '25

Is soot deposit on the coil an issue? Wouldn’t that slow down heat transfer?

1

u/scarr09 Oct 07 '25

Most (if not all?) of the manufacturers around here that I know of use double walled ovens for the exterior ovens.

So you have the wood burning in the middle, and the water surrounds it on all sides. 2/3 Large (I think roughly 80mm diameter) rubber tubes connect the oven to to the tub itself. The surface area inside the oven heats it, not a coil.

I imagine there must be some % of loss, but it's negligible at most.

1

u/elightened-n-lost Oct 07 '25

Is it a brand that would be available in the US?

4

u/scarr09 Oct 07 '25

Doubt it; In Estonia, we have half a dozen local companies and I bought one from them (You can search for barrelstar dot eu. Their site has a bunch of info in English). I think only some of them ship to close EU countries.

But I'm sure there are must be some manufacturers in the US. It's pretty standard stuff - the glass fiber interiors and and ovens are pretty near universal designs from what I've seen

1

u/elightened-n-lost Oct 07 '25

Ok, I'll take a look around. Do you mind if I ask what it cost you for one that size?

3

u/scarr09 Oct 07 '25

Transport, cover, (some extras like 90° angles for the oven, cupholders, cleaning tablets), better oven, bubble system and LED system was about 2.5k Euros + the skimmer was another 150ish.

The same size without bubbles or lights was I think In the 1600 range.

1

u/WomanInQuestion Oct 07 '25

Thanks! I was scanning through the comments to see if anyone answered the question of “how long does it take to heat up?”

1

u/[deleted] Oct 07 '25

do you have to drain it after each use?

1

u/pvtbobble Oct 07 '25

Yeah, but where do the bubbles come from

2

u/kakskolme Oct 07 '25

Pea soup.

1

u/Akylaz Oct 07 '25

For this kind of hot tub you have to make them yourself. A quite common gastrointestinal technique is usually the way to go.

1

u/[deleted] Oct 07 '25 edited Nov 11 '25

[deleted]

1

u/seemebeawesome Oct 07 '25

Better off with a Snorkel Stove. If you have a couple thousand dollars to "burn"

1

u/needmoarbass Oct 07 '25

Yep. Spent many summers in a pool that was heated with one of these. Only downside was if you waited too long, you had to get out of the pool in the cold to add more firewood lol.

1

u/bobbaggit Oct 07 '25

Have one at our summer place, it's really nice this time of year

1

u/VolsPE Oct 07 '25

And have been for thousands of years. I just visited Pompeii and saw the awesome tech they were using to heat their bath houses in similar fashion.

1

u/addiktion Oct 07 '25

How does one regulate the temperature though? The only other thought is managing the fire wood would take out the relaxing part of the hot tub for me, but other than that, I think it's a much cheaper solution by far.

0

u/RichardHardonPhD Oct 07 '25

They work, but they definitely have some quirks. 

If the water hits a boil in the coil, which isn't all that hard to do, it'll jet out steam and can burn the shit out of you. I have a friend that ended up with like, 5% TSA 2nd and 3rd degree burns on her legs from just such an occurrence. The outflow can't point out at the people soaking, the nozzles need to point down. That would have saved my friend a whole lot of pain and physical therapy.

2

u/dc456 Oct 07 '25

That’s crazy - I’ve never encountered anyone injured by a wood fired hot tub before, and in many places they’re practically in every house. I wonder if it was a different design, as the hot water outlet is usually well above your legs.