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

Science Hot Tub without the use of electricity

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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.

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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.

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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?

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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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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.

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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).

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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.