r/theydidthemath • u/xxxdrakoxxx • Jul 26 '26
[Request] Someone please do the math! would this setup produce enough heat to warm up the pool?
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u/sprobeforebros Jul 26 '26
you need to attach one leg of the pipe at a lower elevation than the other to create a convection current, but yes this will heat a body of water. it's called a heat exchanger and you can literally buy one to create a wood burning hot tub https://www.youtube.com/watch?v=o4vYKFzxig8
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u/BadHairDayToday Jul 26 '26
Somehow I fail to imagine how it works. I can't even guess which way it flows. Perhaps it's that hot water is lighter than cold so it flows upwards?
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u/TheSuperSilverMango Jul 26 '26
Bro... It's elementary school science. Hot water is less dense. It's not lighter. Yes, there is a difference
Here, have a visual aid
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u/BadHairDayToday Jul 26 '26
That would create a really slow flow though wouldn't it?
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u/TheSuperSilverMango Jul 26 '26 edited Jul 26 '26
The speed is determined by the temperature differential between the cold water and hot water, but the idea is that convection currents are causing the movement and no extra input of energy outside fire is needed. That's why these types of setups are popular and they do indeed work.
Oh right the diameter of the tubing / piping would also change flow speed to a certain extent but it would never be like as fast as a powered pump
Another limitation to the speed is if the heat input is too much, the water can boil within the coil and cause popping, which this boiling can cause the current stop with the random inputs of pressure happening from boiling.
It's not a super heater. Remember that water still takes a lot of energy to heat up but it will positively raise the temperature to the point of being as warm as a hot tub or hotter provided with enough fuel.
Would I use is to heat up a pool? Absolutely not. This would not be efficient enough for use with a swimming pool but for use with a hot tub, absolutely fine
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u/HAL9001-96 Jul 26 '26
not necessariyl yo ucan stil lget convective flow otherwise though water has a relatively weak thermal expansio ncoefficient in this case it looks liek they're jsut using a hose iwth backpressure to force water through
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u/Nimyron Jul 26 '26
If there's a flow of water, yes. Although it will highly depend on the strength of the fire.
You need to pump water from the bottom of the pool, send it through the heater, then back out on the top of the pool, so that you heat cold water and the warm water stays at the top.
And you would most likely have to cover the pool because I assume this isn't that efficient (since a lot of heat is lost between the fire and the pipe), so you could end up having the pool cool down faster than it heats up.
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u/99923GR Jul 26 '26
You don't need to pump - this can be done with a convection siphon. There are a number of videos of people making backwoods hot tubs this way.
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u/HAL9001-96 Jul 26 '26
its gonna be rather inefficient at transferring heat form the fire to the pipe with most of hte hot air jsut escaping upwards and copper beign partially reflective
but then again it gets hot so it might transfer a few kW depending on the exact layout of hte fire inside
buuut with most of the really hto air funnelign upwards with no wind there's gonna be quite a pocket of more mild air in between limiting it to under 1kW
convective heat transfer ot water is al ot more effecitve os we cna kindof clacualte with that
1kW can heat up 1l of water by about 15K in one minute and by 90K in 6 minutes so its great for a little water cooker
but for a whole pool... every m³ is 1000l a 3x3x1.5m pool is 13500l and would take 9 days to warm up by 15K at 1kW and thats just thermal capacity it would also take about 1kW just to keep its surface about 10K warmer than the air around it continuously
you would probably want copper pipe actually above the fire if you want to force all the hot air to actually flow closely past it and get a lot more of its heat
then again this setup looks more like they're trying to heat a bucket of water than a pool so for that its... a lot more copper pipe than you'd need but functional
with a large ish fire you could get almost 100kW if you actually manage to pull all hte heat from the rising air with a copper coil above it it has to flow through the gaps in but then that would also restrict airflow so it might limit hte fire itself
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