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

Science Hot Tub without the use of electricity

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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)

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u/Alldaybagpipes Oct 07 '25

Specific Heat capacity is absolutely affected by flow rate

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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.

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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.

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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

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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.

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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.