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   Rich Felker (dalias@hachyderm.io)'s status on Tuesday, 11-Nov-2025 07:52:13 JST Rich Felker

   Is there a reason passive heating via geothermal heatpipes isn't feasible? Depth required too great? Unlike ground source heatpump, no external energy input should be needed. You get gravity return for free.

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     Rich Felker (dalias@hachyderm.io)'s status on Tuesday, 11-Nov-2025 08:23:37 JST Rich Felker

     in reply to

     • Steve Canon

     @steve No need to use water. Heat pipe can use any refrigerant (substance that undergoes liquid/vapor phase change at desired temperature under workable operating pressure).

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     Steve Canon (steve@discuss.systems)'s status on Tuesday, 11-Nov-2025 08:23:38 JST Steve Canon

     in reply to

     @dalias IIRC the change in water density across the temperature range for a ground-source heat pump is less than 0.1%, so I expect you’re not going to get that much flow just from gravity unless the pipe is really large, so probably not economical?

     Or are you thinking of using some medium other than water?

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     Rich Felker (dalias@hachyderm.io)'s status on Tuesday, 11-Nov-2025 08:30:48 JST Rich Felker

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     • Alex von Kitchen

     @Dangerous_beans The depth needed should just be the depth at which ground temperature is a livable air temperature (like 17°C or so) provided you can insulate the pipe well enough.

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     Alex von Kitchen (dangerous_beans@aus.social)'s status on Tuesday, 11-Nov-2025 08:30:49 JST Alex von Kitchen

     in reply to

     @dalias cost of drilling a hole deep enough to get a sufficiently large energy gradient for it to work is too high

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     Janne Moren (jannem@fosstodon.org)'s status on Tuesday, 11-Nov-2025 08:31:06 JST Janne Moren

     in reply to

     • Steve Canon

     @steve @dalias
     Also, I think you want the flow to be predictable and adjustable as well. For instance, you probably want to be able to shut it off if needed, without having to prime the passive flow again.

     Wonder if you could make a non-electric active system with a Stirling engine powered by the water/air temperature difference, that directly drives the pump?

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     Rich Felker (dalias@hachyderm.io)'s status on Tuesday, 11-Nov-2025 08:31:06 JST Rich Felker

     in reply to

     • Janne Moren
     • Steve Canon

     @jannem @steve A heat pipe will have zero flow without a temperature differential.

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     Rich Felker (dalias@hachyderm.io)'s status on Tuesday, 11-Nov-2025 09:43:18 JST Rich Felker

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     • Steve Canon

     @steve Yeah but when you use water you use it at very low pressures where the boiling point is at the target temperature, not at atmospheric pressure.

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     Steve Canon (steve@discuss.systems)'s status on Tuesday, 11-Nov-2025 09:43:19 JST Steve Canon

     in reply to

     @dalias Agree no need, but (at least for systems I'm familiar with), it (or water / glycol mix) is definitely the norm for existing systems.

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     Paul Rosen (paulrosen@hachyderm.io)'s status on Tuesday, 11-Nov-2025 09:59:30 JST Paul Rosen

     in reply to

     @dalias I've been wondering that, too. I thought one would probably need an electric pump. And I was wondering about repairs. How do you get to the pipe?

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     Rich Felker (dalias@hachyderm.io)'s status on Tuesday, 11-Nov-2025 11:36:36 JST Rich Felker

     in reply to

     • Steve Canon

     @steve That's not what I consider the definition of "heat pipe". Heat pipe is phase change based. Like the one cooling your laptop CPU.

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     Steve Canon (steve@discuss.systems)'s status on Tuesday, 11-Nov-2025 11:36:37 JST Steve Canon

     in reply to

     @dalias Systems I've seen (including the one in our house) are operating above atmospheric pressure; there's no phase-change in the ground loop, just convection.

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     Rich Felker (dalias@hachyderm.io)'s status on Tuesday, 11-Nov-2025 11:39:58 JST Rich Felker

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     • Alex von Kitchen

     @Dangerous_beans Are you assuming liquid phase? The whole point of a heat pipe is that the heat moves rapidly through the motion of gas particles, but maybe at these scales that's still not sufficient. My default hypothesis for why it's impractical would be the giant volume of gas you'd need to move enough heat per unit time.

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     Alex von Kitchen (dangerous_beans@aus.social)'s status on Tuesday, 11-Nov-2025 11:39:59 JST Alex von Kitchen

     in reply to

     @dalias that's fine if you power the actual pump part externally, but to get it purely passive you'll need a sufficient energy gradient to cover pumping the fluid. via a thermosiphon or some sort of heat engine at the top
     and the deeper you make it the larger the energy needed for pumping, since it's all against gravity

     a KG of water at 17c, if your room is 15c, is only bringing up ~8KJ, if you assume the heat exchanger is 100% efficient. That's 30-60L/min to match a typical split system, which is quite a lot of water. That's without considering fans, pumps, however energy is recovered to drive the system

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     Rich Felker (dalias@hachyderm.io)'s status on Tuesday, 11-Nov-2025 11:49:20 JST Rich Felker

     in reply to

     • Steve Canon

     @steve Yeah. The idea is you take in heat at one point with liquid to gas phase change, rapidly move that heat via the sparseness of gas particles, and harvest the heat at another point via condensation against a lower-temperature surface. Then let the liquid state return to the heat source via gravity.

     (Heat pipes can also use wicking when gravity isn't available/favorable, but that's much more speed limiting.)

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     Steve Canon (steve@discuss.systems)'s status on Tuesday, 11-Nov-2025 11:49:21 JST Steve Canon

     in reply to

     @dalias sees like you’d be trying to move most of the heat at the point in the loop where the phase change happens, while a normal convective loop bleeds heat evenly over its full length.

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     Rich Felker (dalias@hachyderm.io)'s status on Tuesday, 11-Nov-2025 12:00:05 JST Rich Felker

     in reply to

     • Alex von Kitchen

     @Dangerous_beans OK, well that's a completely different thing from what I was asking about, so the reasons for it being impractical are somewhat different. Heat pipe isn't a gas-phase working fluid in what you described, but rather phase changes as the heat carrier.

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     Alex von Kitchen (dangerous_beans@aus.social)'s status on Tuesday, 11-Nov-2025 12:00:06 JST Alex von Kitchen

     in reply to

     @dalias yeah, water in a liquid phase. The numbers get worse for gas or other working fluids. Increasing the thermal gradient makes the fluid flow lower, but requires more digging and pumping work
     The idea could work, it does in places like Iceland where they use geothermal power, but it's not practical cost wise

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     Rich Felker (dalias@hachyderm.io)'s status on Tuesday, 11-Nov-2025 12:10:41 JST Rich Felker

     in reply to

     • Alex von Kitchen

     @Dangerous_beans I think at some level that's ultimately right, but it's not directly the mechanism with a heat pipe, because the heat capacity of the working fluid is vastly amplified. The entire energy of the phase transition is carried, not just the energy of the delta-T. Presence of the phase transition makes the effective gradient very large.

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     Alex von Kitchen (dangerous_beans@aus.social)'s status on Tuesday, 11-Nov-2025 12:10:42 JST Alex von Kitchen

     in reply to

     @dalias it's the same problem, your just suggesting using a heat pipe to get around pumping requirements. With the available heat gradient you have to move too much working fluid for it to be practical
     If you improve the heat gradient the cost goes up