Drain water from 5" pipe buried 9ft underground

[druidhr]

On that topic, if you want good heat transfer on hot days, you would benefit from a number of heat exchangers above ground. That would make the best of any available heat and reduce unwanted leakage at night. I mean a sort of thermal ratchet.

Do you think these radiators would be good for above ground exchangers.

 

[sophiecentaur]

Do you think these radiators would be good

I've no idea from your photo, how big they are b
ut if they're free then go for them.
Problem is that the details are really important here and you need to measure temperatures 'everywhere'. It's not a trivial bit of engineering and you have to give yourself as good a chance as possible to justify all the work involved. I don't think it would be worth while just to 'suck it and see'.

You need to know the mass of water underground and the temperature you can expect down there. That will tell you how much Energy is being stored. Without knowing the temperature you are getting down there you can't work out how much energy is stored.

There are two things to estimate, how much heat the rads can capture and also the heat capacity of all the underground mass. Radiant heat from the Sun will be the best source. You can expect at least a few tens of Watts getting into each small black painted CH radiator whenever the Sun's out. Croatia is warm in the summer iirc. You need to measure the temperature of the water (contact thermometer on a rad?). Even in UK, a coiled up hose, out in the sun can reach at least 40C on the outer coils.

Without the pump, the static water in the rads would be 'warmish' by the end of the day. Just experiment with a single radiator full of water and see how warm it gets on a summer day. Turn on the pump and the whole mass of water would be absorbing the heat so the temperature wouldn't be very high after one summer day. Obviously, you need as much heat as possible from the system when it's needed so controlling the pump is vital. Only have it running when the soil temperature in your beds is low.
How much heat would be available on a winter's night? That depends on the size of the rads and the catalogues tell you how many Watts for a temperature difference of 50C. You will be expecting only a few degrees difference.

How much stored heat? E = M T 4200 where M is the total mass of water, T is the temperature difference and 4200 is the specific heat of water. That's in SI units but there are many sources of the same information in imperial units. You can estimate the mass of water from the length and radius of the big pipe times the density.

Water circulation: I think the solutions higher up the thread could well be overkill. Air locks could be a problem but only the ones in the inner pipe in the narrow inner pipe. Air (static) in the large pipe wouldn't matter.. If you feed the small pipe through and see if mains pressure water will flow through steadily (after a bit of initial bubbling) I can't see the need for a high pressure pump of the kind discussed at the top of the thread. A cheap CH pump should do the job of circulating the water. You would need a vapour trap at the highest point to avoid build up of air bubbles. A pump above ground would do the job and be a lot cheaper.