Making 22mm copper pipe into a coil??

[Gareth J]

I've never tried it but would a variable speed pump work. Slow speed for hot and fast for cool?

If going the route of electrics (I'd imagined thermosyphon, for simplicity, would be desirable), a loading valve "esbe" or similar, controlled by flue gas temp or water temp, would work. But that's all a bit fiddly/expensive.

You could try to separate burn chamber, from HX chamber - rocket mass heater style

 

[Justme]

Keep it simple.

Thermosyphon from a "back boiler" using 2" pipes.

Just like all the ready made external wood fired hot tub use.

The big hot tub we go to uses a 50kw burner unit (they did have 30kw but took too long to heat up).

The smaller one we go to is more like 10-15kw as there is far less water volume.

Temp control via the drain valve & cold top ups / tending the fire.

 

[Rrunnach]

I would have thought a tube roller would be the best tool to create a coil.
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When I made my gates I made an improvised roller using the vice from my milling machine. To roll tube just make sure the rollers have a suitable radius in them.
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Cool gates, I get the impression of skimming stones on a lock, with with the long curves being the ripples of water.