12V DC: Collecting solar, hot water


  1. Used electric boiler, 50 l;
  2. Spiral black pipe, inner volume 5 l max; Frame it (opt. / recomm. );
  3. TWO mechanical Thermostats (T). Either refer to #7;
  4. Solar panel 10W;
  5. Mill type water pump 5W;
  6. Three pipe taps (keys), some T connections;
  7. Optional, most efficient – Battery 12V 5Ah, Programmable Thermostat, Solar Controller 10A max.

Below, more detailed description and scheme.
I have ~0,1 bar in pipes. In case of city pressure, ~2 bar, I have no idea how little pump would circle water.

Solar hot water collector, active system. Scheme.

1: Connect spirals outlet to boiler’s hot water outlet (inner, long pipe) RED. Connect boiler’s inlet BLUE to spiral’s inlet. Connect T fork below RED for hot water outlet. Connect T fork for immediate pump and spirals BLUE?

2: Not needed long pipe. Shorter it is, more frequent pump switch, shorter pumping. Instead, good idea to frame it, to collect more heat. Longer pipe needed only if shade is reaching frame sooner, as to heat water in 5m or 50m hose, takes nearly the same time.

3: Ideal would be to use electric, programmable (but still two) thermostats, but quite more electronics needed (see update #7), esp. if this meant be running exceptionally from solar panel only. No battery is needed.
The first trick, is to remake one of T. Be default, they switch-off when are hot. Have to remake it in to switch-on. Might take a nerve. Make sure the click remains when touching contacts. Slow touch won’t serve.

T1 needs to be injected in to pipe of spiral, at the outlet. It switches-on the pump to rotate hot water. It cuts power when cold water (from boiler) reaching T1.

Second T stays as it is in the boiler. It is needed to cut the power, when collected water in heater is hot. Without – it will be circling the pipe continuously, when temp. in pipes and boiler are equal.

Will take a nerve to adjust them both for the most efficiency. Otherwise, circling of hot water in pipes will be cut only when water will get chilled to cut the T1;

4 / 5: Solar connected straight to the pump. Such a single pump serves me already for two years, never failed, once cleaned b/c of little stones blockage. A filter before it would be preferable. Overcast conditions are still generate enough power to run the pump. Place the solar nearest spirals;

6: To use collected water from boiler only (otherwise it will be also in-taking cold water, from main intake) the spiral should be cut (K2);
If collected water is too hot, K2 can be opened to mix additionally (depending on permeability of pump) cold water. Otherwise, cold water delivery before K3 should be attached;

7: Update. The most efficient way, easier to maintain, few smart electronics needed. Refer to the same scheme, where instead T1, is programmable T used.  I found it cheaper to solve than with no battery.
A little 5Ah battery (approx 10 €) will serve ages. Programmable T with relay switch is about 5€ on eBay (from China), as well as (programmable) solar controller (10A), I’ve bought for 7€. This gadgets worth to buy, as comparing to version above (with no battery) outcome is not efficient, as it is very complicated to remake, firmly, mechanical T, and does not serve as it should.

Remember: in order for pump to work properly, make sure there are no air gaps in closed water circuit.

Pipe spiral. Mechanical thermostat. Solar 10W.
Mechanical rod thermostat. Rod installed through the hole in L connector.
Tiny (45x35x25mm) mill water pupm. 5W.
El. Boiler. Collecting water circles in opposite directions: in-hot-“outlet” and out-cold”inlet”.