Complete scheme

This complete assembly diagram is based on my construction project using the homemade solar collector model 1 and the gravity tank.

In summary of this complete diagram, we will say that 2 water flows will circulate in the tank:

• The drinking water that comes from the house pipe enters the tank through connector ‘B’ as long as its floating buoy does not indicate that the tank is full. And every time a hot water tap in the house is opened, the hot water will come out of the tank through connector ‘E’ (the floating buoy of connector ‘E’ always collects the highest and warmest water).
• The water from the panels, which always comes hot from the solar collector. When the water pump is turned on, it takes the cold water from the tank, which is always the lowest part of the tank, connector ‘F’, and delivers it to the solar collector, while pushing the water that was inside the solar collector and spills it into the tank through connector ‘C’, and so on until the solar collector cools down and is not able to heat the water at that precise moment.

It is important to place plastic mesh on the bottom, to maintain a reserve fund for the water pump. We must ensure that the water pump always absorbs water, and never finds itself without the lack of it trying to pump air, because to begin with it is very bad for the pump itself, and because it requires a great effort to pass the air to rescue the water again, a bubble or air flow could stop the circuit until we manually purge the air from the pump-collector circuit.
We would be at that risk if we allow the fishing tube to have access to the bottom of the tank.
By placing this mesh as seen in the drawing (yellow mesh), we prevent the fishing tube from reaching the bottom, and in this way the water pump will always have a guaranteed water supply.

Below I describe the service of each tank connection, which as you will see are labeled with a pink letter, naming them from top to bottom:

Connector ‘A’:

Connection with a tube that will allow air to enter and exit depending on whether the water level rises or falls.
Never cover this tube, because when cold water enters it needs to push air out and if the air does not escape to the outside the tank could break or burst due to excess pressure.
It is advisable to connect a tube here, as shown in the grey tube, so that it sticks out somewhere where we can see the water falling, in case the mechanism that stops the filling fails, and the tank is flooded with water without stopping. (It does not matter how long this tube is, but it can never exceed the height of the tank itself)

This connection is made with a ‘T’ to connect to this vent and at the same time to a temperature sensor input.
This sensor is optional, it is equivalent to the reading of the water temperature at level (the water that will come out when opening the tap) (if we are not going to put a sensor here, it will not be necessary to put the ‘T’)

(the assembly of the sensor is explained in the Temperature Sensors section)

Connector ‘B’:

Cold water inlet from the house to the tank, it has a little DIY because we have to attach a toilet cistern filling mechanism and invent some way of vertical adjustment in the float to adjust to the level we want to fill the tank, preferably that this level can be adjusted to different levels without difficulty according to the needs and season of the time.
We will fit a non-return valve (if possible) to force the water to flow into the tank, thus preventing any circumstance such as a leak or the removal of the pipe from causing all the water content to escape through that connection.

Because it can also happen that very low domestic water pressure or a supply cut-off causes the water in the tank to flow by gravity to compensate for this lack of flow or pressure in the cold water pipes.
If we do not put this valve, and sometimes we notice very hot water coming out of the cold water tap, this may be exactly what is happening, that the tank is filling the cold water pipes by suction and gravity.

Connector ‘C’:

This connection is the one that carries the heated water from the collector to the inside of the tank, if possible in this connection and inside the tank we will extend with some tube, to avoid the noise of the falling water flow.

Connector ‘D’ and Connector ‘G’:

These temperature sensors are optional, they are part of the management of a differential thermostat. Connector ‘D’ takes the reading of the coldest temperature of the tank (the lowest part), and Connector ‘G’ takes the reading of the warmest temperature of the solar collector, the thermostat compares the temperature of these two sensors and governs the water pump to exchange the waters at the appropriate times.
(The assembly of the sensor is explained in the Temperature Sensors section)
(If you cannot acquire a differential thermostat with its two sensors, you will have to adapt your system to the requirements of the thermosiphon system)

Connector ‘E’:

Inside this connection we have to attach a fishing tube (flexible tube) attached to a floating object: (I have used a vinyl tube, it is very good, flexible and incredibly withstands very high temperatures)

The need for this is because in our tank the water level is configurable, and because the water inlet and outlet work independently, then it is impossible to define an exact point to extract the water. 
The intention of this is that said tube always supplies us with water and not air, as the level is variable and configurable we have to pursue this level to 'suck' the water, and coincidentally the highest part is also the one we are interested in because it is always the hot

If we have installed the sensor of connector 'A', said sensor must be tied to this hose, so that the reading is made on this variable level.
On the outside of this connection a stopcock is necessary, because the leak or disconnection would cause the imminent emptying of the entire tank. But, although the drawing places this stopcock in the tank (it is an indicative drawing), it is not the best place, because in case of emergency and we need to close this valve we will not like having to run to the roof. The ideal is to install this stopcock in the most accessible place in the same pipe, for example at the junction of this pipe to the pipes of the house
This connection will start to work after an initial suction and the pipes that follow it are filled with water, after this event the water will always come out automatically when opening any hot water tap, because the water that comes out of the tap and the entire path that will go down will suck the water from this connection.
In the event that for some reason these pipes are emptied, we will have to apply this suction again and manually, either from a tap, from the junction of the pipe of this connection with the house, or from the tank itself.

Connector ‘F’:

This is where the water pump collects the water, it collects it from the lowest part of the tank (the coldest area) and takes it to the beginning of the collector to be heated.
Here it is imperative to put a stopcock, because a break or leak in the tube or any component of the collector would cause the entire tank to empty.
Here we put (if possible) a non-return valve or solenoid valve, due to the possible dangers that I will discuss below

1. In the case of having the upper part of the collector at the same height or above the upper part of the tank, it may happen that, at night with the recirculation system stopped, and the tank being warmer than the collector, the collector will start to 'steal' hot water from the tank. As the hot water always goes upwards, the upper part of the tank will travel to the upper part of the collector, the collector will deliver its lower-cold part to the lower part of the tank (reverse recirculation effect). This happens in reality, and can only be avoided by placing the tank well above the collector, or by placing a non-return valve, or by placing a solenoid valve.
2. If we empty the tank (either for maintenance, or because we wanted to use the heated water without mixing it with the incoming cold water) the collector will be emptied irremediably, and it is possible that later the water pump will not be able to fill it by itself, and will force us to come forward and do the filling manually. Remember that we are using a fish tank water pump that is more than enough for recirculation, but to raise the water when empty it will surely lack power
3. If the collector had a bad seal in some elbow or union and took in air, or we momentarily released the connection of the connector 'C', the water level would drop or empty. This would happen if the tank lowers its level, and automatically when the water pump stops and if the collector is higher than the tank

Special attention to the non-return valve on the ‘F’ connector (if you put it in):

These valves usually come with an internal spring, and this spring requires a certain amount of pressure to open in the correct direction, and in my case the water pump did not deliver that ‘certain pressure’ and the water simply did not circulate. It is very easy to check this, before installing it we will take it with our hand and blow in the correct direction as if imitating the flow of water.
If you notice that it does not open by blowing very lightly and you use a very low-power water pump, here you may have problems.

I recommend then, that when you have it installed, you compare the flow rate with the valve in place and without it.
If that is your case, that this valve slows down your recirculation, you will have to relieve that tension. It's simple, locate the spring inside, and pull the tip of that spring outwards and pull until you have extracted a small section, cut it and return the tip of the spring to its place, and blow and check successively applying these small reductions of the spring until you consider a logical tension, but make sure that there is a minimum tension and that when blowing in the opposite direction it does not allow the air to pass. (That's what I did and it turned out great).

A – Minimum pressure spring
B – Non-return valve or also known as check valve