Two-point temperature controller BESFUL BF-D215B + for solar collectors. Or how much is free solar energy.

All the time, mankind has tried to use solar energy for good purposes. What devices for this exist today ...
It will be about the solar collector for heating water. My personal opinion is that the use of free solar energy in our latitudes is quite expensive and does not justify itself. Perhaps where there is during daylight hourshigh consumption of hot water (hotels, recreation centers, etc.) is beneficial, but for a single family of, for example, 4 people, this does not give a lot of savings and equipment costs do not pay off or pay off too long.
I will not go into details, if someone thinks differently, this device will help you.
Be that as it may, my friend painted the heat sink black,hung it on the roof and connected it with a combined tank for heating water.
Sample diagram from the manufacturer’s website

As is clear with this arrangement (radiator abovetank), hot water will not circulate on its own, so you need a pump. Of course, the pump can be turned on in the morning, and turned off in the evening. But at the same time, it will work continuously and at a certain moment can trigger a reverse reaction, i.e. the water in the tank will cool.
A conventional temperature controller is not suitable here. It can be configured to turn it on, for example, when the temperature of the collector is above +30 degrees, but the temperature of the water can be +15 in all, and the temperature of the collector is +20 and even then you can start heating the water.
Thus, we need a sensor that responds to the temperature difference between the collector and the water heater.
Searches on Ali led to a not very large result: option without housing and power supply and Besful appliances. There are several options, I did not really delve into the possibilities, I chose the option that arranged and ordered me.
The device arrived in 20 days, is packed in a company box, in a “pimple” on top and another box.
Characteristics (something honestly taken from the site, something added by me):
Number and type of sensors to be connected: 2 x NTC (10K / + 25C thermistor)
Measured temperature range: from minus 50С to + 120С
Antifreeze mode: yes
Supply voltage: 90 ... 250 VAC
Power consumption, no more: 2 Watt
Maximum current of contacts of the output load relay: no more than 5A (28 VDC or 220 VAC) per active load
When working on inductive load (L / R no more than 7 ms), the maximum current through the relay contacts must not exceed 1.0A (28 VDC or 220 VAC)
** Speaking Russian: if a heating element is used - 5A, if, for example, an engine, then no more than 1A
Saving the set parameters when turning off the power: yes
Dimensions of the thermostat: front panel 91x52x40 mm, weight 190 grams
Dimensions of sensors: length 30mm, diameter 6 mm
Thermostat Mounting: 35mm DiN Rail
Sensor cable length: 2.5m (it is allowed to extend the cable to 100).
Inside the original packaging is the device itself, two temperature sensors, a manual in "intelligible" Chinese and a small flat screwdriver for tightening the terminal screws.
Photo of the device in different angles

Also, inside the box was something likebusiness cards with a QR code. A similar code is on the box and the device itself. The scan led to, which turned out to be "dead." But I still managed to find a working site, and on it a link to monitored device. Unfortunately, the site was on the sameChinese language. There is a choice of English on it, but the “click” on it did not lead to any result. Thanks to Google, you can get acquainted with the characteristics and capabilities of the controller, but attempts to download something from the site (I was hoping that there would be instructions) or send a support request did not lead to anything. Contact by email and skype remained unanswered. Judging by some dates, the site was updated around 2013.
The seller, when asked whether he had instructions in English, replied that there was none.
The device is not so common in “our circles” that I found only a couple of intelligible understandings about it on Russian-language sites and the best one is on Kazakh site :).
But the “poke method” always helped us deal with devices even before reading any instructions, so I managed to find out the principle of operation of this device.
On the inclusion diagram (as on the manufacturer’s website,so on the device itself) it is clear that there is a relay with one group of contacts of 3 contacts: the central one is normally closed to one, and the other is normally open.
Switching circuit on the device

When a certain temperature difference is reached, the relay switches, and why it comes back, when a lower threshold is reached.
So, we have 3 settings:
- I-H - temperature difference of the sensors at which the relay is triggered, range 0-80 degrees;
- I-P - temperature difference of the sensors, at which the relay turns off, the range is 1-30 degrees;
- PH - temperature “freezing” (“Antifreeze” mode) of the hot loop sensor, at which the relay turns on again, the range is -10 to +8 degrees or off.
The data is set by successively pressing the SET button and selecting a value with the Up / Down buttons.
Indication of some operating modes

In the normal state, the device shows the temperature difference between the sensors. By pressing the F1 button, you can see the current temperature of each sensor.
In addition, by pressing the Up button, you can force the relay to turn on or off.
As for accuracy, I don’t have a referencea thermometer, but a bunch of non-standard ones showed a difference with the device of about 0.5 degrees. The temperature readings of the sensors, if they are in the same conditions, are the same.
In appearance of the relay and sensors, we can say that it is made quite soundly (unfortunately, there will be no viscera).
Time will tell how it will work and how much it will help to save. Although it will be quite difficult to calculate the savings, a lot of effort and time should be devoted to this.