It is recommended to make a heat exchanger, guided by the following:

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welding inverter;
"Bulgarian"
other tools.
Calculation of sizes
The heater power must match the power of the furnace itself.

The heat exchanger should not consume more than 10% of the capacity of the stove itself. In this case, the stove capacity will be enough to heat the room. The fuel will be used as efficiently as possible. The mint data time will not increase.
The total length of the pipes should not exceed 3 meters. It is necessary to make the necessary slope, which is 2500 on the direct feed, and on the return - more than 300 towards the stove. So the coolant will be effectively supplied by gravity. Otherwise, you cannot do without a pump.
There should be enough water. It should heat up for no more than two hours, but not boil. If you make the tank too small, the coolant will boil very quickly. If the boiler is located directly in the steam room, it will be very difficult to wash there. Heavy steam will fill the room and it will be impossible to breathe normally. If the metal tank is too large, the coolant may not heat up at all.
Don't forget that when the stove burns out, the water should still remain hot for some time. Therefore, you need to include the necessary power reserve.
Taking into account all of the above, let's try to select the optimal metal heat exchanger. We assume that 5 kW is needed to heat the air in the bath. Then the power of the metal heat exchanger will be 10 kW, and its area is 1 m2. We will attribute 3 kW to possible heat losses. Then the installation of 1 m2 of the heat exchanger will give us 7 kW of energy. The water heating time is 2 hours. Then we will determine the required area of ​​the heat exchanger using the formula: S = 1/7 * W, where

W = 0.00117*V*(tk-tн) /T= 0.00117*65*(70-10) /2= 2.3 kW, hence S = 1/7*W=1/7*2.3 = 0.33 m2, where S is the required area of ​​the heat exchanger, m2;

Let's determine a suitable place for the remote tank. It is hung on the wall, 100-300 mm above the heat exchanger.
The tank should not be hung directly on the wall of the bathhouse. First, we attach the guides with dowels - 2-3 wooden planks. Then, we hang the tank on them. The gap between the wall and the tank should be at least 20 mm.