When the difference between the supply and return temperatures at the boiler is large, then the temperature on the walls of the combustion chamber of the boiler approaches the dew point temperature and condensation may occur. It is known that when fuel is burnt, different gasses are emitted, including carbon dioxide (CO2). If this gas joins with dew which has formed on the boiler walls, then acid will be formed which will corrode the «water jacket» of the furnace of the boiler. As a result, the boiler can easily stop functioning. Therefore you must design the heating system to prevent dew from forming and so the temperature difference between the supply and return should not be too great. Typically this is achieved by heating the return of the warmth carrier and/or inclusion of water from the hot water supply into the return, with priority.
To heat the warmth carrier between the boiler return and supply, a bypass is added with a circulational pump installed on it. The power of the recirculational pump is usually chosen to be one-third the power of the main circulational pump (or sum of several pumps) (see Figure 41). To prevent «pushing» of the recirculational contour in the opposite direction by the main circulational pump, a check valve is installed behind the recirculational pump.
Another way of heating the return flow is to install a hot water supply heater in the immediate vicinity of the boiler. This water heater is put on a short heating ring and placed so that hot water from the boiler, right after it passes through the main distributive collector, goes into the hot water heater, and from it goes back to the boiler. However, if the demand for hot water is not too great, then in the heating system, both the recirculational ring with a pump and a heating ring with a water heater are installed. If the calculations are well done, then the recirculational ring with a pump can be replaced by a system with three- or four-way mixers (see Figure 42).