The installation of thermoregulators on heating devices creates new conditions for the whole heating system. For example, suppose it got abruptly warm outside, and the warmth losses from the house are decreased, and so the room temperature begins to go up. In this case, all the automatic thermoregulators understand this situation clearly. They need to stop the supply of warmth carrier to the radiators and they do so. As the supply of warmth carrier through the radiators tries to get to zero, then the pressure in the supply pipeline increases dramatically. It is necessary to take measures to equalize the pressure. For this, right after the circulational pump in the system, a relief valve (see Figure 35) is installed, and it is connected to the supply and return pipelines. In essence it is the same check valve as we installed in the safety group right after the boiler, but in this case the outlet of the relief valve is connected not with the drain pipe but with the return pipe of the heating ring (see Figure 36).
Thermoregulators turn off radiators, the pressure in the heating contour increases, the relief valve is activated and transfers the warmth carrier into the return pipeline, bypassing the heating contour; the pressure in the contour is reduced and equalized. In complex heating systems with a large number of heating contours which were made according to the classical scheme, it is better to put the relief valves after each circulational pump, as they will help to avoid problems connected with imbalances in the contours. In order to avoid motion of the warmth carrier in the opposite direction when the dramatic increase in differential pressure occurs, the check valve is installed on the supply right after the pump. This mechanism, equipped with a weak spring and a closing flap or a cone, allows the warmth carrier to go, unimpeded, in one direction, and prevents it from going in the other direction. Whether or not to install the check valve is determined by the calculation of hydraulic resistance and differential pressure in this part of the heating contour. The hydraulic resistance of the valve must be taken into account when selecting the circulational pump for this heating contour.