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The objective of this paper is to raise awareness of the risk of boiling liquid expanding vapor explosions (steam explosions) in heating systems in Serbia caused by a combination of the following factors: solid fuel burning, older boiler design, closed systems, and non-installation of heat dissipation devices. The practice is in accordance with neither Standard SRPS EN 303-5:2012 nor subject literature, which both demand that this type of heating be installed in open systems. Explosions do occur; there was one in 2014 in Futog, Serbia, with fatal consequences. The main protection element, safety valve, is designed for temperatures up to 110 °C. Its operation above 110 °C is unknown. The experiment physically simulated the worst case scenario, where there is no circulation in the heating system. It used a 90 L water-filled vessel with six 3 kW electric heaters installed and safety valves attached. This paper presents the first results for the case where the set pressure of the safety valve was 1.5 bar and one heater of 3 kW was in operation. The results showed that the safety valve did not prevent boiling. The recorded pressure peaks were at 2.2 bar and the lows were at 0.8 bar, so its operation intensified boiling. Therefore, the system cannot be considered safe even with a brand new safety valve and at low overheating rates. Better air removal in the system is to be solved in future experiments. Tests will be done with different safety valves and overheating rates. © 2020 Society of Thermal Engineers of Serbia.