Abstract In recent years, the literature on HVAC has been predominantly devoted to the problems of increasing the efficiency of buildings, reduction of CO2 emission or ensuring user-friendly indoor air quality. However, in many countries, including Poland, ensuring safety for the users of gas appliances is a key issue. That particularly concerns the gas-fired water heaters with open combustion chamber (Type B). The problem can be solved by replacing the traditional heaters with combustion chamber sealed with respect to the room (Type C) equipped with air supply and combustion product ducts systems. Two pilot plants were built, corresponding with the conditions of a typical Polish multi-storey residential building. During the designing process, the CFD Fluent software was used, making it possible to optimally select the dimensions and configuration of the tested plants. The tests were conducted for different operation variants, during which the appropriate functioning of the appliances was evaluated as well as the energy efficiency of hot water production. Moreover, selected ways of enhancing the operation of such appliances as well as the impact of wind on their work were examined. The experimental results presented in the paper should make it easier to implement gas-fired instantaneous water heaters Type C, particularly in the already-existing multi-storey residential buildings.