In residential installations, hot-water boilers are the most commonly used type due to their efficiency and reliability. Steam boilers are less common, while low-pressure and high-pressure boilers are not suitable for residential use. Hot-water systems are preferred for heating due to their convenience and effectiveness. ;
Hot-water boilers are the preferred choice for residential installations due to their efficiency, convenience, and safety. Steam, low-pressure, and high-pressure boilers are less suitable for home use. Therefore, the answer is D. Hot-water.
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