What term is used to describe an appliance that supplies hot liquid or vapor for space heating?

The term used to describe an appliance that supplies hot liquid or vapor for space heating is indeed "boiler." A boiler is designed to generate hot water or steam, which can then be circulated through pipes or radiators to provide heating to a space. This process is essential in many heating systems, particularly in larger buildings or homes that require a centralized heating source.

In contrast, a furnace also provides heat but primarily does so by heating air that is then circulated through ducts. While both systems serve the purpose of heating, the fundamental difference lies in how they distribute that heat; a furnace focuses on warm air, whereas a boiler utilizes hot water or steam.

Heat exchangers are components found within many systems, including boilers or furnaces, that facilitate the transfer of heat from one medium to another. However, they do not themselves act as independent heating appliances.

Water heaters are typically designed for heating water for domestic use (such as showers or washing dishes) rather than for space heating. While some water heaters can also be used in a heating system, they aren’t primarily intended for that purpose, differentiating them from boilers.

This clarification helps underline the specific role of a boiler in the context of space heating appliances.