Hot-water heating systems transmit only sensible heat to radiators, as distinguished from steam systems that work principally by the latent heat of evaporation. The result is that the radiator temperature of a steam system is relatively high compared to that of a hot-water system. In a hot-water system, latent heat is not given off to a great degree, so more heating surface is required.

Advantages of hot-water heating include:
1. Temperatures may widely vary, so it is more flexible than low-pressure (above atmospheric) steam systems.
2. The radiators will remain warm for a considerable time after the heat-generating fire has gone out;thus, the system is a reservoir for storing heat.

Disadvantages include:
1. There is a danger of freezing when not in use.
2. More or larger heating surfaces (radiators) are required than with steam systems.

There are actually two types of hot-water systems, depending on how heated water flows: thermal and forced circulation. The word “thermal” refers to systems that depend on the difference in the weight of water per unit of volume at different temperatures to form the motive force that results in circulation. This type is rightfully called a gravity hot-water system. The difference in the density or weight of hot and cold water causes natural circulation throughout the system. This circulation is necessary in order for the water to carry the heat from the boiler to the radiators.

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