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.

In the forced-circulation type of hot-water system a pump is used to force the water through the piping.
Thus, the flow is entirely independent of the difference in water temperature.

Gravity hot-water systems are used mainly in small buildings such as homes and small business places.

Advantages of this type of system include:
1. Ease of operation.
2. Low installation costs.
3. Low maintenance costs.

Disadvantages include:
1. Possible water damage in case of leaks.
2. Rapid temperature changes result in a slow response from the system.
3. Properly balancing the flow of water to radiators is sometimes difficult.
4. Nonattendance when the heat-generating unit fails may result in a freeze-up.
5. Flow depends on gravity, and as a result larger pipe sizes are required for good operation.

Forced-hot-water heating systems require a pump that forces the water through the piping system. Limitations of flow, dependent on water-temperature differences, do not exist in this type of system. It may be of either the one- or two-pipe variety.

In two-pipe systems, either direct or reversed returns and up-feed or down-feed mains may be used. The path of the water from the boiler into and through the radiator and back again to the boiler is almost the same length for each of the radiators in the system.

It is common to use one-pipe forced-circulation systems for small and medium-sized buildings when hot water is used as the heating medium.

Advantages of forced circulation include:
1. There is rapid response to temperature changes.
2. Smaller pipe sizes may be installed.
3. Room temperatures can be automatically controlled if either the burner or the flow of water is thermostatically controlled.
4. There is less danger of water freeze and damage.

Disadvantages include:
1. All high points must be vented.

Radiant-panel heating is the method of heating a room by raising the temperature of one or more of its interior surfaces (floor, walls, or ceiling) instead of heating the air.

One of the most common methods of achieving radiant heating is by the installation of specially constructed pipe coils or lengths of tubing in the floor, walls, or ceiling.

These coils generally consist of smallbore wrought-iron, steel, brass, or copper pipe, usually with an inside diameter of 3/8 to 1 inch. Every consideration should be given to complete building insulation when radiant panel heating is used.

Air venting is necessary to the proper control of any panel hot-water heating system. Collection of air in either the circuit pipe or pipe coils results in a shortage of heat.

Because of the continuous slope of the coil connections, it may be sufficient to install automatic vents at the top of the return riser only, omitting such vents on the supply riser.

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