HOT-WATER HEATING
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 sys- tem 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 vol- ume 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 nat- ural 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 ra- diator 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 con- structed pipe coils or lengths of tubing in the floor, walls, or ceiling. These coils generally consist of small- bore wrought-iron, steel, brass, or copper pipe, usually with an inside diameter of 3/8 to 1 inch. Every con- sideration 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.
The following are some advantages of radiant-panel heating:
1. Radiant-panel heating eliminates radiators and grills, thus providing more floor space and resulting in better furniture arrangements and wall decorations.
2. There is less streaking of walls and ceiling due to lower velocities of air currents.
3. It provides warm floors in homes with no basement.
4. It simplifies interior architectural and engineering building designs.
5. A well-designed and installed radiant-panel heating system provides low operating and maintenance costs.
Hot-water radiant panels can be installed in nearly any type of building, with or without a basement or excavated section. Conventional hot-water boilers are used. Units of this type are available in compact types that fit into small spaces and are fired by gas or oil. Additions can be installed at any time, provided that the limitations of the boiler unit and circulating pump are not exceeded.
Radiant panels should never be used with steam; too many complications arise. In addition, domestic hot water should not be taken from the system for use in bathrooms or kitchens.
Provision must be made for draining the system if the need should arise. Care must be taken in design and installation to ensure that the system can be completely drained, with no water pockets that will hold water and result in damage in the event of a freeze.
The ceiling is the most satisfactory location for radiant-heating panels. A combination grid and continu- ous coil are used in a large installation where the heat requirements make it necessary to install several coils.
Ceiling panels should not be installed above plywood, composition board, or other insulating types of ceil- ing material. Surfaces of this type have an undesirable insulating effect that diminishes full heat output of the panel.
Radiant panels are often installed in floors. When this is done, the best arrangement is to place the coils in the concrete floor slab. Good results are obtained when the pipe or tubing is placed at least 2 inches be- low the floor surface or deeper if a heavy traffic load is anticipated. Allow a minimum of two weeks for the concrete to set before applying heat, and then apply heat gradually. Floor covering of terrazzo, linoleum, tile, or carpeting can be installed. If the water temperature is kept below the prescribed maximum of 85 degrees F, no damage will result to rugs, varnish, polish, or other materials. A typical piping diagram for a radiant floor panel avoids low places in the coil.
Heat loss to the ground from a floor panel laid directly on the ground can be expected. This loss is estimated to be from 10 to 20 percent of the heat provided for the room. Heat loss from outside slab edges can be greater than this amount. Slab edges should receive from l/2 inch to 2 inches of waterproof insulation.
It is not customary to install radiant wall panels except to provide supplementary heat where ceiling and/or floor panels do not provide a required degree of comfort. Wall panels are occasionally installed in bathrooms where higher than normal heat temperatures may be desired. In a typical radiant wall-panel installation diagram it must be remembered that circulating pumps for use with radiant heating should have a higher head rating than for convector systems of the same capacity. This requirement exists because the coil pressure drop is considerably higher than the drop in a radiator or convector.
Certain fittings or devices are essential to the proper operation of a boiler. The piping diagram is a typi- cal steam boiler with the control and indicating devices that are essential to its proper operation.
One of the most important features is the safety valve. Every boiler installation must have a safety valve installed to protect the boiler itself and the building occupants in case of malfunction. These valves are adjusted to open and relieve the internal pressure should it rise above a safe predetermined level. Numerous valves, gauges, and safety devices are found on all boilers.
The level of the water in a boiler must be maintained between certain limits; otherwise, serious damage to the boiler and building may result, as well as possible injury to the building occupants. Various safety de- vices are incorporated to protect against this possibility. Water gauges are provided as a means of visually checking the level. More sophisticated gauges employ floats that actuate a whistle or other alarm when the water level drops to a dangerous point.
Pressure-relief valves and fusible plugs are also used to protect the boiler in case of malfunction. Both of these devices will relieve dangerous high pressure under certain conditions.
Injectors are used to supply water against the high pressure existing within a boiler. This is done by means of the jet principle. Steam loops are often provided to return condensate to a boiler. These devices are en- tirely automatic and have only one moving part, a check valve at the bottom of the drop leg.
Heating pumps are usually used in steam heating systems to improve efficiency. Two principal types are available—condensation pumps and vacuum pumps. The type of heating system, cost, and individual requirements dictate which of these pumps must be used.
This heating section is a brief summary of steam and hot-water heating principles and in general contains the information necessary to answer questions pertaining to steam and hot-water heating that may be found in plumbing code examinations.