A hot-water heating system consists of a heater or furnace, radiators, piping systems, and circulator.
A gravity system without circulating pumps is rarely installed. It depends on a difference in density of the hot supply water and the colder return water for working head. Piping resistance must be kept to a minimum, and the circulating piping system must be of large size. A forced circulation system can maintain higher water velocities, thus requires much smaller pipes and provides much more sensitive control.
Three types of piping systems are in general use for forced hot-water circulation systems:
One-pipe system (Fig. 13.5). This type has many disadvantages and is not usually recommended. It may be seen in Fig. 13.5 that radiator No. 1 takes hot water from the supply main and dumps the colder water back in the supply main.
This causes the supply-water temperature for radiator No. 2 to be lower, requiring a corresponding increase in radiator size. (Special flow and return fittings are available to induce flow through the radiators.) The design of such a system is very difficult, and any future adjustment or balancing of the system throws the remainder of the temperatures out.
Two-pipe direct-return system (Fig. 13.6). Here are radiators get the same supply-water temperature, but the last radiator has more pipe resistance than the first. This can be balanced out by sizing the pump for the longest run and installing orifices in the other radiators to add an equivalent resistance for balancing.
Two-pipe reversed-return system (Fig. 13.7). The total pipe resistance is about the same for all radiators. Radiator No. 1 has the shortest supply pipe and the longest return pipe, while radiator No. 3 has the longest supply pipe and the shortest return pipe.
Supply design temperatures usually are 180F, with a 20F drop assumed through the radiators; thus the temperature of the return riser would be 160F.
When a hot-water heating system is designed, it is best to locate the radiators, then calculate the water flow in gallons per minute required by each radiator. For a 20F rise
A one-line diagram showing the pipe runs should next be drawn, with gallons per minute to be carried by each pipe noted. The piping may be sized, using friction-flow charts and tables showing equivalent pipe lengths for fittings, with water velocity limited to a maximum of 4 ft / s. (See ‘‘ASHRAE Handbook— Fundamentals.’’) Too high a water velocity will cause noisy flow; too low a velocity will create a sluggish system and costlier piping.
The friction should be between 250 and 600 milinches/ ft (1 milinch = 0.001 in). It should be checked against available pump head, or a pump should be picked for the design gallons per minute at the require head.
It is very important that piping systems be made flexible enough to allow for expansion and contraction. Expansion joints are very satisfactory but expensive.
Swing joints as shown in Fig. 13.8 should be used where necessary. In this type of branch takeoff, the runout pipe, on expansion or contraction, will cause the threads in the elbows to screw in or out slightly, instead of creating strains in the piping.
(W. J. McGuinness et al., ‘‘Mechanical and Electrical Equipment for Buildings,’’ John Wiley & Sons, Inc., New York.)
Hot-Water Radiators. Radiators, whether of the old cast-iron type, finned pipe, or other, should be picked for the size required, in accordance with the manufacturer’s ratings. These ratings depend on the average water temperature. For 170F average water temperature, 1 ft2 of radiation surface is equal to 150 Btu/hr.
Expansion Tanks for Hot-Water Systems. All hot-water heating systems must be provided with an expansion tank of either the open or closed type.
Figure 13.9 shows an open-type expansion tank. This tank should be located at least 3 ft above the highest radiator and in a location where it cannot freeze.
The size of tank depends on the amount of expansion of the water. From a low near 32F to a high near boiling, water expands 4% of its volume. Therefore, an expansion tank should be sized for 6% of the total volume of water in radiators, heater, and all piping. That is, the volume of the tank, up to the level of its overflow pipe, should not be less than 6% of the total volume of water in the system.
Figure 13.10 is a diagram of the hookup of a closed-type expansion tank. This tank is only partly filled with water, creating an air cushion to allow for expansion and contraction. The pressure-reducing valve and relief valve are often supplied as a single combination unit.
The downstream side of the reducing valve is set at a pressure below city watermain pressure but slightly higher than required to maintain a static head in the highest radiator. The minimum pressure setting in pounds per square inch is equal to the height in feet divided by 2.31.
The relief valve is set above maximum possible pressure. Thus, the system will automatically fill and relieve as required.
Precautions in Hot-Water Piping Layout. One of the most important precautions in a hot-water heating system is to avoid air pockets or loops. The pipe should be pitched so that vented air will collect at points that can be readily vented either automatically or manually. Vents should be located at all radiators.
Pipe traps should contain drains for complete drainage in case of shutdown.
Zones should be valved so that the complete system need not be shut down for repair of a zone. Multiple circulators may be used to supply the various zones at different times, for different temperature settings and different exposures.
Allow for expansion and contraction of pipe without causing undue stresses.
All supply and return piping should be insulated.
In very high buildings, the static pressure on the boiler may be too great. To prevent this, heat exchangers may be installed as indicated in Fig. 13.11. The boiler temperature and lowest zone would be designed for 200F supply water and 180F return. The second lowest zone and heat exchanger can be designed for 170F supply water and 150F return, etc.
Control of Hot-Water Systems. The control system is usually arranged as follows: An immersion thermostat in the heater controls the heat source, such as an oil burner or gas solenoid valve. The thermostat is set to maintain design heater water temperature (usually about 180F). When the room thermostat calls for heat, it starts the circulator. Thus, an immediate supply of hot water is available for the radiators. A low-limit immersion stat, usually placed in the boiler and wired in series with the room stat and the pump, is arranged to shut off the circulator in the event that the water temperature drops below 70F. This is an economy measure; if there is a flame failure, water will not be circulated unless it is warm enough to do some good.
If the boiler is used to supply domestic hot water via an instantaneous coil or storage tank, hot water will always be available for that purpose. It should be kept in mind that the boiler must be sized for the heating load plus the probable domestic hot-water demand.
High-Temperature, High-Pressure Hot- Water Systems. Some commercial and industrial building complexes have installed hot-water heating systems in which the water temperature is maintained well above 212F. This is made possible by subjecting the system to a pressure well above the saturation pressure
of the water at the design temperature.
Such high-temperature hot-water systems present some inherent hazards.
Most important is the danger of a leak, because then the water will flash into steam.
Another serious condition occurs when a pump’s suction strainer becomes partly clogged, creating a pressure drop. This may cause steam to flash in the circulating pump casing and vapor bind the pump.
These systems are not generally used for heating with radiators. They are mostly used in conjunction with air-conditioning installations in which the air-handling units contain a heating coil for winter heating. The advantage of high-temperature hot-water systems is higher rate of heat transfer from the heating medium to the air. This permits smaller circulating piping, pumps, and other equipment.
(H. E. Bovay, Jr., ‘‘Handbook of Mechanical and Electrical Systems for Buildings,’’ McGraw-Hill Publishing Company, New York; B. Stein et al., ‘‘Mechanical and Electrical Equipment for Buildings,’’ 7th ed., John Wiley & Sons, Inc., New York.)