Hot Water System And Loops

Hot-Water Systems

Hot-water systems use hot water to convey heat to a conditioned space. This is done using suitable heat transfer units located in the duct work or in the conditioned space. Hot-water heating systems are also called hydronic heating systems.

Water Temperature

Hot-water systems vary in the temperature of supply water provided and can be classified as follows

• Low temperature systems operate below 250° F and maintain a typical operating pressure of about 30 psi. Specifically designed low temperature water systems rated for higher pressures may be used up to 160 psi.
• Medium temperature systems operate at temperatures below 350°F with pressures not exceeding 150 psi. The usual water supply temperature is 250 to 325° F with a pressure rating of 150 psi.
• High temperature systems operate below 450° F with pressures not exceeding 300 psi. Typical operating temperatures are over 350° with pressure around 300 psi. 

Each component in the high temperature system must be selected with this higher temperature pressure requirement in mind. Low temperature water systems are widely used in commercial and institutional applications where space heating is the primary load, while medium temperature water is most commonly used in large commercial buildings or in process control work. High temperature water is better suited to multiple building applications such as a campus, where long runs between the heat source and heat transfer devices may result in significant heat loss. Higher temperature systems circulate more water through smaller pipes and are generally more efficient than low temperature systems. However, as system temperature, pressure, and efficiency increase, so does purchase, installation, and operating costs.

System Circulation

Once water is heated in the boiler, it must be circulated to heat transfer devices through the 

supply piping, accomplished in one of two ways

• Gravity circulation uses the difference in density between the warmer supply water and the cooler return water to force water flow around the system. This is an older design and is rarely seen in new commercial applications. 
• Forced circulation uses a pump, usually driven by an electric motor, to maintain flow. Pumps are most often located after the heat transfer devices in order to minimize damage and wear from hotter water.

Hot-water systems are completely filled with water which expands or contracts as water temperature rises or falls. This change in volume affects system pressure.

System Pressure

An expansion tank permits the system's water volume to change without undesirable pressure changes. Excess water volume is stored in this tank as long as high operating temperatures and pressure exist. This water is then returned to the system as water temperature decreases. The tank is designed to store the maximum excess water volume at system design conditions without exceeding the safe, maximum operating pressure.

Heat Transfer Units

Most of the same types of heat transfer devices used for steam are available with hot water. As with steam, the amount of heat given off by these devices is dependent on the flow of water through the heat transfer device, the water temperature, and the type of heat transfer surface and area exposed.

Piping Arrangements

In a two-pipe direct return system, return water from each heat transfer device flows opposite to the flow of supply water, taking the most direct path back to the heat source. This requires the least amount of piping but provides poor heat distribution because of varying lengths of return mains. This can be lessened by the use of balancing valves. The two-pipe reverse return system, on the other hand, provides nearly equal flow circuits. It provides proper heat distribution but requires additional piping.

Converters

Besides furnaces and boilers, converters (sometimes called heat exchangers) also supply hot water in commercial applications. A converter takes heat from one medium (such as steam or high temperature water) and transfers it to another medium (such as water). 

Converters are relatively simple. They consist of a shell containing tubes which hold the water or secondary medium. These are called shell and tube converters, the most common in commercial heating applications. Steam enters the converter through the steam supply line and fills the shell with steam. The steam is held in the converter by the steam trap in the condensate line. The steam transfers its heat to the water flowing in the tubes. A thermostat senses the temperature of the water within the system water supply line and as the temperature of this water drops, a control valve opens to permit more steam to enter the converter. This increases the rate of heat transfer. Heat transfer will take place as hot water (secondary medium) is applied to heating coils or radiator units located elsewhere in the system. After the hot water has passed through the coils or the radiators it is returned to the converter for reuse.