While hot water systems are frequently used to supply domestic hot water, or the hot water needed for personal washing and in some cases heavy and light industrial usage , this application may also be done using a heat exchanger working from a steam system.

Most often, low-temperature water systems are seen as the preferred method specified for the heating of commercial and industrial non-residential construction. These systems are used for comfort conditioning in buildings ranging in size from small commercial structures to very large complex industrial facilities.

Since the same water system can convey heat to or from a conditioned space or process with either hot or chilled water, the water often flows through piping which connects both a boiler and a chillier to the terminal heat transfer units located at the space or process.
These heating/cooling systems are often called "single pipe" systems. Special care and consideration should be given to the following when the same distribution system is shared for both heating and cooling:

• Type of boilers selected
• Control systems selected
• Type of load heat in the morning, cooling in the afternoon, heat in the evening.

To complete the comfort intent of the system satisfactorily, neither boiler nor chillier can be inoperative when their function is requested by the building occupant.
This could happen when heating is required in the morning and evening. During the day cooling is requested. To meet the demand, the boiler is maintained in the stand-by mode. When the system demands heat, the pumps and control valves shift to send boiler water to the system. What comes back to the boiler is the "chilled" water from the system. The result - thermal shock, or the death of the boiler...and the death of the chillier is just as likely to occur.

Thermal Shock

All pressure vessels are under constant stress during operation. Any sudden change in the distribution of stresses, due to excessive or sudden heating or cooling will set up stresses of large magnitude called thermal shock. Thermal shock may cause loosened tube ends, cracked tube sheets or even break welds in boilers.
The best way to prevent thermal shock damage to a boiler is to prevent rapid changes in water temperature within the boiler.
Certain boilers are better able to withstand thermal stress than others. Boilers with two rear tube sheets as used in a three pass water-backed boiler will eliminate unequal stresses found with a single tube sheet unit.
Temperature differences within the boiler can be reduced by providing for constant circulation of water through the system and boiler to prevent stratification during the off and low heat periods.
When possible, the system itself should be designed so that high temperature drops do not occur. To minimize the potential for this problem, the following is suggested:

• Provide slow acting valves - that slowly bleed the system water into the boiler during cooling/heating mode changeover.
• Specify "Shock Proof" boilers.
• Select and specify boiler controls - that maximize equipment protection such as an energy management system.
• Add an accumulator tank to the system.
• Add a blend pump arrangement

Improving System Efficiencies

Efficiency is an important topic in this market. Three suggestions for enhancing large hydronic heating systems are:

• Higher Supply Temperatures
• Primary/Secondary Pumping
• Terminal Equipment designed for Smaller Flow Rates (used either singly or in combination)

Primary/secondary or compound pumping reduces the size and cost of the distribution system. It may also use larger flows and lower temperatures in the terminal circuits. A primary pump circulates the water in the primary distribution system while one or more secondary pumps circulate terminal circuits.

Using terminal heat transfer units designed for smaller flow rates with temperature drops up to 100° F in LTW systems in conjunction with a fan apparatus is most adaptable to such designs.