Hydronics is the use of water as the heat-transfer medium in heating and cooling systems. A hydronic piping system is used to circulate chilled or hot water with the connections between the piping and the terminal units made in a series loop. The terminal units are the heat exchangers that transfer the thermal energy between the water and the spaces to be cooled or heated.
Hydronic systems may be used for both a chilled and a heated water loop with chillers and cooling towers used separately or together as a means to provide water cooling, while boilers heat the water.
Types of hydronic piping systems are:
- The Series Loop – This system is aptly named because all of the units are in series, and one loop is formed. In this system the entire water supply flows through each terminal unit and then returns to the generator and pump. Although it is a simple arrangement, this setup has its disadvantages:
- To maintain or repair any terminal unit, it requires a shutdown of the entire system.
- The number of units is limited because in heating systems the water temperature continually decreases as it gives up heat in each unit in series. That can cause a low temperature in the far units in the system which may not provide adequate heat for comfort.
The series loop arrangement is basic, inexpensive and mostly used for residences.
- One-Pipe Main – With this system, each terminal unit is connected by a supply and a return branch pipe to the main. By locating valves in the branch lines, each unit can be separately controlled and serviced. In this system, like in the series loop, if there are too many units the heated water going to the far units may be not sufficient for room comfort.
- Two-Pipe Direct Return – This is generally used for larger systems and consists of two mains. One main is used for supply and one main is used for return. This system is more expensive than the one-pipe main and series loop, but it allows each terminal unit to be separately controlled and serviced because the supply water temperature to each unit is the same. The two-pipe system is called direct return because the return main is routed to bring the water back to the source by the shortest path.
- Two-Pipe Reverse Return – Here we have a supply and a return that are equal in length and size. The first terminal supplied is the last terminal returned and vise-versa, making it is easy to balance the flow rates.
Combination arrangements can also be made to create a three-pipe or four-pipe system. In the three-pipe arrangement, simultaneous heating or cooling can be made available. There are two-supply mains, one circulating chilled water, the other hot water. Three-way control valves in the branch to each terminal unit will determine whether the unit receives hot or chilled water and the return main receives the water from each unit. However, the three-pipe system can waste energy because the return main mixes chilled and hot water. In this mixing process, the chilled water is warmed and the hot water is cooled, which results in extra heating and cooling in the boiler and/or chiller. The four-pipe arrangement is expensive, but it separates two-pipe systems – one for chilled water and one for hot water. Therefore, no mixing occurs making it an ideal arrangement to avoid wasted energy.