Hydronics (from Ancient Greek hydro- 'water') is the use of liquid water or gaseous water (steam) or a water solution (usually glycol with water) as a heat-transfer medium in heating and cooling systems. The name differentiates such systems from oil and refrigerant systems. Historically, in large-scale commercial buildings such as high-rise and campus facilities, a hydronic system may include both a chilled and a heated water loop, to provide for both heating and air conditioning. Chillers and cooling towers are used either separately or together as means to provide water cooling, while boilers heat water. A recent innovation is the chiller boiler system, which provides an efficient form of HVAC for homes and smaller commercial spaces.
District heating
Many larger cities have a district heating system that provides, through underground piping, publicly available high temperature hot water and chilled water. A building in the service district may be connected to these on payment of a service fee.
Types of hydronic system
Basic types Hydronic systems can include the following kinds of distributions:
Chilled water systems Hot water systems Steam systems Steam condensate systems Ground source heat pump systems
Classification Hydronic systems are further classified in five ways:
Flow generation (forced flow or gravity flow) Temperature (low, medium, and high) Pressurization (low, medium, and high) Piping arrangement Pumping arrangement
Piping arrangements Hydronic systems may be divided into several general piping arrangement categories:
Single or one-pipe Two pipe steam (direct return or reverse return) Three pipe Four pipe Series loop
Single-pipe steam
In the oldest modern hydronic heating technology, a single-pipe steam system delivers steam to the radiators where the steam gives up its heat and is condensed back to water. The radiators and steam supply pipes are pitched so that gravity eventually takes this condensate back down through the steam supply piping to the boiler where it can once again be turned into steam and returned to the radiators. Despite its name, a radiator does not primarily heat a room by radiation. If positioned correctly a radiator will create an air convection current in the room, which will provide the main heat transfer mechanism. It is generally agreed that for the best results a steam radiator should be no more than one to two inches (2.5 to 5cm) from a wall. Single-pipe systems are limited in both their ability to deliver high volumes of steam (that is, heat) and the ability to control the flow of steam to individual radiators (because closing off the steam supply traps condensate in the radiators). Because of these limitations, single-pipe systems are no longer preferred. These systems depend on the proper operation of thermostatic air-venting valves located on radiators throughout the heated area. When the system is not in use, these valves are open to the atmosphere, and radiators and pipes contain air. When a heating cycle begins, the boiler produces steam, which expands and displaces the air in the system. The air exits the system through the air-venting valves on the radiators and on the steam pipes themselves. The thermostatic valves close when they become hot; in the most common kind, the vapor pressure of a small amount of alcohol in the valve exerts the force to actuate the valve and prevent steam from leaving the radiator. When the valve cools, air enters the system to replace the condensing steam. Some more modern valves can be adjusted to allow for more rapid or slower venting. In general, valves nearest to the boiler should vent the slowest, and valves furthest from the boiler should vent the fastest. Ideally, steam should reach each valve and close each and every valve at the same time, so that the system can work at maximal efficiency; this condition is known as a "balanced" system.
Two-pipe steam systems In two-pipe steam systems, there is a return path for the condensate and it may involve pumps as well as gravity-induced flow. The flow of steam to individual radiators can be modulated using manual or automatic valves.
Two-pipe direct return system The return piping, as the name suggests, takes the most direct path back to the boiler.
Advantages Lower cost of return piping in most (but not all) applications, and the supply and return piping are separated.
Disadvantages This system can be difficult to balance due to the supply line being a different length than the return; the further the heat transfer device is from the boiler, the more pronounced the pressure difference. Because of this, it is always recommended to: minimize the distribution piping pressure drops; use a pump with a flat head characteristic, include balancing and flow-measuring devices at each terminal or branch circuit; and use control valves with a high head loss at the terminals.
Two-pipe reverse return system The two-pipe reverse return configuration which is sometimes called 'the three-pipe system' is different from the two-pipe system in the way that water returns to the boiler. In a two-pipe system, once the water has left the first radiator, it returns to the boiler to be reheated, and so with the second and third etc. With the two-pipe reverse return, the return pipe travels to the last radiator in the system before returning to the boiler to be reheated.
Advantages The advantage with the two-pipe reverse return system is that the pipe run to each radiator is about the same, this ensures that the frictional resistance to the flow of water in each radiator is the same. This allows easy balancing of the system.
Disadvantages The installer or repair person cannot trust that every system is self-balancing without properly testing it.
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