Hydronics

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Hydronics is the name for the use of water as the heat-transfer medium in heating and cooling systems.

Some of the oldest and most common examples are steam and hot-water radiators. In large-scale commercial buildings such as high-rise and campus facilities, a hydronics 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 separately or together as means to provide water cooling, while boilers heat water. In addition, many larger cities have a district heating system that provides, through underground piping, publicly available steam and chilled water. By paying a service fee, a building in the service district may be connected to these.

Hydronic Systems are of three basic types:

Steam
Hot Water
Chilled Water

Hydronic systems are 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

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

[edit] Single-pipe steam

Single-pipe steam radiator

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 steam 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 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 installed.

In order to work correctly, these systems are completely dependent on the proper operation of thermally-closed air venting valves located on radiators throughout the heated area. When not in use, the single-pipe steam heating system's valves are open to the atmosphere, and the steam pipes and radiators contain regular air. When heat is called for by the thermostat, the boiler starts, and produces steam from the water contained in the boiler. This steam expands and rises, and displaces the air contained in the steam pipes from them. The displaced air rapidly exits from the air venting valves located on the radiators, and, in larger systems, valves placed on the steam pipes themselves. When steam reaches each air venting valve located on each radiator, a small quantity of solvent (usually alcohol) contained within the valve is heated by the steam, and rapidly turns into vapor, exerting enough mechanical force to close the valve, thus holding the steam inside the radiator, and preventing it from exiting the radiator into the room being heated. After the heating cycle is over, the boiler shuts down, and the steam in the radiators cools. Once the air venting valve is sufficiently cool, the solvent located within each radiator valve will turn back into a liquid, and open the valve on the radiator to the atmosphere, which allows air to enter the steam pipes, and the steam and water contained within to drain back down to the boiler.

To increase heat delivered to an area served by a radiator, a larger air-venting valve, allowing for more rapid venting of air (and therefore faster delivery of steam) from the individual radiator, can be installed. Some more modern valves can also be adjusted so as 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; however, a good degree of patience, trial, and error are necessary to fully optimize a single-pipe steam system to this level. The most common problems with radiator valves occur when they are painted over (common in older houses), bashed, or otherwise abused, often leading to manifest homeowner frustration; untrained service technicians often respond to complaints by increasing steam pressure at the boiler rather than replacing air venting valves, making matters worse, by causing high pressure steam to leak or otherwise disturb steam pipes, causing "knocking" sounds in the middle of the night (which can easily disrupt sleep as well as other nocturnal--or diurnal--activities), as well as wasting heating oil and energy. Investing in new radiator venting valves for an old or troublesome single-pipe steam system, as well as taking the time to correctly size and adjust them, will reduce or eliminate many headaches once completed and lower heating fuel use and bills.

[edit] Two-pipe steam systems

In two-pipe steam systems, there is a separate 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.

[edit] Two Pipe Direct Return System

The return piping as the name suggests takes the most direct path back to the boiler.

[edit] Advantages

Low cost of return piping in most but not all application and the supply and return piping are separated.

[edit] Disadvantages

This system can be difficult to balance due 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.

[edit] Two Pipe Reverse Return System

The return piping takes the same basic path as the supply back to the boiler.

[edit] Advantages

This system is often considered "self balancing" however valves should always be included.

[edit] Disadvantages

Cannot trust that every system is self balancing without properly testing it.

Very large scale systems can be built using the two-pipe principle. For example, rather than heating individual radiators, the steam may be used in the reheat coils of large air handlers to heat an entire floor of a building.

[edit] Water loops

Modern systems often use heated water rather than steam. This opens the system to the possibility of also using chilled water to provide air conditioning.

In homes, the water loop may be as simple as a single pipe that "loops" the flow through every radiator in a zone. In such a system, flow to the individual radiators can not be modulated as all of the water is flowing through every radiator in the zone. Slightly more complicated systems use a "main" pipe that flows uninterrupted around the zone; the individual radiators tap off a small portion of the flow in the main pipe. In these systems, individual radiators can be modulated. Alternatively, a number of loops with several radiators can be installed, the flow in each loop or zone controlled by a zone valve connected to a thermostat.

In any water system, the water is circulated by means of one or more circulator pumps. This is in marked contrast to steam systems where the inherent pressure of the steam is sufficient to distribute the steam to remote points in the system. A system may be broken up into individual heating zones using either multiple circulator pumps or a single pump and electrically operated zone valves.

[edit] Boiler water treatment

Domestic (home) systems may use ordinary tap water, but sophisticated commercial systems often add various chemicals to the system water. For example, these added chemicals may:

Inhibit corrosion
Prevent freezing of the water in the system
Increase the boiling point of the water in the system
Inhibit the growth of mold and bacteria
Allow improved leak detection (for example, dyes that fluoresce under ultraviolet light)

[edit] Air Elimination

All hydronic systems must have a means to eliminate air from the system. A properly designed system that is air-free should provide many years of excellent performance.

Air causes irritating system noise in addition to interrupting proper heat transfer as the system fluids circulate throughout the system. In addition, unless reduced below an acceptable level, the oxygen found within water will cause corrosion. This corrosion can cause rust and scale to build up on the system piping. Over time these particles can become loose and travel throughout the system. The particles can reduce flow and even clog the system in addition to causing damage to pump seals and other system components.

[edit] Steam system

In steam systems, individual radiators are usually equipped with a thermostatic bleed valve. At room temperature, the valve opens the radiator to the air, but as hot steam flows into the radiator and pushes the contained air out, the valve heats and eventually closes, preventing steam from escaping into the room.

[edit] Water loop system

Water-loop systems can also experience air problems. Air found within hydronic water-loop systems may be classified into three forms:

[edit] Free air

Various devices such as manual and automatic air vents are used to address Free air which floats up to the high points throughout the system. Automatic air vents contain a valve that is operated by a float. When air is present, the float drops, allowing the valve to open and bleed air out. When water reaches (fills) the valve, the float lifts, blocking the water from escaping. Small (domestic) versions of these valves in older systems are sometimes fitted with a Schraeder-type air valve fitting and any trapped, now-compressed air can be bled from the valve by manually depressing the valve stem until water rather than air begins to emerge.

[edit] Entrained air

Entrained air is the air bubbles that travel around in the piping at the same velocity as the water. Air "scoops" are one example of products which attempt to remove this type of air.

[edit] Dissolved air

Dissolved air is also present in the system water and the amount is determined principally by the temperature and pressure (see Henry's Law) of the incoming water. On average tap water contains between 8-10% dissolved air by volume. Removal of Dissolved air and Free / Entrained air can be achieved with a high-efficiency air elimination device that includes a coalescing medium that continually scrubs the air out of the system.

[edit] Accommodating thermal expansion

Water expands and contracts as it heats and cools. A water-loop hydronic system must have one or more expansion tanks in the system to accommodate this varying volume of the working fluid. These tanks often use a rubber diaphragm pressurised with compressed air. The expansion tank accommodates the expanded water by further air compression and helps maintain a roughly-constant pressure in the system across the expected change in fluid volume.

[edit] Automatic fill mechanisms

Hydronic systems are usually connected to a water supply (such as the public water supply). An automatic valve regulates the amount of water in the system and also prevents backflow of system water (and any water treatment chemicals!) into the water supply.

[edit] Safety mechanisms

Excessive heat or pressure may cause the system to fail. At least one combination over-temperature and over-pressure relief valve is always fitted to the system to allow the steam or water to vent to the atmosphere in case of the failure of some mechanism (such as the boiler temperature control) rather than allowing the catastrophic bursting of the piping, radiators, or boiler. The relief valve usually has a manual operating handle to allow testing and the flushing of contaminants (such as grit) that may cause the valve to leak under otherwise-normal operating conditions.