A boiler may be defined just as a pressurized vessel in which heating of water takes place. It can be used for production of either hot water or steam. The operating principle of hot water boilers i.e. hydronic boilers and steam boilers is more or less same. However, there are few significant points of distinction lying in their equipment and processes which are mentioned in the section below.

In general, both hydronic boilers and steam boilers results in heating of water and generation of steam by burning of fuel. The heated water is then passed via boiler tubes while the steam generated is circulated in the building with the help of radiators. Though everything works in a similar fashion yet sometimes, the steam produced in steam boilers gets collected in a separate container. The burning fuel employed in a hydronic boiler is typically the same as used in steam boilers. However, owing to cost-effectiveness, use of natural gas is sometimes preferred in hot water boilers.

Steam boilers and hydronic boilers tend to find some differences in their application areas too. Since steam boilers are more robust, they are frequently put into use for industrial applications where hot water or steam is required whereas the use of hot water boilers is generally limited to domestic heating applications.

Major Differences

Some important point of differences between steam boilers and hot water boilers are mentioned below:

Heating Points

In hot water boilers, the intensity of water boiling is extremely low. Their heating temperature usually doesn’t go beyond 180 or 200 degrees Fahrenheit. On the other hand, in steam boilers high intensity of water boiling takes place in order to convert water into steam. Hence, their heating temperature is very high i.e. around 212 degrees Fahrenheit.

Accessories

A hot water boiler typically requires extra boiler accessories as compared to a steam boiler, for example, a flow check valve, a circulator, an expansion tank and especially a motorized pump.

Water Level and Circulation

A hydronic boiler must always be completely filled with water whereas in steam boilers, there is no such requirement. Besides, a hot water boiler requires an electric pump for circulation of hot water whereas in steam boilers, natural expansion of steam takes place which automatically fills pipes.

Efficiency and Safety

The operation of hot water boilers is usually considered to be safer as compared to that of steam boilers. Also, the efficiency of steam boilers is found to be less in comparison to hot water boilers. This is due to the fact that steam boilers don’t employ motorized pumps for circulation. However, the boiler efficiency also depends upon the kind of boiler.

Cost

The cost involved with steam boilers and hydronic boilers along with their accessories is approximately equal. The basic cost difference depends upon the choice of required fuel which can be either gas or oil.

Water Levels & Controls

Unlike steam boiler systems, water level in hot water boiler systems is usually maintained very high i.e. upto top of the water jacket. On the other hand, the steam boilers are not completely full of water; their water level is normally maintained below the top of the water jacket in order to facilitate steam generation. Steam boiler designs generally incorporate a float valve along with a sight glass which automatically controls the level of water. Steam is distributed in a building by means of radiators and eventually returned back to the boiler utilizing the gravity effect. In this way, the whole boiler cycle keeps on repeating without need of any special control devices or fittings such as circulators, flow check valves or automatic bleeders etc.

However, hot water boilers require several fittings and accessories for their proper operation. They incorporate a special temperature regulating device known as an aquastat which basically meets two major boiler objectives. “It controls the temperature the boiler operates at, and provides a low voltage transformer and relay to control the circulator.”¹
If more than one circulator is employed then extra relays or a multiple control panel would be required for a hot water boiler operation. The pressure of water is usually kept in check by means of a valve which is referred to as a regulator. A special device known as check valve or backflow preventer is also required to be incorporated into hydronic boiler systems for preventing the migration of boiler water back to the supply water in case the pressure within the boiler rises above the domestic water pressure. Besides, a temperature-pressure relief valve is employed to keep the system temperature and pressure within maximum limits. Also an expansion tank is needed to facilitate expansion and contraction of water depending upon heating or cooling effect applied.

Due to the impracticality of hermetically sealing the heating system, air bleeders whether it be automatic or manual should always be incorporated into the water circulating loop of the boiler. For circulation of water in the heat loop, a motorized pump is typically put into use. There are basically two means by which hot water can be generated for domestic heating purposes. “A tank-less coil (a coil of copper tubing with aluminum fins attached) can be immersed into the water jacket to create allow a heat exchange. A hot water maker is a storage tank with a coil inside through which hot water from the boiler is circulated for the heat exchange.”²
Since heated water produced by a hydronic boiler may consist of anti-freeze or corrosion preventing chemicals, it is not permissible to draw hot water directly out of the boiler system.

Scale Build-up & Corrosion

Steam boilers and hot water boilers are usually subjected to diverse water treatment techniques.

Steam boilers are mainly provided with treatment which avoids build up of scale and corrosion. Scale build-up takes place inside a boiler due to the accumulation of dissolved minerals present in the boiler feed water. This boiler water loaded with minerals tends to replace the clean steam and condensate leaks. “Strategies to prevent scale attempt to keep the components of scale such as calcium and magnesium suspended in the boiler water or to reduce their concentrations in the boiler water.”³

One of the common techniques used to trim down the concentration of solids inside a boiler is known as bottom blowdown. In this method, the water simply gets discharged from the bottom of the boiler. To avoid scale deposits, the sludge build up must also be frequently discharged from the boiler system. Now for eliminating corrosion, oxygen scavenging chemicals are generally mixed up into the boiler water. Finally, to reduce the possibility of scale build-up and corrosion inside a boiler, the regular checking of boiler water is required to be carried out.

The problem of scale build up is typically not encountered in hot water boilers. It doesn’t mean that the water fed inside a hydronic boiler is free of dissolved minerals. Actually the only difference is that the hydronic boiler systems are capable of discharging equal quantity of dissolved minerals as much is entering the system. Hence, the possibility of mineral accumulation inside the boiler gets totally eliminated. However, the hydronic boiler systems still face the problem of corrosion which can be solved via addition of oxygen scavenging chemicals only.

References

1. Aquastat
2. Domestic Hot Water Maker
3. Scale Build-up

Sources

Blurtit
ehow
Warm Air
CBS

They are also referred to as hydronic boilers. They are frequently applied in residential & commercial buildings for heating purposes. They are usually manufactured in small sized portable units for domestic applications whereas large sized units are put into use for industrial applications.

One can chose among variety of fuels such as propane, electricity etc. for hot water boilers operation but the most commonly used fuel source happens to be natural gas owing to its cost effectiveness. These boilers are extremely durable and offer long operational life. Besides, their usage involves less complication as compared to other heating systems. However, the installation procedure of hydronic boilers is quite expensive.

Just like steam boilers, hot water boilers also exist in two different configurations i.e. the fire tube boiler configuration and the water tube boiler configuration. The fire tube boilers are also referred to as shell type boilers owing to their construction.

Major components

Below mentioned are the key components used in the construction of hot water boilers:

1. Thermostat which is mainly employed for controlling heat and temperature inside the system.
2. A gas valve for regulating fuel gas flow
3. A pressure gauge which is basically used to indicate the pressure of the water in the boiler.
4. Water feed valve for adding water into the boiler system.
5. Pressure reduction valve for lowering pressure inside the system.
6. An air vent which allows the escape of redundant air from the system.
7. An expansion tank which enables expansion of water upon heating.
8. A flow control valve for controlling the flow of boiler water
9. A pressure relief valve for pressure control
10. A circulator which is usually a motorized electric pump used for circulation of water throughout the system.
11. A drain valve

Working

In a typical hot water boiler system, the fuel is introduced into a pressurized tank where combustion process takes place. A temperature controlling device called thermostat is included in the system which keeps a check on the fuel temperature. Within the pressurized tank, the water combined with a regulated quantity of air is supplied which initiates the fuel burning process. The products of combustion are then passed via pipe towards the cylinder which contains the water. Due to the heat supplied by the hot gases, the water inside the system gets heated. The resulting hot water is then finally distributed by means of an electric pump. The heated water is sent via another pipe to all the parts in the building requiring heat.

In hot water heating systems, the whole system is usually divided into different heating zones in a building. This zoning method offers following benefits:

1. It results in efficient heating.
2. It makes the living extremely comfortable.
3. It makes the boiler operation very simple.
4. It offers extremely cost-effective heating solution.

Radiant heating is one of the oldest techniques for generating hot water heating. It is usually applied owing to its highly efficient heating effect. With the advent of new technologies, radiant piping systems have been made extra robust as well as inexpensive. “Plastic tubing (which is a more economical choice than other piping materials) has made it conveniently possible for homeowners to have their floors, walls, driveways and pools heated hydronically.”¹ Distribution of heated water inside a hydronic boiler system can take place via following techniques:

1. Radiators
2. Baseboards
3. Convectors and
4. Vents, also referred to as hydro-air systems

Types of Hot Water Boiler Systems

There are four major types of hot water boiler systems available which are mentioned below:

1. Closed System: In these types of systems, the water which gets evaporated and converted into steam is again put back into use by condensing the steam back to liquid form. This means that the 100 percent reutilization of the water takes place in closed systems.
2. Open System: In these systems, the water is heated up but the evaporated water is not put back for reutilization inside the system.
3. Single-pipe System: In these systems, two pipes are utilized. One pipe is used for carrying the heated water to the required location whereas the second pipe is used for returning the cool water back towards the boiler by means of a motorized pump.
4. Gravity System: These are the older hot water boiling systems in which the water is carried back after heating via the gravity effect. Hence, in these systems, circulators are not required.

Main Features

Following are the significant features associated with the use of hot water boiler systems:

• Although the installation of hot water boiler systems is extremely costly yet they are frequently employed for heating applications owing to their high efficiency and cost in comparison to forced-air boiler systems.
• Since the hot water boiler systems are made up of aluminum fins and copper pipes, they tend to employ less metal for their construction and hence occupy comparatively smaller area.
• Hydronic boilers typically offer small and uniform values of temperature as compared to forced-air boilers. Moreover, in these systems, the on-off switching action is usually prevented because of the ability of baseboard pipes to grasp the heat and liberate it over longer periods of time.
• Another important feature regarding hot water boilers is that they dry the air inside the system to a very small extent only.
• “A Hydronic boiler does not allot allergens, dust, combustions or mold by any products that enter living space. This is always an advantage, especially for a family that is sensitive with allergies.”²
  
• The piping mechanism within a hot water boiler system is very exceptional and its correct implementation is very much responsible for the proper operation of the boiler.
• A single hydronic boiler system per home is usually found adequate to produce necessary heating effect, unless it’s a big triple story house.
• Hot water boilers are mainly employed for domestic heating applications, generally in the northern parts of Europe.
• Hydronic boiler systems are mainly applied as central heating system inside buildings to give heating effect to the areas, which are otherwise very cold.
• “A typical hot water boiler operates throughout the winter with a water temperature determined by the Aquastat, or low limit, setting (an Aquastat is like a thermostat that senses the water temperature inside the boiler and shuts off the burner when the water is hot enough).”³
  

Maintenance

It is very important to carry out periodic maintenance of a typical hot water boiler system; otherwise the boiler may burst out resulting in fires or explosions. The cylinder used inside the boiler system must be regularly drained and cleaned to avoid clogging of pipes because of surplus mineral deposits. This mineral build up takes place due to the nitrate components present in the water.

Oiling of pipes must be regularly done in order to provide lubrication to them. Also, the pipes must be carefully inspected at regular intervals for detecting leakage points. Moreover, the pressure gauge incorporated within the system needs to be cautiously checked for any type of irregularity in the pressure. To ensure proper boiler operation, one must get the boiler inspected and certified by a qualified boiler professional.

References

1. Plastic Tubing
2. Hydronic Boiler Advantages
3. Aquastat

Sources

Oilheat America
ezine articles
ehow
Faryal

Steam boilers are frequently employed in various industrial and domestic applications where saturated or superheated steam is required. Steam generated via steam boilers is put into use for heating, cooking and reboiling purposes, mainly because of its heat energy. On the other hand, the pressure energy associated with steam primarily finds its use in reciprocating steam engines and steam turbines.

Steam boilers can produce steam by utilizing heat from variety of sources such as burning of fuel, nuclear reactions, solar energy and heat dissipated from several other processes. A steam boiler should be designed in such a way that it can handle exceedingly high pressure steam generated at immensely high temperatures. In order to achieve this, special care must be taken regarding steam boiler construction. Following factors must be specifically considered while designing a steam boiler:

• Building materials
• Construction methods
• Mechanisms like consistent heat-up and cool-down

In residential applications, one-pipe steam systems are usually employed. In these systems, a temperature controlling device called thermostat is used which basically generates a signal of heat requirement. Upon receiving this signal, the steam boiler commences its operation and steam starts to spread out in the whole system. “As the steam expands, it displaces air contained in the pipes and radiators, and that air must be allowed to escape through air vents typically located at the top of each steam radiator.”¹

As soon as the steam arrives at the radiator, these air vents close up while the steam is locked inside. The heat energy from the steam is then transferred via radiators. After this, the steam gets condensed and returned back in the form of water towards the boiler. The water flows back due to gravity action via the same pipe, hence it is known as single pipe steam system. A typical single pipe steam heating system is shown in the figure² below.

Two-pipe steam systems consisting of a separate condensate-return pipe are also available but they are rarely put into use for residential purposes.

Types of Steam Boilers

In early days, the boiler configurations were very simple which included only iron tanks placed over a wood fire. However, nowadays, advanced boiler designs have been developed with following two major configurations:

1. Fire Tube Boiler Configuration
2. Water Tube Boiler Configuration

Both configurations mentioned above consist of a fire box inside which burning of fuel takes place via continuous introduction of fuel and air. The hot exhaust gases released from the fire box are then utilized for heating the water and converting it into steam. The steam generated is then passed via radiators to provide necessary heat.

Main Components

Following are the major components incorporated inside a steam boiler system:

• Steam valves: These valves are mounted upon radiators to allow the expulsion of air from the pipes and keeping the steam inside. However, in cases where steam starts escaping from the system, a white calcium layer is formed around the steam valves. Hence, this situation can be easily detected and corrected.
• Gauge glass: This device is basically used to indicate the level of water inside the boiler. Typically, the average water level is in the center of the gauge glass. But once the burner starts operating, the level of water changes to small extent.
• Low-water cutoff: According to code requirements, this device is generally used to stop the burner in situations where the level of water drops below the required water level limits.
• Automatic water feeder: This is an automatic device which is used for the purpose of feeding water into the boiler when the level of water decreases. Use of this device is optional and entirely depends upon the preference of the homeowner. However, a homeowner must always keep a periodic check on the water level in boiler even if an automatic water feed is employed.
• Pressure relief valve: This valve is used for safety purpose since it protects the boiler system from getting over pressurized.
• Steam limit control: This device is also referred to as pressuretrol. Its sole purpose is to cut off the burner as soon as the adequate steam pressure is build up.

Safety Controls

A steam boiler is capable of generating superheated steam having temperature more than 212 degrees F. However, this temperature can go beyond unsafe limits and result in fire breakout, if adequate safety controls are not incorporated within the boiler system. Hence, for safe and sound operation of steam boilers, following safety controls must always be provided:

• Limit switches: These are the safety switches which are basically included in the system to detect the increase of temperature beyond a certain limit. As soon as the temperature inside the boiler crosses the defined limits, a limit switch operates resulting in the shut down of the whole system. It is usually mounted near the draft diverters. It proves to be a useful device in preventing fire breakouts and plugging up of chimney or flue gases. It helps in avoiding carbon monoxide poisoning or any other dangerous situation related to temperature rise.
• Low water cut-off: This safety feature should always be incorporated in the boiler system for keeping a check on the level of water in the boiler. The normal level of water is in the half-way in the gauge/ sight glass which should always be maintained. For carrying out a regular visual inspection of water level, the sight glass indicating the level must be kept clean and free from dirt. Lower than average water level in the boiler may lead to undesirable results.
• Automatic water feeder: This device is generally employed for automatically feeding water into the boiler system whenever the water level drops below a certain point. Appropriate water level will result in generation of high quality steam heat. However, small adjustments are needed in the system to maintain correct water level i.e. neither too much high nor too much low. Since excess water will not heat up the system whereas scarcity of water may cause hazardous situation.
• Pressure cut-off control: Various devices such as pressure relief valves and blow-down valves etc. are employed in the boiler system for exercising control over excess steam pressures.

Major Problems

Following are the major problems associated with the use of steam boilers:

• Water Hammer or Banging Pipes: This is a very frequent problem which mainly takes place due to the mixing of condensed water with the steam. Owing to the cold nature of water and hot nature of steam, the pipes in the system tend to expand and contract very quickly. Because of this pipe action, a sound resembling to pipe beating with a hammer somewhere down in the basement is produced, hence it is named water hammer problem. The problem can be raised in the system when the condensed water flows back via the Hartford loop towards the boiler. “It could mean a strainer or trap is plugged with sediment and limiting the return of condensate water returning to the boiler.”³
  The automatic water feed employed in the system will continuously work to maintain the proper water level in the boiler. However, the water level in the system will become too high that the mains will overflow. Hence, to avoid this problem, it is necessary to maintain the efficiency of return water piping system. It is very difficult to totally eliminate this problem but it can be reduced to a great extent with the help of a professionally qualified HVAC consultant and use of appropriate devices.
• Leakage of Water via Steam Vents: In case of single pipe steam systems, steam radiators are usually not kept on level. Hence, when the floor tends to settle, the radiator results in leakage of water from the steam valve. To avoid this, a shim can be positioned beneath the radiator legs opposed to the pipe which will enable the condensed water discharged from the radiator to flow back towards the boiler via the return piping system. If the problem persists even after employing a shim in the system, an HVAC professional must be consulted.
• Radiators not heating: There can be various reasons lying behind this problem based upon the type of radiator employed in the system i.e. whether it is two-pipe or single-pipe radiator. To detect the cause of this problem, first of all one should verify that the valve is in open position at the bottom of radiator except in cases where a thermostatically controlled valve is used. In case of single pipe steam systems, the thermostatically controlled valve is located at the steam vent. On the other hand, in case of two-pipe systems, this valve is usually mounted upon the supply pipe approaching towards the radiator. If the thermostat is completely turned down the valve will close whereas if the thermostat is altogether turned up, the valve will accordingly open up. Even if after applying all these techniques, nothing solves the problem then HVAC professional needs to be consulted for advice.
• Total Dissolved Solids (TDS): This problem usually occurs in the boiler when a proper and regular blow-down of the boiler is not being performed. Blow-down always needs to be done at regular intervals and a pre-defined frequency. In a boiler system, when water converts into steam, the dissolved solids are left behind in the boilers. If these solids are not timely discharged, then they tend to build up and result in various problems. Besides, they affect the efficiency and safety of the boiler system. “This is something usually done by the homeowner or maintenance person (if it is a business) but should be done according to the advice of an HVAC professional as far as blow down frequency is concerned.”⁴
• “The build-up of carbonic acids can eat through hydronic piping and needs to be addressed by either venting oxygen and CO2 or having water treatment.”⁵
  

References

1. Steam Heating
2. Single-pipe Steam Heating
3. Water Hammer
4. TDS
5. Steam Boiler_Problems

Sources

ehow
Oilheat America
High-performance HVAC
Faryal

“External water treatment, as the term is applied to water prepared for use as boiler feed water, usually refers to the chemical and mechanical treatment of the water source.”¹
Boiler feed water is usually treated for impurities like hardness, silica, oxygen etc. before entering the boiler system. However, in some cases, the water source treatment outside the boiler system is not required and the treatment can be carried out within the boiler system only.

Evaporation

The basic method of water purification is referred to as evaporation. In this process, water is heated to a level that it transforms into vapors leaving impurities behind. These water vapors are then collected and condensed back to liquid state which is then used as boiler feed water. “Evaporators are of several different types, the simplest being a tank of water through which steam coils are passed to heat the water to the boiling point.”²

However, at times, two water tanks can also be utilized in order to boost the evaporation efficiency and generate extra heat. Then, the water vapors from the one tank will be transferred to the other tank via coils. Use of evaporators is mainly preferred in areas where steam can be easily applied as the required heating source.

In boiler systems, evaporation is often used as one of the pre-water treatment technique. Some other advanced water treatment techniques are also available which are mentioned in section below. Evaporation technique has its own benefits especially in cases where the quantity of dissolved solids in the raw water is too much.

External Treatment Methods

Following are the major techniques employed for external water treatment:

1. Clarification process
2. Filtration process
3. Softening action
4. De-alkalization action
5. De-mineralization technique
6. Deaeration method
7. Heating method

All the above mentioned techniques are helpful in purification and treatment of boiler feed water.

Also, several natural and artificial elements are available that are proficient in removing and replacing mineral ions present in the water source. “For example, in passing water through a simple cation exchange softener all of calcium and magnesium ions are removed and replaced with sodium ions.”³

Another valuable external water treatment process is called the hot lime-zeolite. In this method, the water for boiler use is first treated with lime to remove impurities like hardness, alkalinity and silica etc. and after that it is treated with a cation exchange softener. Since cation exchange process used alone can not remove the total solids present in the water, it is usually employed in combination with precipitation type softening for getting better water treatment results “This system of treatment accomplishes several functions: softening, alkalinity and silica reduction, some oxygen reduction, and removal of suspended matter and turbidity.”⁴

References

1. External Water Treatment
2. Evaporators
3. Cation Exchanger
4. Hot lime-zeolite process

Sources

Lenn Tech

CIP