Steam Boilers Vs. Hydronic Boilers

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.


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.


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.


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



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