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Boiler Types
Boiler Types - There are many different types of boilers in the boiler room today in a variety of heating applications. There are two main categories of boilers among the different boilers and those two categories are steam boilers and hot water boilers. Either of those categories can be fueled by oil, gas, or electric (although electric is uncommon for steam boilers).

Boiler Types

Boilers have different designs and piping configurations. A steam boiler system is designed to turn the water into steam and uses gravity and pressure to deliver the heat. A hot water boiler is designed to simply make hot water that is circulated (by a circulator pump) through a piping system to provide heat. Typically, hot water boilers are more efficient than steam boilers for a few reasons. First, there is less heat loss throughout the hot water piping and the shell of the boiler because the hot water boiler operates at a lower temperature than the steam boiler. That means there is less heat loss throughout the entire boiler and piping system. Secondly, because the hot water boiler operates at a lower temperature, it requires less fuel or energy to convert into heat. What kind of boiler do you have in your boiler room?

Boiler Types - Water Tube Boilers

Watertube boilers have many different tubes inside of it that have water circulating through them. Hot combustion gases surround these tubes, and an exchange of heat happens. The heat exchange is between the combustion gases and the tubes and water. The water tube boilers can generally are for higher capacities and pressures than the fire tube boilers because of the steam and hot water confines in the tubes.

Watertube Boiler sizes range from 10 million BTU/h boilers all way up to 300 million BTU/h. These boilers are generally in medium to large commercial/industrial applications. They can be either steam or hot water boilers in low to high-pressure boiler applications. These boilers can be either an oil boiler, coal boiler, or gas-fired boiler. Also, they pass hot flue gases around tubes filled with water. Boiler combustion efficiency depends on several factors for these boilers. These include whether it is a steam boiler or hot water boiler, combustion controls, flue dampers. Additionally, the frequency of tune-ups and/or air and/or water pre-heaters (boiler economizers) can determine efficiency.

Boiler Types - Fire Tube Boilers

Firetube boilers confine the combustion process and gases in tubes, and water circulates these tubes. Some fire tube boilers have turbulators inside of the tubes to cause turbulence of the flue gases. That increases the heat absorption into the water, which makes the boiler more efficient.

Related Link: How important is boiler maintenance? Check out our boiler maintenance article here.

HVAC Safety Cleaning Boiler Tubes

A large water tube boiler with the tubes being cleaned

Firetube Boilers range in sizes from .6 million BTU/h up to 50 million BTU/h. These boilers use hot flue gases passing through tubes submerged in water. They are generally for medium to large boiler commercial/industrial applications. These boilers can be either steam or hot water boilers in low to medium pressure applications. Again as with the water tube boiler, combustion efficiencies depend on several factors, as noted above. There are various names applied to different fire tube boilers. These names include Scotch Marine, locomotive, firebox, and vertical or horizontal return tube.

Condensing Boilers

Between steam and hot water and water tube and fire tube boilers, there exist conventional atmospheric boilers and condensing boilers. The condensing boiler is far more efficient than the conventional atmospheric boiler. A condensing boiler typically has two heat exchangers and absorbs more heat from the flue gases. It absorbs so much heat from the gases that moisture in the flue gases condenses. This condensation needs a drain to drain off. This condensed liquid is highly corrosive. The manufacturer must build the condensing boiler out of special materials. That prevents corrosion of the equipment.

Condensing Boilers Venting

condensing gas furnace

The condensing gas furnace works much the same way a condensing boiler works.

Typically, the flue is PVC pipe or stainless steel and is resistant to the corrosive effects of the condensation.
Additionally, because so much heat leaches out from the flue gases, the products of the combustion process need help to be safely vented. Usually, a fan is used to either push or pull these gases out of a vent. Condensing boilers are high efficiency at 90 plus AFUE ratings, whereas the conventional atmospheric boilers rate at around 80 percent plus AFUE. Conventional atmospheric boilers do not use a blower motor to remove the combustion byproduct gases.

Instead, they rely on the combustion gases to have enough heat to cause the gases to rise through the flue. The flue channels the gases safely outside the dwelling. If for some reason, too much heat leaches from the flue gases, condensation can occur. Lack of heat causes condensation inside the flue or chimney. This condensation can have corrosive effects on both the flue and the chimney and cause severe problems.

Boiler Types | Maintenance

A qualified HVAC technician must inspect the boiler. That they check the temperature of the flue gases. Using a combustion analyzer, a qualified technician can tune the boiler. That is done to make sure that the temperature of the flue gas settings is optimum levels. That is important so that the gases will properly vent. It is also important that the gases do not have too much heat in them. If the flue gases are too hot, then you are losing efficiency up the flue. If the flue gases do not have enough heat in them, then you can have condensation issues. Condensation issues cause corrosion.

Boiler Types | Conclusion

Have your boiler checked and inspected at least annually to maintain optimum efficiency. Electric Boiler One other type not mentioned above is the electric boiler. One could say that an electric water heater is an electric boiler. Although there are electric boilers that can heat water to steam temperatures. Boilers can use many types of fuels (oil, gas, coal, wood, and electric power). Boilers are mainly used to heat water, but they also heat other liquids. The main focus of Boiler Types article was to categorize the mechanical aspects and differences.High Performance HVAC

Boiler Types

Questions from our readers:

What is the best type of boiler?
This depends on the application and the desired efficiency. Most residential applications where the customer wants high efficiency will want a condensing boiler. Condensing boilers have efficiency levels above 95% AFUE.
Is a boiler a furnace?

Technically, no. A boiler heats water for heating purposes. The water is then circulated throughout a piping loop and through convectors. A furnace heats air for heating purposes. The heated air is channeled through ducts to heat the desired space.

How long do boilers last?

Depending on the type of boiler, a boiler should last at least 15 or 16 years. As a technician, I’ve seen boilers still in use that were over 30 years old. One particular customer I visited had a boiler that was 80 years old. He resided in a historic district, and the old boiler was very reliable for him.


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