Boiler Types Explained: How Do They Work?

Oscar Garner

By Oscar Garner

Last Updated on February, 2025

Boilers are integral to various industries and homes, providing essential heating and power through steam or hot water. However, not all boilers are created equal.

This article will explore the different types of boilers available, exploring their unique designs, operations, and applications. 

We will also discuss the pros and cons of the different types of boilers, helping you understand which boiler might be the best fit for your special needs.

Whether you’re considering a new boiler installation or simply want to learn more about these heating systems, this guide is your go-to!

Quick Summary

  • There are three main types of boilers: regular, combi, and system.
  • Boiler designs, operations, and uses all differ depending on the type.
  • It is important to consider factors such as heating needs, available space, and fuel options when choosing a boiler, and to ensure proper installation and maintenance for safety and efficiency.

What is a Boiler?

A boiler is a device designed to produce heat by steam or hot water for heating and powering various industrial, residential, and commercial buildings. Different types of boilers have different designs, operations, and uses.

A boiler is a pressurised heat exchanger consisting of water-filled tubes within a shell. It heats a fluid, typically water, but sometimes soil or other substances, using hot gases or air, converting it into steam for various purposes.

The steam generated by the boiler can serve as a driving force, act as a primary mover, or function as a heat source for systems downstream of the steam pipelines.

While the boiler is central to steam generation, it is just one larger steam generation plant component. The boiler must be integrated with other equipment, such as condensers, cascade tanks, preheaters, fuel oil lamps, and pressure-reducing valves, to operate efficiently.

Useful guides:

Most Common Types of Boilers

There are three common types of boilers: Regular, combi, and system.

Regular Boiler

a picture of a regular boiler

A regular or conventional boiler, also called a heat-only boiler, has powerful heating systems commonly found in many homes. These boilers are known for their robust heating capabilities and are among the largest types available. 

In addition to the boiler unit, a regular boiler requires a separate hot water cylinder and cold water tank. These boilers feature a firebox, where fuel is burned.

Hot gases pass through tubes immersed in water. The heated water is circulated to radiators or other heating appliances throughout the home. 

How it Works?

Conventional boiler systems use a heat exchanger to warm water sourced from a cold water tank, usually in the loft.

This heated water is then stored in a hot water cylinder, which remains directly available for distribution throughout the home’s central heating system or directly to taps.

Unlike other different types of boilers that rely on mains water pressure, conventional boilers rely on gravity to facilitate water flow from the mains supply into the storage tank. 

Pros of Regular/Conventional Boilers

  • Reliable heating: Regular boilers provide consistent and reliable heating performance, making them a popular choice for many homeowners.
  • Ample hot water: A conventional boiler can supply a large volume of hot water, making it suitable for homes with high hot water demands.
  • Proven technology: With a long history of use, the technology behind regular boilers is well-established and understood, making them easier to return and repair. 

Cons of Regular/Conventional Boilers

  • Size and space requirements: Regular boilers are typically larger and require additional space for the hot water cylinder and cold-water storage tank.
  • Less energy-efficient: Regular boilers can be less energy-efficient than newer boiler models, leading to higher energy bills. 
  • Installation complexity: Installing a conventional boiler, including the hot water cylinder and storage tank, can be more complex and time-consuming, potentially increasing installation costs. 

Combi Boilers

a picture of a combi boiler

A combi boiler, short for combination boiler, integrates both a high-efficiency water heater and a central heating boiler into a single unit. This compact heating system is often called a wall-mounted boiler due to its space-saving design.

Combi boilers are highly regarded for their energy efficiency and are well-suited for small spaces.

How it Works?

Combi boilers use mains water pressure to heat water on demand, eliminating the need for a separate water cylinder or tank. The instantaneous combustion process ensures a constant supply of hot water without the need for storage.

However, it’s important to note that combi boilers may struggle to meet high demand from multiple taps or showers simultaneously. 

Pros of Combi Boilers

  • Energy-Efficient: Combi boilers heat water on demand, reducing energy waste compared to traditional boilers.
  • Space-saving: Their compact design, which can heat water on demand, eliminates the need for a separate hot water cylinder, saving valuable space.
  • Quick heating: Instantaneous hot water delivery ensures you never run out of hot water.
  • Cost-effective: Reduced energy consumption can lead to lower utility bills over time.

Cons of Combi Boilers

  • Limited capacity: Combi boilers may struggle to meet high-demand scenarios, such as using multiple taps and showers simultaneously.
  • Pressure sensitivity: Performance can be affected by fluctuations in mains water pressure.
  • Complex installation: Wall-mounted installation may require professional expertise, adding to installation costs.

System Boilers

a picture of a system boiler

A system boiler consists of two main components: the boiler, typically in the kitchen or air cupboard, and a separate cylinder for storing hot water.

This setup allows for a constant hot water supply throughout the home, making them suitable for larger residences with higher heating and hot water needs. 

How System Boilers Work?

They function differently depending on whether they are gas-fired boilers or electric boilers.

A gas-fired system boiler heats water in an enclosed system using a flame, with heating coils in the water cylinder carrying central heating water through the cylinder to heat its contents, similar to a central heating boiler unit.

An electric-powered system boiler uses an electric heat exchanger to warm circulated water instead of a gas flame. Heat pumps pump water from the cylinder to taps and radiators in both cases.

Pros of System Boilers

  • Hot water on demand: They can simultaneously supply hot water to multiple taps, making them ideal for homes with high hot water demand or multiple bathrooms.
  • Good water pressure: Hot water comes directly from the mains, ensuring good water pressure throughout the home.
  • No loft tank: Unlike regular boilers, system boilers do not require a tank in the loft, making them suitable for homes without loft space or for those considering loft conversions.
  • Compatibility with solar energy: The separate hot water cylinder in system boilers allows for compatibility with solar panels or thermal systems, enhancing efficiency. 

Cons of System Boilers

  • Energy efficiency: While system boilers are efficient, they may not be as energy-efficient as combi boilers due to the need for pre-heating and storing water. 

Different Classification of Boilers

a picture of a watertube boiler

Based on Use

  • Main boiler: Generate steam for propulsion on ships or primary steam in industrial settings.
  • Auxiliary boiler: Provides various functions like cargo discharge, tank heating, and machinery support on ships or supplements main boilers in settings.

Based on Tube Contents

  • Water Tube Boiler: Water flows within tubes, surrounded by hot gases, and is ideal for high-pressure applications.
  • Fire Tube Boiler: Hot gases flow within tubes surrounded by water and are suitable for lower-pressure applications.

Based on Steam Pressure

  • Low-pressure steam boiler: Steam/hot air pressure up to 10 bar.
  • Medium-pressure steam boiler: Pressure between 10-25 bar.
  • High-pressure steam boiler: Steam pressures above 25 bar.

Based on Heating Medium

  • Steam boilers: Steam boilers are common, available at various pressures, and they provide heating by either saturated or superheated steam.
  • Thermal fluid boiler: Uses liquid fluids such as water, oil, or a water-glycol mixture for heat transfer.

Based on Furnace Location

  • Externally fired boiler: Furnace outside the shell.
  • Internally fired boiler: Combustion chamber within the shell.

Based on the Number of Tubes

  • Single-Tube: Contains one fire or water tube (also called fire tube boilers and water tube boilers).
  • Multi-tube: Contains multiple tubes for higher capacity for generating steam.

Based on Orientation

  • Horizontal boiler: Shell axis horizontal.
  • Vertical boiler: Shell axis vertical.
  • Inclined boiler: Shell axis aligned to the ground.

Based on Mobility

  • Stationary boiler: Fixed in place, common in large industrial settings like power plants.
  • Portable boiler: Movable, self-contained with wheels or manual carrying.

Based on Energy Source

  • Solid fuel boiler: This boiler uses fossil fuels like coal, wood, or biomass (called biomass boilers). However, carbon emissions are an issue, and it’s always best to stick to renewable energy sources. 
  • Oil-fired boilers: Runs by heating oil, diesel, or hybrid fuel.
  • Waste heat boiler: Utilises waste heat from exhaust gases, exhaust fumes, or industrial processes.
  • Electric boiler: An electric boiler generates heat from electricity.
  • Gas-fired boiler: This uses natural gas or propane.

Based on the Circulation Method

  • Natural Circulation Method: Water circulates naturally due to density difference.
  • Forced circulation boiler: Water circulation is forced by a pump for faster steam generation from the steam drum.

Based on Number of Passes

  • Single-Pass boiler: Flue gases pass through once.
  • Multi-pass boiler: Flue gases pass through multiple times for enhanced heat transfer 

Based on Reverse Chamber Design

  • Dryback boiler: Partially surrounded by water, prone to radiation losses.
  • Wetback boiler: Fully surrounded by water, minimizing heat losses and improving steam production.

Benefits of Using Boilers

  • Efficient heating: Boilers offer efficient and consistent heating throughout the home. 
  • Energy Savings: Modern boilers are designed to be energy-efficient, reducing heating costs.
  • Versatility: Different types of boilers cater to various heating needs and space requirements.
  • Reliability: Boilers are known for their reliability and longevity with proper maintenance. 

Risks of Using Boilers

  • Installation costs: Initial installation can be costly depending on the type and complexity.
  • Maintenance: Regular maintenance is essential to ensure efficiency and safety.
  • Space requirements: Some boiler types require additional space for a hot water tank, storage tanks, or cylinders.
  • Safety concerns: Improper installation or maintenance can pose safety risks, including gas leaks or carbon monoxide poisoning.

How to Choose the Right Boiler for Your Needs?

a repaired boiler

Choosing the right type of boiler involves assessing your heating needs, available space, and fuel options.

Assess needs: Determine hot water production quantity and available space.

Fuel options: Choose between natural gas, oil, propane, or electricity based on availability and efficiency.

Boiler Types: Different boiler types cater to different requirements, either for home use or industrial use etc.

Energy efficiency: Look for high-efficiency ratings.

Professional advice: Consult a heating engineer for personalised recommendations and professional boiler installation

Conclusion

Selecting the right boiler is a crucial decision that can significantly impact your home’s comfort, energy efficiency, and operating costs. Whether you opt for a combi, system, or regular one depends on your heating needs, available space, and fuel options. 

Prioritising energy efficiency and consulting with a heating engineer can help you make an informed choice that meets your requirements.

Investing in the right boiler ensures smooth heating performance, reduces energy consumption, and enhances overall comfort and well-being at home.

FAQs

Leave a Comment