A Simple Guide to Boilers, Radiators and Hot Water
A gas boiler uses energy from burning gas to heat your home and, in most systems, provide hot water.
For central heating, the boiler heats water that circulates through pipes and radiators around your home. As the radiators release heat into the rooms, the water cools and returns to the boiler.
The boiler then heats it again.
This creates a continuous cycle while your heating is running.
However, not every gas-boiler system works in exactly the same way. In particular, the way your home produces hot water for taps and showers depends on the type of boiler and heating system you have.
The Basic Central Heating Process
The process usually begins when your heating controls call for heat.
For example, your thermostat may detect that the room temperature has fallen below the temperature you have selected.
The boiler then starts operating.
Gas is burned inside the boiler, while a heat exchanger transfers the resulting heat into water circulating through the central-heating system.
A pump then moves the heated water through the pipework towards your radiators.
As the radiators warm up, they transfer heat into the rooms. Meanwhile, the water inside them gradually cools.
The cooler water then returns to the boiler, where it is heated again.
This process continues until the heating demand is satisfied.
Inside a Gas Boiler
Several components work together inside a modern gas boiler.
Although designs vary, understanding the main parts makes the overall process much easier to follow.
Burner
The burner is where gas is mixed with air and burned in a controlled way.
Modern boilers can often adjust their output rather than simply operating at full power all the time.
This is known as modulation.
As a result, the boiler can reduce its output when less heat is required, which can help it operate more efficiently.
Heat Exchanger
The heat exchanger is one of the most important components in the boiler.
It transfers heat produced by combustion into the water circulating through the heating system.
Importantly, the flame and heating water do not mix.
Instead, heat passes through the heat exchanger’s metal surfaces.
The heated water can then carry that energy around your home.
Pump
The pump circulates water through the central-heating system.
It sends heated water away from the boiler and towards the radiators. At the same time, cooler water returns through the pipework so that it can be heated again.
Without circulation, the heat produced by the boiler could not be distributed effectively around the home.
Flue
Burning gas produces combustion gases that need to leave the property safely.
The flue provides the route for these gases to leave the boiler and discharge outside.
Modern room-sealed boilers also obtain the air needed for combustion from outside rather than relying on air from the room.
The flue is therefore an important part of the boiler’s combustion and safety system.
Condensate Pipe
Modern gas boilers are normally condensing boilers.
As the boiler recovers additional heat from the combustion gases, water vapour can condense into liquid.
This liquid, known as condensate, needs to drain away.
Therefore, a condensing boiler normally has a condensate pipe connected to a suitable drain.
How the Water Travels Around Your Home
The central-heating system is essentially a circuit.
One pipe carries heated water away from the boiler, while another returns cooler water.
These are commonly known as the flow and return.
Flow
The flow carries heated water from the boiler into the heating system.
From there, the water travels through the pipework towards the radiators.
Radiators
Hot water passes through the radiators, heating their metal surfaces.
The radiators then transfer heat into the rooms.
As this happens, the water flowing through them loses some of its heat.
Return
The cooler water leaves the radiators and travels back towards the boiler through the return pipework.
The boiler then heats the returning water again.
Therefore, the central-heating water is continually circulated and reheated rather than being used once and thrown away.
Your Radiator Water Does Not Come Out of Your Taps
This is an important distinction.
The water circulating through your radiators is part of the central-heating circuit.
It is not normally the water that comes from your hot tap or shower.
The two systems have different purposes.
Heating circuit:
Carries heated water between the boiler and radiators.
Domestic hot water:
Provides water for taps, baths and showers.
How the boiler produces domestic hot water depends on the type of system installed in your home.
The Three Main Types of Gas Boiler
Gas-boiler systems in UK homes generally fall into three broad categories.
Understanding which type you have can make the rest of your heating system much easier to understand.
Combi Boilers
A combination boiler, usually called a combi boiler, provides central heating and heats domestic hot water when you need it.
When you turn on a hot tap, cold mains water enters the boiler.
The boiler then heats that water before it travels to the tap or shower.
Therefore, a combi boiler does not normally need a separate hot-water cylinder.
This can make the overall system relatively compact.
However, the hot-water performance depends partly on factors such as the boiler’s capacity and the available mains-water flow.
System Boilers
A system boiler provides central heating but normally works with a separate hot-water cylinder.
Instead of producing all hot water instantly at the tap, the system heats water for storage in the cylinder.
That stored hot water can then be supplied to taps and showers when required.
This arrangement can be useful in homes with greater hot-water demand.
However, once the stored hot water has been used, the cylinder needs time to heat again.
Conventional Boilers
A conventional boiler may also be called a regular, heat-only or traditional boiler.
These systems normally work with a hot-water cylinder and may also have water storage tanks, often located in the loft.
They are commonly found in older properties and heating systems.
Therefore, if you have a boiler, a hot-water cylinder and tanks in the loft, you may have a conventional system rather than a combi.
How a Combi Boiler Produces Hot Water
A combi boiler needs to recognise when somebody opens a hot tap.
When this happens, the boiler gives priority to producing domestic hot water.
Cold mains water passes through the boiler and is heated before travelling to the open tap or shower.
Once the tap is closed, the demand for hot water ends.
The boiler can then return to providing central heating when required.
This is why you can usually have hot water without storing a cylinder full of it.
How a Boiler Heats a Hot-Water Cylinder
With a system or conventional boiler, domestic hot water is normally stored in a cylinder.
However, the boiler water and the water you eventually use from the hot tap are still kept separate.
Typically, heated water from the boiler passes through a coil inside the cylinder.
Heat transfers through the coil and warms the stored domestic water surrounding it.
The boiler water then returns to the boiler.
Therefore, the boiler heats the stored water indirectly rather than mixing central-heating water with the water you use for washing and bathing.
What Does the Thermostat Do?
Your room thermostat helps control when the central heating is required.
Suppose you set it to 20°C.
If the measured room temperature falls below the target, the heating controls can call for heat.
The boiler then operates as required to supply heat to the heating system.
Once the target is reached, the demand can stop.
However, the thermostat is only one part of the control system.
Timers, programmers, thermostatic radiator valves and modern smart controls can also influence when and where heating is used.
Learn how to use your boiler and heating controls properly →
What Are Thermostatic Radiator Valves?
Many radiators have thermostatic radiator valves, commonly called TRVs.
These help regulate the temperature of individual rooms by controlling the flow of heating water through the radiator.
For example, you may want a living room warmer than a spare bedroom.
TRVs allow you to adjust rooms separately rather than heating every room to exactly the same level.
However, they do not normally control the boiler in the same way as the main room thermostat.
Instead, they help regulate individual radiators.
Why Does Boiler Pressure Matter?
Most modern sealed central-heating systems operate under pressure.
The pressure allows water to circulate correctly around the system.
Your boiler will usually have a pressure gauge or digital pressure reading.
If the pressure becomes too low or too high, the boiler may not operate correctly and may sometimes display a fault.
However, the correct pressure depends on the boiler and system.
Therefore, follow the manufacturer’s instructions rather than assuming one pressure setting applies to every boiler.
We will cover this in more detail in our guide to boiler pressure and radiators.
What Does a Condensing Boiler Do?
Modern gas boilers are designed to recover heat that older boilers allowed to escape through the flue.
They do this by cooling the combustion gases sufficiently for some of the water vapour within them to condense.
When this happens, additional heat can be recovered and transferred into the heating system.
As a result, less energy is wasted.
This is why modern boilers are called condensing boilers.
Why Return Temperature Matters
A condensing boiler can recover more heat when the water returning from the heating system is sufficiently cool.
Therefore, simply running a boiler at the highest possible flow temperature does not necessarily make it operate most efficiently.
A well-designed and properly controlled heating system may be able to keep the home comfortable using lower water temperatures for much of the time.
However, the appropriate settings depend on the property, heating system and weather.
Learn how to use your boiler and heating controls properly →
What Happens to the Combustion Gases?
Once heat has been extracted, the remaining combustion gases leave the boiler through the flue.
You may sometimes notice a visible white plume outside when a modern condensing boiler is operating.
This can be normal.
The plume is largely associated with water vapour in the flue gases becoming visible as it cools in the outside air.
However, anything unusual about your boiler, flue or combustion should be checked by an appropriately qualified professional.
How Does the Boiler Know Something Is Wrong?
Modern boilers contain several safety and monitoring systems.
Depending on the boiler, these can monitor things such as:
- Flame operation
- Water temperature
- System pressure
- Water circulation
- Combustion
- Flue operation
If the boiler detects certain problems, it may stop operating and display a fault code.
This is often called a lockout.
The shutdown is there to protect the boiler and, importantly, the people in the property.
Some simple faults may allow a user reset according to the manufacturer’s instructions.
However, repeatedly resetting a boiler without understanding why it is failing is not a solution.
Learn how to reset your boiler safely →
The Heating Cycle in Simple Steps
For central heating, the basic process can be summarised as:
- Your heating controls call for heat
- The boiler starts
- Gas burns inside the boiler
- The heat exchanger transfers heat into the heating water
- The pump circulates the heated water
- Hot water travels through the flow pipework
- Radiators release heat into the rooms
- Cooler water returns to the boiler
- The water is heated again
- The cycle continues until the heating demand is satisfied
Once you understand that loop, the basic operation of a gas central-heating system becomes much easier to picture.
Gas Safety
Gas boilers contain combustion equipment and should be treated accordingly.
Installation, servicing and gas-related repair work should be carried out by a Gas Safe registered engineer where the work legally requires one.
Do not remove the boiler casing or attempt to work on internal gas components yourself.
If you smell gas or suspect a gas leak, do not try to diagnose the problem by dismantling the boiler. Follow the appropriate gas-emergency procedure instead.
Likewise, a carbon monoxide alarm should never be treated as a boiler fault that can simply be reset.
In Short
A gas boiler converts energy from gas into useful heat.
For central heating, it:
Burns gas → transfers the heat to water → circulates that water through radiators → receives the cooler water back → heats it again.
Domestic hot water works differently depending on the system.
A combi boiler heats mains water when you need it, while system and conventional boilers normally heat water stored in a cylinder.
Modern condensing boilers also recover additional heat from their combustion gases, helping reduce wasted energy.
Most importantly, the water circulating through your radiators is not normally the same water that comes from your taps.
Understanding these basics makes it much easier to understand your heating controls, boiler pressure, radiators and hot-water system.
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