Your home may heat water when you turn on a tap, or heat it earlier and store it in a cylinder. Knowing which system you have makes it much easier to understand the controls.
Start With One Question
Ask:
Do I have a hot-water cylinder?
This is one of the quickest ways to start identifying your system.
If you do not have a cylinder, your water may be heated when you need it.
If you do have a cylinder, your hot water is normally heated and stored before you use it.
There are exceptions, but this is a useful starting point.
Hot Water on Demand
Some systems heat water only when you need it.
A combi boiler is the most common example.
When you open a hot tap:
Cold mains water → boiler → hot water → tap
So, there is normally no separate cylinder full of stored hot water.
Instead, the boiler heats the water as it passes through.
Stored Hot Water
Other systems heat water in advance and store it in a hot-water cylinder.
The cylinder may be heated by:
- a gas boiler
- an immersion heater
- an electric boiler
- a heat pump
- solar thermal
- more than one heat source
So, finding a cylinder tells you that water is being stored.
However, it does not tell you what is heating it.
That is the next thing to find out.
Boiler With a Cylinder
If you have both a boiler and hot-water cylinder, the boiler may heat the stored water.
This is common with system and conventional boilers.
The boiler normally heats water that passes through a coil inside the cylinder.
Heat then passes from the coil into the stored water.
So:
Boiler → cylinder → stored hot water → taps
The water flowing through the boiler does not normally mix with the water you use from the tap.
System Boilers
A system boiler normally works with a separate hot-water cylinder.
It heats your home and can also heat the water stored in the cylinder.
Unlike a combi boiler, it does not normally heat all your tap water instantly as you use it.
Therefore, once the stored hot water has been used, the cylinder needs time to heat again.
Conventional Boilers
A conventional boiler is also called a regular or heat-only boiler.
It commonly works with a hot-water cylinder.
You may also have one or more water tanks, often in the loft.
So, if you have:
Boiler + cylinder + loft tank
you may have a conventional system.
Immersion-Heated Water
A hot-water cylinder may also contain an electric immersion heater.
This is an electrical element that heats the stored water directly.
So:
Electricity → immersion heater → stored hot water → taps
In some homes, the immersion heater is the main source of hot water.
In others, it is a backup for a boiler or another heating system.
Learn more: Immersion Heaters Explained →
Why Have I Got a Boiler and Immersion Heater?
Because your cylinder can sometimes be heated in more than one way.
For example:
Gas boiler → normal hot-water heating
Immersion heater → electrical alternative or backup
So, finding an immersion switch does not necessarily mean that electricity normally heats your water.
Likewise, switching off the immersion does not necessarily mean you will lose your hot water.
Your boiler may still be heating the cylinder.
Electric Heating Without a Boiler
Some homes have no boiler at all.
For example, a property might have:
Storage heaters → room heating
Immersion heater → hot water
This is common in some all-electric homes.
Therefore, don’t assume that a hot-water cylinder must be connected to a boiler.
Learn more: Electric Heating Explained →
Heat Pumps
A heat pump can also provide hot water.
It normally heats water stored in a suitable cylinder.
So:
Heat pump → cylinder → stored hot water → taps
The system may also contain an immersion heater for backup or other purposes.
Heat pumps work differently from standard electric heating. Therefore, we explain them separately.
Learn more: Heat Pumps →
Solar Thermal
Solar thermal panels use energy from sunlight to help heat water.
They normally work with a hot-water cylinder.
However, solar energy may not always provide all the hot water you need.
So, the cylinder may also be heated by a boiler, immersion heater, heat pump or another source.
Learn more: Solar Thermal →
What Type of Cylinder Do You Have?
If you have stored hot water, you may have a vented or unvented cylinder.
Vented Cylinder
A vented cylinder normally receives water from a cold-water storage tank, often in the loft.
So, if you have a cylinder and a water tank above it, you may have a vented system.
Unvented Cylinder
An unvented cylinder is normally supplied directly from the mains.
It does not need the same cold-water storage tank in the loft.
Unvented cylinders contain important pressure and temperature safety controls.
So, don’t alter or interfere with their safety devices.
How Do I Identify My System?
Start by looking around your home.
Boiler but no cylinder
You may have a combi boiler.
Boiler and cylinder
You may have a system or conventional boiler with stored hot water.
Boiler, cylinder and loft tank
You may have a conventional system.
Cylinder but no boiler
Your water may be heated by immersion heaters, a heat pump or another source.
Heat pump and cylinder
The heat pump may provide most of your hot water.
Cylinder with several heat sources
You may have a system that combines technologies.
For example, a cylinder might be heated by solar thermal and a boiler.
Check the Controls
Once you have found the main equipment, look at the controls.
You may find:
- a hot-water timer
- a programmer
- a cylinder thermostat
- immersion-heater switches
- a Boost control
- heat-pump controls
- boiler controls
Don’t change controls simply because you are unsure what they do.
Instead, identify them first.
Look for labels, maker names and model numbers. If necessary, check the instructions for the equipment.
Why Have I Run Out of Hot Water?
If you have a cylinder, it stores a limited amount of hot water.
So, several showers or a bath can use the stored supply faster than the system heats more.
You then need to wait for the cylinder to heat again.
A combi boiler works differently because it heats water as you use it.
However, a combi can only heat water at a certain rate. So, using several hot-water outlets at once can affect performance.
Why Is There Still Hot Water When Heating Is Off?
If you have a cylinder, the water may simply still be hot from an earlier heating period.
Alternatively, another heat source may be working.
For example, an immersion heater could be heating the cylinder even though the boiler is off.
So, hot water at the tap does not necessarily mean your boiler is currently heating it.
Hot Water and Your Energy Bill
Once you know what heats your water, you can start looking at what it costs.
For example:
Gas boiler → check gas use and tariff
Immersion heater → check electricity use and tariff
Off-peak immersion → check when the cheaper rate applies
Heat pump → understand how the whole system is designed to operate
So, identify the system before trying to reduce its running costs.
Otherwise, you may change the wrong control.
Know Your System
Before you have a problem, find out:
- What heats my water?
- Do I have a hot-water cylinder?
- Do I have an immersion heater?
- Where are the controls?
- Does more than one system heat the water?
Take a photo of the equipment labels if that helps.
Then, if something goes wrong, you already know what equipment you have.
Key Points
Hot-water systems broadly fall into two groups:
Water heated when needed
or
Water heated and stored for later
A combi boiler normally heats water when you use it.
Meanwhile, system and conventional boilers normally work with a cylinder.
A cylinder can also be heated by an immersion heater, heat pump, solar thermal system or more than one heat source.
So, start by finding out whether you have a cylinder.
Then, identify what actually heats the water inside it.
Once you know those two things, the rest of your hot-water system becomes much easier to understand.
Next: How to Use Your Boiler and Heating Controls →