A Simple Guide to How Your Home Produces Hot Water
You turn on a hot tap and, a few moments later, hot water appears.
However, what happens before the water reaches the tap depends on the heating system in your home.
Some homes heat water only when it is needed. Others heat it in advance and store it inside a hot-water cylinder.
The heat itself might come from:
- A gas boiler
- An immersion heater
- An electric boiler
- A heat pump
- Solar thermal panels
- More than one heat source
Understanding your system can help you use hot water more efficiently, choose appropriate controls and recognise when something is not working properly.
Most importantly, it helps answer a simple question:
What actually heats the water coming out of my tap?
Two Main Ways of Providing Hot Water
Most domestic hot-water systems work in one of two broad ways.
Instantaneous Hot Water
The water is heated when you need it.
A combi boiler is a common example.
You open a hot tap, cold mains water enters the boiler, and the boiler heats it before sending it to the tap.
Therefore, you do not normally store a cylinder full of hot water.
Stored Hot Water
The water is heated in advance and kept inside a hot-water cylinder until you need it.
The cylinder might be heated by a boiler, immersion heater, heat pump or another heat source.
Therefore, when you open a hot tap, you are drawing from the hot water that has already been stored.
Understanding which of these systems you have is the first step towards understanding your hot water.
How a Combi Boiler Produces Hot Water
A combination boiler, usually called a combi boiler, provides central heating and hot water from one appliance.
When you open a hot tap, the boiler detects the demand.
Cold mains water enters the boiler and is heated before travelling to the tap or shower.
As a result, there is normally no separate hot-water cylinder.
This has several advantages.
For example:
- Hot water does not need to be stored
- You do not need to schedule a cylinder to heat
- There is no stored supply to run out in the traditional sense
- Less space is needed for hot-water storage
However, a combi boiler can only heat water at a certain rate.
Therefore, hot-water performance depends partly on the boiler’s output and the flow of mains water into the property.
If several outlets demand hot water at the same time, the available flow may be divided between them.
How a Hot-Water Cylinder Works
A hot-water cylinder stores water so that it is ready when you need it.
You may find the cylinder in:
- An airing cupboard
- A utility room
- A plant room
- Another cupboard or storage area
Modern cylinders are normally insulated to reduce heat loss.
However, stored water will gradually lose some heat over time. Therefore, the system may occasionally need to heat it again even when nobody has used hot water.
How Does a Gas Boiler Heat a Cylinder?
System and conventional gas boilers commonly work with hot-water cylinders.
However, the water circulating through the boiler does not normally mix with the water coming from your taps.
Instead, boiler water usually passes through a coil inside the cylinder.
The coil becomes hot and transfers heat into the stored domestic water surrounding it.
The boiler water then returns to the boiler.
Therefore:
Boiler water heats the cylinder → stored domestic water heats up → domestic water goes to your taps.
The two bodies of water remain separate.
What Is an Immersion Heater?
Many hot-water cylinders contain an immersion heater.
An immersion heater is an electrical heating element installed inside the cylinder.
It works rather like the heating element inside an electric kettle.
When electricity passes through the element, it becomes hot and transfers heat directly into the surrounding water.
This means a cylinder may be able to produce hot water without the gas boiler operating.
That can be particularly useful as a backup.
However, some homes use immersion heaters as their main source of hot water rather than as backup heating.
We will explain these in detail in our separate guide:
Immersion Heaters Explained →
Why Might a Cylinder Have Two Immersion Heaters?
Some hot-water cylinders have two immersion heaters.
This is particularly common in homes designed around off-peak electricity.
Typically, one element heats a larger volume of water, while another can provide a smaller amount of additional hot water when required.
The exact arrangement varies.
Therefore, don’t assume that two switches beside a cylinder perform exactly the same job.
If you are unsure, check the system instructions or have the controls identified.
What Is a System Boiler?
A system boiler provides central heating and normally heats water stored in a separate cylinder.
Unlike a combi boiler, it does not usually provide all domestic hot water instantly as you open the tap.
Instead, the boiler heats the cylinder according to the hot-water controls.
This arrangement can provide a substantial stored supply of hot water, which can be useful in homes with several bathrooms or higher simultaneous demand.
However, once the stored hot water has been used, the system needs time to heat more.
What Is a Conventional Boiler?
A conventional boiler may also be described as a regular or heat-only boiler.
Like a system boiler, it commonly works with a hot-water cylinder.
However, some conventional systems also use water storage tanks, often located in the loft.
These systems are particularly common in older properties.
Therefore, if you have a boiler, cylinder and tanks in the loft, you may have a conventional heating and hot-water system.
What About an Electric Boiler?
An electric boiler uses electricity rather than burning gas.
Depending on the system, it can heat water for central heating and may also work with a hot-water cylinder.
The basic principle of circulating heated water can therefore look similar to a gas central-heating system.
However, the source of the heat is different.
Instead of burning gas, electrical energy is converted into heat.
We will cover electric boilers as part of our guide to electric heating.
What About Homes Without a Boiler?
You do not need a gas or electric boiler to have stored hot water.
For example, an electrically heated home may have a hot-water cylinder heated entirely by immersion heaters.
In this type of property:
Electricity → immersion heater → stored hot water → taps and showers.
This is particularly important in homes using storage heaters, where there may be no central-heating boiler at all.
Therefore, finding a hot-water cylinder does not automatically mean you have a boiler.
How Does a Heat Pump Produce Hot Water?
A heat pump can also heat domestic hot water.
Typically, the heat pump transfers heat into water stored in a suitable hot-water cylinder.
However, heat pumps generally operate differently from traditional gas boilers.
They are designed to move heat efficiently rather than create it through combustion.
The system, cylinder and controls therefore need to be designed to work together correctly.
Some systems also use an electrical immersion heater or another heat source when required.
We cover heat pumps separately within Renewables, where we can explain the technology properly without duplicating it here.
What About Solar Thermal?
Solar thermal panels can use energy from sunlight to help heat domestic water.
Typically, the system transfers solar heat into a hot-water cylinder.
However, solar energy may not always provide all the hot water the household requires.
Therefore, another heat source, such as a boiler, immersion heater or heat pump, may also be connected to the system.
Solar thermal can therefore contribute to hot-water heating rather than necessarily replacing the main heat source completely.
We cover solar thermal in more detail within Renewables.
Vented and Unvented Hot-Water Cylinders
Not all hot-water cylinders work in the same way.
Two broad types are vented and unvented cylinders.
Vented Cylinders
A traditional vented cylinder normally receives water from a cold-water storage tank, often located in the loft.
Therefore, the hot-water pressure can depend partly on the height of the storage tank above the outlets.
These systems are common in older properties.
Unvented Cylinders
An unvented cylinder is supplied directly from the mains water supply.
As a result, it can often provide hot water at higher pressure without needing a cold-water storage tank in the loft.
However, unvented cylinders contain important pressure and temperature safety controls.
Therefore, installation and certain work on them require appropriately qualified people.
Do not interfere with their safety devices.
How Do I Know Which Hot-Water System I Have?
Start by looking at the equipment in your home.
If You Have a Boiler but No Hot-Water Cylinder
You probably have a combi boiler, although other arrangements are possible.
When you open a hot tap, the boiler should normally heat the water as required.
If You Have a Boiler and a Hot-Water Cylinder
You probably have a system or conventional boiler arrangement.
The boiler may heat the stored water according to a timer, programmer or other controls.
If You Have a Cylinder but No Boiler
Your hot water may be heated electrically using one or more immersion heaters, although another heat source could also be present.
If You Have a Cylinder and a Heat Pump
The heat pump may be your main source of domestic hot-water heating, potentially supported by an immersion heater.
If you are still unsure, look for the manufacturer’s information on the equipment or ask a suitably qualified professional to identify the system.
Why Do I Have an Immersion Switch If I Have a Gas Boiler?
This is a very common question.
A cylinder heated by a gas boiler may also contain an electric immersion heater.
The immersion can provide an alternative way of heating the water.
For example, if the boiler is unavailable, the immersion heater may still allow you to produce hot water.
However, that does not necessarily mean the immersion should be switched on all the time.
If your boiler normally heats the cylinder, accidentally leaving an immersion heater operating may use electricity unnecessarily.
Therefore, understand what the switch controls before changing it.
Why Do I Have Hot Water When My Boiler Is Off?
There are several possible explanations.
If you have a cylinder, the water may simply still be hot from an earlier heating period.
Alternatively, an immersion heater may be heating the cylinder electrically.
Some systems may also have another heat source.
Therefore, hot water at the tap does not always prove that the boiler is currently operating.
Stored hot water can remain usable for many hours, depending on the cylinder, temperature and household use.
Why Have I Run Out of Hot Water?
If you have a cylinder, the amount of hot water available is limited by factors such as:
- Cylinder capacity
- Water temperature
- How much hot water has already been used
- How quickly the system reheats the cylinder
- Shower and tap flow rates
Therefore, several showers or baths in quick succession can use the stored supply faster than the system can replace it.
Once that happens, you need to wait for the water to heat again.
A combi boiler works differently because it heats water on demand rather than relying on a stored cylinder.
Does a Bigger Cylinder Mean More Hot Water?
Generally, a larger cylinder can store more hot water.
However, size is not the only consideration.
A larger volume of water may also take longer or require more energy to heat.
Therefore, cylinders should be appropriately sized for the household and heating system rather than simply being as large as possible.
Should You Heat Hot Water All Day?
Not necessarily.
If you have stored hot water, the best heating schedule depends on:
- Household routine
- Cylinder size
- Insulation
- Heat source
- Tariff
- Hot-water demand
For example, there may be little reason to continually reheat a cylinder when nobody is going to use hot water for many hours.
However, modern insulated cylinders can retain heat well, and the best strategy depends on the particular system.
Therefore, avoid universal rules such as “always leave it on” or “always heat it only once a day.”
Instead, match the controls to the household and system.
Hot Water and Off-Peak Electricity
Timing can be particularly important in electrically heated homes.
If you have an electricity tariff with cheaper off-peak periods, the hot-water system may be designed to heat most of the cylinder during those cheaper hours.
For example, an immersion heater may operate overnight and store hot water for use the following day.
A second heater may then provide a boost if the stored water runs out.
Therefore, understanding your electricity tariff is an important part of understanding your hot-water system.
Heating a large cylinder at an expensive time may cost considerably more than heating it during an available cheaper period.
What Does the Boost Switch Do?
Some electrically heated hot-water systems have a boost control.
This allows additional water to be heated when the normal stored supply is insufficient.
For example, you might use the boost after several showers have used most of the hot water.
However, boost electricity may be charged differently from the main off-peak heating period, depending on the tariff and wiring arrangement.
Therefore, understand your tariff and controls rather than assuming every immersion heater uses the same electricity price at every time.
How Hot Should Stored Water Be?
Stored hot water needs to be managed carefully.
Setting the temperature unnecessarily high can increase heat loss and the risk of scalding.
However, setting stored-water temperatures too low can create hygiene concerns, including conditions that may allow Legionella bacteria to multiply.
Therefore, hot-water cylinder temperatures should not simply be turned down as far as possible to save energy.
Follow the guidance for your particular system and seek professional advice if you are unsure about safe temperature settings.
Why Is the Water at My Tap Cooler Than the Cylinder?
Some systems use a thermostatic mixing valve to blend hot and cold water before it reaches certain outlets.
This allows water to be stored at an appropriate temperature while reducing the temperature delivered at the tap.
Therefore, the temperature of the stored water does not necessarily equal the temperature you experience at the outlet.
Mixing valves can be particularly important for reducing scalding risk.
Don’t Confuse Hot-Water Temperature With Boiler Flow Temperature
This is another important distinction.
The temperature used for your central-heating radiators is not necessarily the same as the temperature required for stored domestic hot water.
Therefore, advice about lowering a condensing boiler’s heating flow temperature should not simply be applied to a hot-water cylinder.
The two settings perform different jobs and have different considerations.
Learn how to use your boiler and heating controls properly →
How Can You Reduce Hot-Water Energy Use?
You do not necessarily need to make the water cold or stop using it.
Instead, reduce unnecessary consumption and heat loss.
For example:
- Take shorter showers where practical
- Repair dripping hot taps
- Avoid running hot water unnecessarily
- Use appropriate hot-water schedules
- Insulate older cylinders and accessible hot-water pipes where appropriate
- Use off-peak electricity effectively if your system and tariff allow it
- Avoid repeatedly heating more water than the household needs
Small improvements can add up because water heating is a regular household energy demand.
A Dripping Hot Tap Can Waste More Than Water
A dripping cold tap wastes water.
However, a dripping hot tap can waste both water and energy.
Every litre of heated water that disappears down the drain has already required energy to heat it.
Therefore, repairing hot-water leaks can reduce both water consumption and energy use.
What If My Hot Water Suddenly Stops?
Start by identifying what type of system you have.
If you have a combi boiler, check whether:
- The boiler has power
- A fault code is displayed
- The gas supply is available
- The boiler is otherwise operating
If you have a hot-water cylinder, check whether:
- The boiler or other heat source is operating
- The hot-water controls are correctly scheduled
- An immersion heater is available where appropriate
- A circuit or control has been switched off
However, don’t remove covers or work on internal electrical, gas or pressurised components yourself.
If basic user checks do not identify the problem, seek appropriate professional help.
Who Is Responsible for the Hot-Water Equipment?
In an owner-occupied property, the homeowner is generally responsible for the domestic heating and hot-water equipment within the property.
However, rented properties have different responsibilities between landlord and tenant.
Likewise, responsibility for meters, network equipment and utility supplies is separate from responsibility for your internal hot-water system.
We explain those boundaries in our separate guide:
Learn who is responsible for your energy supply →
Know Your System Before Something Goes Wrong
One of the best times to understand your hot-water system is before you have a problem.
Find out:
- What heats your water
- Whether you have a cylinder
- Whether you have an immersion heater
- Which switches control it
- Where the hot-water controls are
- Whether you have off-peak electricity
- How to identify the equipment
- Who you would contact if something failed
Then, if the hot water stops, you already know where to begin.
Your Hot-Water System in Simple Terms
You can think of the most common systems like this:
Combi boiler
Cold mains water → boiler heats it when required → hot tap
Gas boiler with cylinder
Boiler → heats cylinder → stored hot water → hot tap
Electric immersion system
Electricity → immersion heater → stored hot water → hot tap
Heat pump system
Heat pump → heats cylinder → stored hot water → hot tap
Solar thermal system
Solar energy → contributes heat to cylinder → stored hot water → hot tap
Some homes combine more than one of these.
In Short
Hot water can be produced in several different ways.
The most important distinction is whether your home heats water when you need it or heats and stores it in advance.
A combi boiler normally produces hot water on demand.
Meanwhile, system and conventional boilers generally work with a hot-water cylinder.
Immersion heaters use electricity to heat water inside a cylinder, while heat pumps and solar thermal systems can also contribute to hot-water production.
If you have stored hot water, when and how you heat it matters.
This is particularly important when electricity prices vary according to the time of day.
Most importantly, learn what equipment you already have.
Once you know what heats your water, where it is stored and what controls it, managing your hot-water system becomes much easier.
Energility
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