Hot Water Systems

There are many methods of generating and storing hot water. A few of the most common are listed below.

Hot Water Cylinders

The Cylinder, usually found in an airing cupboard is your home’s store of hot water. Heated either through your boiler, an immersion heater or linked to solar thermal panels, water gets stored here ready for use.

The main benefits are:

  • Potential for higher water flow rates from your taps
  • The ability to run several taps at the same time
  • Pumped shower capability depending on your choice of cylinder
  • Hot water can get to the taps quicker, you don’t have to wait for the water to ‘heat up’ as with a ‘Combi’ boiler
  • The option of immersion heater back-up, if your boiler breaks down you will lose your Central Heating but still be able to have a hot shower!

Types of hot water cylinders

There are two common types of hot water cylinder

Vented Hot Water Cylinders

A vented hot water cylinder requires a store of cold water, usually in the loft that feeds the hot water system. The pressure is provided through gravity, so the height of the cold-water store above the hot water cylinder creates the momentum that pushes that water out of your taps.

The benefits for this type of system are:

  • Easy to install
  • Low maintenance
  • Simple mechanism

The potential drawbacks are:

  • You have to have a cold water store, usually in the loft
  • Hot water flow rates may be low especially if pipes are not adequately sized
  • Shower pump required to provide power-shower.

Unvented Hot Water Cylinders

Whilst vented hot water cylinders need a store of cold water, unvented cylinders get their water directly from the water mains.

The mains water supply fills and feeds the hot water cylinder, this water is then heated, using either your boiler, or an immersion heater. Then when you need hot water it releases its store of hot water to your taps.

The Benefits are:

  • No need for a store of cold water
  • Higher water pressure
  • Power showers without the need for a pump
  • Ability to be easily sited anywhere in the property


  • Pressure and flow rate dependent on incoming water main
  • Additional safety controls required
  • Maintenance required

Thermal store

What is a thermal store?

A thermal store is a way of storing and managing heat until it is needed to provide space heating and mains pressure hot water.

In a domestic setting, heated water is usually stored in a large well-insulated cylinder. A thermal store may contain one or more heat exchangers, usually in the form of internal coiled pipes or external flat-plate heat exchangers. Cold mains pressure water passes through the heat exchanger when a tap is opened. The stored heat is transferred providing mains pressure hot water.

As thermal stores don’t require additional safety controls (as the hot water is not stored under pressure), they can be sited away from external walls, making them particularly useful in communal living environments.

Combination boilers

(also known as “combi boilers”) have become more and more popular in the UK over the past few years, with over 70% of homes choosing to install a combi boiler, over regular hot water or system boilers.  

What is a Combi Boiler?

Combi boilers are essentially high efficiency water heater and central heating units, which means that they can heat your home and provide hot water, without the need for a separate water tank. They tend to be more compact than other central heating systems, as they combine these two household functions into one appliance (hence the name “combi boiler”).

Due to their small size and ability to heat water on demand, combi boilers are most suited to homes with limited space, or for couples or small families. It is possible to purchase an oil-fired combi boiler, however, gas and LPG models are more common.

The benefits of a combi boiler

More Efficient

Combi boilers are highly efficient when compared to older boiler models.


Combi boilers are far more compact than traditional heating systems, which means that installations tend to be cheaper. It is also easier to find replacement parts if a problem occurs.


With no water tank to heat up, combi boilers can provide an unlimited supply of hot water on demand. No waiting time!

More Compact

As there is no need for a separate water tank, combi boiler systems take up much less space than conventional heating systems. This makes them ideal for homes that have limited room.


The cold water from a combi boiler system is always safe to drink as it is fed directly from the mains. However, this is not always true of traditional systems, which may store cold water in a tank before it is piped through the house.

Possible disadvantages of a combi boiler

Dependence on Mains Pressure

To ensure that your combi boiler is working properly, you need good mains pressure and flow rate. If you are having a new boiler fitted, PHP Services will check that the mains pressure at your property is adequate before advising the installation.

Limited Use

With a combi boiler it is not always possible to run more than one shower or bath at a time, or to turn on the hot tap while showering. This may be an issue if you live in a large house with a number of bathrooms and en suites, or with lots of people.

No Immersion Heater

As there is no water cylinder, if your combi boiler breaks down then you will be left without hot water as well as central heating.

Contact PHP Services to discuss the best option for your hot water supply.