Central Heating Systems

Here is an introduction to the various types of domestic central heating system that we install, suitable for every home - from a one bedroom flat to a large family home, or a multi-storey property. 

Traditional central heating systems
This is typical of the most of the systems in the UK. It is generally suitable for average to larger homes where hot water is needed at more than one tap at a time, such as families of three or more whose homes have more than one bathroom / shower room. When fitted with fast recovery/high efficiency hot water cylinders, this type of system will be almost as efficient as a combination boiler but will serve several taps at a time.
A traditional central heating system comprises a cold water storage tank (normally in the loft), a hot water storage cylinder and a separate boiler (often installed in the kitchen).
The advantages of this system are:

  • Hot water to several taps at the same time
  • Greater volume of water, so a bath fills quickly
  • When used in conjunction with a fast recovery hot water cylinder, hot water can be available almost 24/7 and efficiently
The disadvantages of this system are:
  • If you require a powerful shower, a mechanical pump is needed to increase the shower pressure
  • It takes up more room i.e. in the airing cupboard and loft
  • There is a risk of freezing pipes in the loft
Unvented hot water system
Also known as a pressurised or mains pressure hot water system. Unvented hot water systems are ideal where high performance is required or where there is no or limited loft space for a conventional cold water storage tank.
When an unvented hot water system is installed, both the hot and cold water supplies within the dwelling deliver water at the taps or shower outlet at mains water pressure. Cold water from the mains is delivered to the base of the cylinder, the water is heated, then when a tap or shower is turned on the mains water pressure pushes the hot water out.
To allow the water that is heated in the cylinder to expand as it is heated, an internal or external expansion vessel is used, and a pressure relief valve is also fitted should the vessel fail.
An unvented hot water storage cylinder can be heated directly via an immersion heater or indirectly by most heating systems, providing it is not a solid-fuel burning appliance.
The advantages of an unvented hot water system are:
  • Balanced pressures at hot and cold taps
  • High performance showering throughout the house
  • High pressure, therefore high flow rate of hot water
  • Fast filling baths
  • No cold water storage so no fear of frozen loft pipes
  • Quiet running
  • Fast recovery (typically 15-20 minutes to heat 150L)

The disadvantages of an unvented hot water system are:
  • No storage back up if mains water fails
  • Poor mains water pressure will cause poor performance
  • Requires a discharge pipe (overflow)
  • Requires an annual inspection
Combination or combi central heating systems
Combination boilers are so called because they combine most of the components of a conventional system (i.e. boiler, cylinder, pump, control valves and thermostats) into one unit – the combi. They are generally more suited to smaller homes with up to three independent hot water users.
The advantages of a combi central heating system are:
  • It is the most economical type of system, only heating the hot water you use
  • No stored water in the loft or airing cupboard, saving space and there’s no risk of freezing pipes
  • Instantaneous hot water is available 24 hours a day
  • High pressure showers do not need an additional pump

The disadvantage of a combi central heating system is:
Combi’s primarily serve one tap at a time, dependent on mains water pressure, so if someone is in the shower upstairs, and someone else turns on the hot tap in the kitchen, the flow to the shower will reduce. However, higher output combis (up to 40kW) and higher output combis with a small hot water storage facility are now available to deal with higher hot water demands.