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This comprises a boiler and a hot water tank. The water in the tank is heated by an internal coil which is fed by the boiler. The tank is fed from a cold water storage cistern, contained in the loft, with a capacity equal to or greater than it.

A conventional boiler has two water tanks in the loft:

  1. A large cold water storage tank: draws cold water from the mains to refill and itself feeds the hot water tank.
  2. A smaller feed and expansion tank: this acts as a feed and expansion vessel for the boiler circuit and space heating system.

The boiler heats the water directly for the space heating requirements and passes this through the hot water tank coil (also known as a heat exchanger), which then heats the water in the tank for DHW use. The water from the boiler is then passed around the central heating system and finally back into the boiler. The water tank stores the hot water for DHW use and is refilled by the cistern in the loft (which gets water from the mains). The Feed and Expansion tank feeds the boiler system, which in itself feeds the radiators or under-floor heating system for space heating requirements. Thus the DHW and space heating water are kept separate, avoiding problems of scaling or corrosion.


This type of system is called a “vented system”, whereby it the feed pipe is open to atmospheric pressure (i.e. the tank in the roof uses gravity to create the pressure in the water system). The pressure created isn’t as good as it could be and sometimes pumps are needed to increase the water pressure.

In 1989 “un-vented systems” were introduced and these operate purely from the mains water. The principles of the system are the same, although as the system is fed from the mains, there is no need for a cold water storage tank in the loft. The water pressure tends to be a lot higher, and because of this there are many safety features built in to the system.

Moving on to the boiler itself: A conventional boiler comprises a burner (gas or oil) to transfer the heat to the water. Of the heat put into the boiler, around 80% is transferred to the water, with the rest being lost through the flue. The water that is returned to the boiler, from either the hot water tank or radiator system, cannot fall below 55 to 60 degrees as this would cause condensation which will lead to premature failure of the heat exchanger due to corrosion.


The advantages of such a system are that:

  • They are easy to install
  • Can be used with renewable systems
  • Do not require a pressurised system (making them safer and less expensive to service)
  • Tend to be very efficient and reliable in all temperatures

The disadvantages are that:

  • They take up a lot of space
  • Are inefficient and costly to run
  • Water pressure can be low if the storage/feeder tanks aren’t high enough

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