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Boiler Output - The boiler must have sufficient output to meet its maximum load, which includes the radiators, the domestic hot water cylinder and the heat losses from the distribution pipe work. It should also have sufficient extra capacity to warm the house up in a reasonable time when the system is switched on from cold. It should not be oversized, however, as that will increase its capital cost. Oversizing can also adversely affect efficiency and hence running cost, although most modern boilers are capable of operating efficiency under part load conditions.

Boiler and Flue Location - A number of factors need to be considered in deciding the location of boilers and flues:
  • access to an external wall or roof, especially for room-sealed appliances
  • the supply of combustion air for conventional flued appliances
  • ventilation around the boiler casting
  • access for servicing and maintenance
  • provision for safe discharge from boiler safety valves
  • the need for condensafe drain ( for condensing boiler )
Combination boilers can save space in two ways: they need no hot water storage cylinder or cistern as they are fed directly from the cold water supply; and they are usally intended for use in a sealed primary system, with no feed/expansion cistern. Another advantage of combination boilers is that they provide the option of a dry loft, as they need no tanks to be placed there to provide the necessary pressures. The same attribute can be especially helpful in a flat, where there may be no opportunity to obtain adequate pressure from a cold water tanks.

The limitations of combination boilers for hot water supply should also be understood by both the installer and the client. In particular, the client should be aware of:
  • the time taken for the hot water to reach an acceptable temperature:
  • the maximum flow rate at which hot water can be drawn off, especially for a sustained period.
  • susceptibility to scaling by hard water
  • the inability to fit an immersion heater for back-up hot water supply
  • the limitations of flow rate when serving more than one point simultaneously.