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Emitters (radiators etc.)

Central heating

Emitters tend to be of two types – traditional radiators and underfloor heating. It’s a bit of a toss-up as to which is better. They have their respective advantages and drawbacks.


  • a pleasanter heat because it is better distributed
  • no wall space taken up by radiators
  • possibly more efficient because of lower flow temperature (this works better with heat pumps and full-on solar systems but may not match with domestic hot water temperature requirements).
  • usually slightly more expensive to install
  • higher pumping costs
  • possibly less efficient (in suspended ground floor situations)
  • slow warm-up time (especially if pipes are bedded in screen)
  • carpets cut down heat output
  • less prone to rust as it is all plastic or non-ferrous
  • difficult to install in existing floors (they need raising)
  • harder to install – manifolds need carefully positioning
  • damage or blockage to pipes can be very expensive to repair
  • timber floors are liable to shrink (or expand!)


  • much better localized control that underfloor
  • more responsive
  • can overheat locally and cause discomfort (probably the worst condition is close to a bed)
  • take up wall space (and this can limit layout of furniture)
  • easy to install
  • easy to repair
  • steel radiators can cause rust sludge in system if not maintained properly
  • possible inefficiency on outside walls depending on insulation value (need reflective surface to be kept clean)

The sizing and positioning of radiators in a very well insulated house is not quite the same as traditional design practice would recommend. Typically radiator sizes can be reduced very considerably and it matters less where they are.

As an example a 3/4 bedroom house with 200mm of insulation all round and reasonably good air tightness (say 4 or 5 acph) might only need a total central heating input of 3 or 4 kW in freezing weather conditions to maintain 21ºC inside. This might equate to half a dozen very small radiators in the region of 60cm x 60cm or so.

With good insulation and high quality double or triple glazing it becomes much less important to position radiators around the edge of a room or below windows. This means that pipe runs can be reduced along with pumping costs. This has a bearing on service duct design and location.

The Passivhaus concept takes this to its logical conclusion where no central heating system is necessary.

Walls and roofs

Walls and roofs act as emitters when heat from the sun finds its way through into the house. The amount of heat and the timing of heat arrival on the inside surface of the wall is determined by the decrement delay value. By getting this right a considerable amount of energy saving can be achieved.

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