Some of the heat from a radiator will heat up the wall on which it is mounted. If that wall is an outside wall, a lot of this heat will be conducted to the outside of the building and lost. Luckily, it is very cheap and simple to prevent this from happening, by placing reflective foil on the wall behind the radiator. Research in the UK has shown that almost any kind of reflective foil (even ordinary aluminium kitchen foil) can reduce this heat loss by 5%. Properly designed reflective panels, which combine reflective foil with a thin layer of insulation, can reduce the heat lost through walls by up to 18%.
Any object that is hotter than its surroudings will transfer energy to the surroundings. So, if you have a pipe carrying hot water from one place to another (e.g. from a hot water tank or boiler to your taps) some of the heat energy the water contains will be lost.
Normally, heat flows from warmer objects to cooler objects just as water naturally flows from high places down to lower places. If you want to force water to move upwards, you need to use a water pump. Similarly, if you want heat to move ‘uphill’ (i.e. from a cooler place to a warmer place) you need to use a heat pump. A refrigerator is basically an insulated cabinet containing a heat pump. Heat is continually leaking into the refrigerator (because the interior is cooler than the exterior) so the heat pump has to continually force this heat out again.
This article contains a list of simple low-cost / no-cost techniques for saving energy in the homes and offices. Not all of these techniques will necessarily be appropriate in all circumstances – the aim was to make the list as broad as possible.