In one of the many different types of domestic heating systems, hot water is pumped around the house and heats rooms via "radiators". The fact that the dominant heat transfer mechanism is actually convection, rather than radiation, is easily demonstrated by putting your hand just above the radiator and just below. It is of course much hotter above the radiator, showing that convection is dominant.

Why is it necessary (and recommended) to turn up the water temperature in the boiler and radiators during the cold winter months?

Pick the most accurate answer.