My lecture last week, to first year students studying thermodynamics, was about energy flows and, in particular, heat transfer. I mentioned that, despite being called radiators, radiation from a typical central heating radiator represents less than a quarter of its heat output with rest arising from convection [see post entitled ‘On the beach‘ on July 24th, 2013 for an explanation of types of heat transfer]. This led one student to ask whether black radiators, with an emissivity of close to one, would be more efficient. The question arises because the rate of radiative heat transfer is proportionate to the difference in the fourth power of the temperature of the radiator and its surroundings, and to the surface emissivity of the surface of the radiator. This implies that heat will transfer more quickly from a hot radiator but also more slowly from a white radiator that has an emissivity of 0.05 compared to 1 for black surface.
Thus, a black radiator will radiator heat more quickly than a white one; but does that mean it’s more efficient? The first law of thermodynamics demands that the nett energy input to a radiator is the same as the energy input required to raise the temperature of the space in which it is located. Hence, the usual thermodynamic definition of efficiency, i.e. what we want divided by what we must supply, does not apply. Instead, we usually mean the rate at which a radiator warms up a room or the size of the radiator required to heat the room. In other words, a radiator that warms a room quickly is considered more efficient and a small radiator that achieves the same as large one is also considered efficient. So, on this basis a black radiator will be more efficient.
Recent research by a team, at my alma mater, has shown that a rough black wall behind the radiator also increases its efficiency, especially when the radiator is located slightly away from the wall. Perhaps, it is time for interior designers to develop a retro-Georgian look with dark walls, perhaps with sand mixed into the paint to increase surface roughness.
Sources:
Beck SMB, Grinsted SC, Blakey SG & Worden K, A novel design for panel radiators, Applied Thermal Engineering, 24:1291-1300, 2004.
Shati AKA, Blakey SG & Beck SBM, The effect of surface roughness and emissivity on radiator output, Energy and Buildings, 43:400-406, 2011.
Image details:
Verplank 2 002<br />
Working Title/Artist: Woodwork of a Room from the Colden HouseDepartment: Am. Decorative ArtsCulture/Period/Location: HB/TOA Date Code: Working Date: 1767<br />
Digital Photo File Name: DP210660.tif<br />
Online Publications Edited By Steven Paneccasio for TOAH 1/3/14
Realize Engineering 
I have recently fitted a black Venetian blind to my large south facing lounge window. Today there is a frost and bright sunlight. When I opened the curtains this morning the heat coming off the blind was very obvious. In an earlier experiment I attached a black plastic multi celled seed tray across a small double glazed window which resulted in the inner pane cracking due to thermal stress.