The radiators in church are essentially very old-fashioned convection heaters. Hot water heat touches the metal from the inside and transfers thermal energy to the metal by conduction. The metal is a good conductor because free electrons are able to go racing through the structure and collide with ions, making them vibrate faster than waiting for the usual domino toppling model. The hot metal transfers thermal energy to the air. The air expands, becomes less dense and rises, which is supposed to warm the air in the building. A high ceiling is not a great idea! But the intricate shape means a larger surface area of hot metal touches the air at once so the rate of energy transfer from metal to air is greater.
