Had the radiator removed here about 2 years ago now for refurbishment work and planning to install new one shortly. When the central heating is switched on in house, all radiators heat up but I noticed both copper pipes here are cold and not hot. The old radiator used to heat up fine. I'm assuming the right one is the flow as this was where the thermostat was so should heat up and the left is return. Any ideas why its not heating up?