Radiators Are Hot Downstairs but Cold Upstairs Cause: This could be caused by a blockage in the ball valve, or if there is a lack of water in the feed or expansion cistern. Quick Fix: Check the ball valve and expansion cistern for blockages, and bleed the radiator to remove any air that could be causing issues.Click to see full answer. In this way, why do some radiators not heat up?If just one (or a few) of your radiators aren’t heating up, the most common reason for this is trapped air. If you’ve just turned the heating back on after the summer, air can become trapped in your radiators, causing them to be warm at the bottom but cold at the top. Your radiator should soon be nice and warm.One may also ask, why is my hot water hotter downstairs than upstairs? If so it is fairly common for a faulty shower valve or mixer tap to allow cold water to flood the hot pipe work, in turn making the hot water seem cooler, this will affect outlets within close proximity to the cause of the fault. Likewise, people ask, why are my radiators cold when the heating is on? If the radiator is cold at the top and warm at the bottom it could be a case of having a trapped air bubble. When a central heating system is signalled to switch on, it sends hot water to radiators. If the top half of the radiator is warm but the bottom half cold it usually means a sludge problem.What to do if a radiator is not working?If the radiator is cold at the bottom it could be sludge in the radiator. Close the valves on the radiator, remove it and then clean out the inside with a hose until the water runs clear. Always remember to check the lockshield valve on the radiator is open and that the TRV pin is not stuck down.