There are a few simple measures you can take to stop radiators from getting cold at the bottom, or at least reduce the risk of them building up sludge and therefore not working to full effect.
You can add central heating inhibitor to your devices to combat sludge, or infiltrate the system with a scale reducer, which works to tackle the risk of a limescale build up. Also, you could add a magnetic filter to your radiators to deal with the metallic fragments and bits of rust found within sludge to deal with the potential issue.
For further information, explore our blog, Why is my radiator cold at the bottom?