Radiators cold at bottom

Hi, looking for some info on radiators. My central heating system has 6 radiators, 4 of which have individual thermostats located at the top of the radiator. These 4 rads only get hot at the top [about 6 or 7 inches max.] while the bottom remains stone cold. I read the other posts on cold rads advising that the system get flushed but its only 4 or 5 years old & the 2 rads which don't have individual thermostats are hot all over so I'm wondering if its maybe because the thermostat is at the top & if so, how difficult it would be to get them moved to the bottom?

Really grateful for any advice.

Thanx

it may only b 4 or 5 years old but if there wasnt any chemicals put in, it will be very sludged up

Poor flow through the radiator will also cause a cold arc at the bottom. Try turning off all the radiators except one of the bad ones and see if you can get some flow through it.

Silly question, but the other pipe doesnt come out of the top of the radiator aswell does it?

If not, remove them and flush.

David