why does one radiator not get hot

number 1,it is not good enough to leave a heating system that way!i would presume that they would have dropped the water out of the system to fit the new radiator?so then it should be refilled,all rads vented,heating put on full and checked for air locks!try shutting down most of the rads,so as to boost the heating water to the one in question!!!tom clydebank plumbing & heating

have you checked for air locks try to bleed all rads,also try to close all the outlets on each rad not all the way but just enough to let abit of the water away ,sounds like all the rads are taking to much before it hits the last rad is the rad in the lounge along way from your boiler ?

The most likely reason is that when the system was drained down that bits were dislodged within pipe work and have got stuck in Radiator Valves. You will need to drain down again and refill system checking that CH water is entering the radiator from both valves. Then turn on CH and close all other radiators and check if Radiator heats up, if not then you have a blockage. Bill Pipe Dreams Plumbing Services Bournemouth

There are a couple of reasons for this happening. Firstly is this an open vented or a sealed system? 1, Airlocks may have occurred when the system was re-filled, checks should be made to ensure there are no air locks. 2, Sludge in the pipework has moved to cause a blockage in the radiator in question. Solution A, Either of these can be rectified by either repeatedly draining and re-filling the system until the blockake or airlock has been removed. Solution B, If the problem persists you may need to have your system powerflushed. This involves conecting a pumped flushing mechanism to your system and using a chemical to breakdown the sludge, flushing the whole system through until the water runs clear. This can have side effects in old systems in that if there is a weak point in the system a powerflush will usually find it. Cliff Easy-Plumb