There are a few causes for the radiators not to get hot.
1) Pump not working or needs bleeding.
2) Insufficient or no water to pump round system. Check boiler pressure if it’s a sealed system - should be 1 bar, plus say a third of a bar for every floor more than 3 that there is above the position of the pressure gauge. For a vented system check the F & E tank has water.
3) Air lock in heating pipe work (not radiators). Turn off all the other rads to force the flow into the cold one.
4) Air in radiator, which should be removed by bleeding via the bleed screw. This will normally be on one end of the rad, but sometimes they’re on the back, covered (or not) with a plastic dome.
5) Radiator valve not open or TRV stuck. See faq 1
6) Zone valve or 3 port valve not opening allowing flow to radiators on Y plan or S plan systems. See faq 3
7) System needs balancing which is here.
8) Check the bypass which should only be open just a little.
An open by pass valve will provide an easy route for the water back to the boiler without going along the full length of the flow and return pipes. This means radiators furthest away either stay cold or just warm.
9) Sludge in radiator or pipe work or valve causing blockage which will need to be flushed out:
When you remove a radiator to flush it out it is a good opportunity to make a fitting with a hose to attach to each valve in turn so you can run off a bucketful or two of water. A garden hose fitting, or a washing machine hose will do the trick and will prove that the valves do open.
If you have a clear hose, you can fit at either end or across the two valves in lieu of the radiator, you can then see direction of flow, cleanliness of water and any air passing along.
This will remove any air lock in pipe work and flushes out sludge from pipe work. This leaves only the bleeding of the radiators.
10) Newly installed not heating up - look at how it has been piped up.
Better flow and easier balancing is achieved if it is fed from larger flow and return pipes nearer the boiler/pump, rather than if it has been daisy-chained off another radiator’s tails.
