cold rad

I have one cold rad in the house thats doing my head in. Its always been a bit dodgy, however I recently had a nice new boiler and pump fitted thinking that this would solve the problem, but to no avail. I have 14 rads with all the downstairs rads fed from the upstairs circuit. The rad in question is the biggest in the house and is situated in the downstairs living room, it is NOT the furthest from the pump. I have balanced the whole system and it is still cold, but if I shut down all the other rads it eventually gets hot but does take its time, sometimes 35 mins from cold!. I have cleaned and flushed the rad using an anti-sludge compound fed via the bleed valve and circulated round the rad for 4 hours then released into the system and it still stays cold when the other rads come online. I have checked that there is a good flow from both the TRV (new) and the lockshield. So apart from buying a new one any ideas?

How has it been piped up?

Is it being fed straight off the 22 mm F &R or off a 15 mm feed.

Is it microbore feed?

All the piping to the rads is 15mm, with the main feed from the boiler, through the switching valve and into the upstairs flooring at 22mm. Somewhere under the flooboards it then gets reduced to 15mm.

Try changing the LS valve and trv. It does sound like a blockage of some sort or restriction of flow to it.
If you shut down all the rads as you have said the hot water HAS to go to this rad unless there is a restriction or resistance of some sort.


See what the pros think

Tried again tonight to get the rad hot with only two other rads on; no joy after an hour. I will now undo both LS and TRV and check the flow with the pump on at the TRV and open the LS and see if water flows out readily. I will also power flush the rad (in the past only used a mains fed hose)
Question: if I power flush the rad both ways and a good flow comes out is this an indication that the rad should work? Is there a solution that I could add to improve the throughflow when the rad is isolated

Rana, you should be able to get the rad to work before going to the expense of a power flush, not that a PF isn't good for the system (because it is), I just want you to fix the primary problem first

Have you checked the rad valves yet? If they prove clear check the pump impellor isn't blocked with gunge, that would seriously impair pumping and may give rise to the symptoms you describe.

After all the advice given I decided to flush out the radiator, check the flow from both the TRV and the LS; both fine, reconnect the rad with all other rads turned off and see what happens. The rad quickly warms up and seems OK, I then introduce one rad at a time in the correct sequence by opening up the LS to ensure the system is balanced. Two hours later the whole system is working fine with the pump [Grunfoss brand new] set to position 2 of 3. The system works fine for the rest of the day and evening…but the following day the rad begins to get cold again after it switches on in the evening. So the next day I go through the whole routine again with the pump set to three this time; empty the rad, refill with all rads off, get it nice and hot again, bring the other rads in and again it works fine for that day but once again it goes cold the following evening!!
So it seems that the rad will work fine for a total of about 8 hours then the flow just stops. Even if I shut down the whole system except one small rad in the same room I cannot get any flow through the rad. I need to drain it completely, reconnect it and off it goes for about 8 hours.
Do I now invest in a new rad with the possibility that I may get the same problem or is this something that someone feels they may have an answer for