In an earlier contribution to this discussion,
rob wrote:

I've just had a plumber fit 2 new bigger radiators to replace 2 old
ones. The system was drained then refilled- all the other rads are OK
but the 2 new ones won't heat up at all.
We tried everything we could think of.
Bled the air out - they are full of water.
Turned all the other rads off and ran the pump.
Put a hose on the drain cocks that are fitted to both rads and ran
the water through till it was hot. Closed the drain cocks but no
hot water into the rads.
The property is a bungalow with solid floors and the pipework runs in
the loft and down the walls to the rads. These 2 rads are served by
the one pair of pipes until they reach skirting board level where
they are teed off to one rad in the lounge and the other through the
wall to the hall.
The tank is an old style primatic, central heating and domestic hot
water in the same tank, separated by an air pocket - but it seems to
work ok for all the other rads.
The valves are all new (one of them is thermostatic) - it seems a bit
unlikely that 2 of them would be blocked. The system has recently been
drained and flushed before they were fitted.
Any suggestions please?

Sounds like you got an air-lock in one or other pipe - stopping any
circulation. Bleeding them with both radiator valves open won't cure this,
because water will come via the non-air-locked pipe. You need to turn off
each radiator valve in turn, and bleed a couple of litres of water out each
time. That should ensure that *both* pipes are full of water.
--
Cheers,
Roger
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