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Radiators hot upstairs BUT cold downstairs?



I moved into a new house 5 months ago and the downstairs rads never get
hot, whilst the upstairs ones get warmish. It's an 'open' system that's
around 15 years old and I don't know if inhibitor was used correctly.
Where's the pump, is it nearest the upstairs rads or the downstairs rads? If
the pump is nearest the upstairs rads it might be that the lock shield
valves on the upstairs rads are fully open or open too far so that they are
hogging the hot water and there is not enough pressure for hot water to
reach the downstairs rads. In that case all the rads MAY need balancing. But
that doesn't explain why the upstairs ones are only warm if their control
valves are fully open. What happens to the upstairs rads when you turn the
pump up to 3 and turn the control valves on all the downstairs rads
completely off? Do the upstairs rads get hot?

I decided to drain the system down and remove all the rads, flush them
out with a hose, replace them and add some sentinel x400. The upstairs
ones are definitely warmer and I'm happy with them, but the downstairs
ones are still stone cold, so I'm not sure where to go next?
There was some very black sediment in all the radiators, but this
mainly came out in suspension when flushing through, rather than really
thick sludge that I'd been expecting. I only refilled the system last
night so should I wait and see if the Sentinel does anything? The fact
that the downstairs rads are stone cold makes me think that this is a
long shot!
The pump has 3 settings and it's currently on the lowest- I was advised
not to turn this up but could that be the problem?

This is my first attempt at plumbing and I was prompted by the high
quotes and un-convincing advice given by the plumbers that I've had
round (no offence to all you decent proffesionals out there!)
Is the pump actually working, you should be able to hear it running if you
put a screwdriver on it, and the handle on your ear (You can just stick your
ear on the pump, but it will probably be hot!) - if not, then the pump is
shot and your upstairs rads are getting heated by gravity circulation.

If the pump is running, turn off the rads upstairs, all but one downstairs -
does this cause the downstairs rad to heat up?
As others have said, you may be able to clear the airlocks in the downstairs
rads by applying *all* the urge to one rad at a time - by turning all the
others off. If that doesn't work, try this:

On each cold rad, turn off one valve. Open the bleed screw and bleed off a
couple of litres of water (into a suitable container!). Then close that
valve and open the other one, and repeat the bleeding operation. That will
ensure that the flow and return pipes are *both* full of water. [If you
bleed with both valves open, the water only needs *one* path - and there can
still be an airlock.] If your system has inhibitor in it, pour the 2 litres
back into the F&E tank after each operation to avoid wasting it.

One other thing to check: I think you mentioned earlier that the rads are
fitted with TRVs. Remove the thermostatic heads and make sure that the pins
move up and down smoothly, and are not stuck down.
When you re connected the rads down stairs, did all the rad fill with
water ?