Vertical Radiator Flow

Radiator turned over and now lovely and hot. But I am disappointed by the 'trickling' noise as the water flows. I keep trying to bleed it but just water flows out. The bleed screw is opposite the flow and the lockshield is open 2 turns.

I can see that the bleed screw is on the end of the top pipe so the water will flow out when the top pipe is only half full (the vent is centred on the top pipe). Is there an off centre vent screw that can empty the top pipe completely?
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If you have upturned the rad and obviously assuming you have kept the flat side of the rad to the front, the input flow diverter connection will now be on right hand and at the bottom - *this has to be the incoming flow to the rad* via a TRV ..... your flow connection pipe appears to be on the bottom left!

In other words, it is against convention to reverse flow on a diverter of any kind weather it be a plate type or tube feed type even though in some cases it may work and will usually cause the trickling water sound.

One obviousl remedy is to swap (repipe) your flow and return pipe connections.

Nearly right. So the diverter is now bottom right. But the flow is in through the lockshield and out through the TRV. The TRV is marked as bi-directional (Drayton TRV4).

It obviously makes better aesthetic design to have it out on the open side for easier adjustment but would it make better engineering design to have it on the flow side? The right hand side has a full length curtain hiding most of the pipes.
Are you bleeding with the pump turned off ?
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So if the flow is at right then that is correct and will not in itself be the cause of the noise.

Is the first pipe/tube hot showing that the flow is being taken up to the top and returning from all the other tubes to the left?

I suspect it is just the design of the rad preventing all the air getting out of the bleed. It might slightly or completely reduce over time. The 1/5 of oxygen will dissolve quite quickly but the 4/5 nitrogen hardly dissolves at all.
Nige F - I'm bleeding with the pump off.
Agile - Yes, the first tube (RHS) gets hot first then the last tube (LHS) gets hot. Over time the middle six then start to catch up. After about 15 minutes (or so) the radiator has an even temperature all over.

This afternoon I've changed the type of bleed valve (and improved things). But curiously the change was too easy. I closed off the main valves and opened the bleed screw. This caused a trickling sound. Then removing the whole fitting no water came out! It was totally dry in the top tube. I put the new valve in and bled again. I couldn't hear a normal hiss but eventually water came out.

I'm wondering whether to fit the top right hand side with a normal lockshield upside down so that I fully fill the top tube! (on a temporary basis). Or maybe wait until it's cold and the heating blows all the air out.
But you don't say if it is quiet now.

Sounds as if the old bleed screw was not opening.
I don't see that whatever valves you have it is going to make any difference.

But it sounds as if you have too much flow through the rad.

Assuming a condensing boiler, the flow rate should be adjusted to provide a temperature drop across the rad of about 15-20 C by reducing the flow using the lockshield.

If adjusted properly the sound might well totally disappear.

Correctly set, the inlet tube should be hot at the bottom and the remaining tubes should be noticeably cooler as measured by hand.

Agile; you might be onto something with flow speed. I've put the lockshield back to where it was originally. But now I have a radiator with a different flow pattern since I turned it over. Also the other radiators had TRVs meaning they were probably off. I'll do the temperature drop next.
Yes, of course the flow pattern is different now.

Not only different but correct!

Now all you need to do is set it for the correct differential temperature!

How is the noise after closing the lockshield?

You hardly need a thermometer, hands can easily feel the difference and could probably be described by "hot" and "warm" depending on your CH flow temperature.

The noise stops when the lockshield is closed. I did a test with it and the TRV closed when I was changing the bleed valve. I thought it might encourage a little more air out of the system. I have a meter with temperature probes for the balancing.

Might also revisit the bypass valve settings(Danfoss ARV22). When you helped me originally on getting this radiator to perform I wondered whether the bypass valve was allowing too much flow and preventing full flow to the end of the run. I need to look up what we set it at and what the Vaillant 728 needs. The pump is at the highest (3) setting and I seem to remember that the Vaillant needs a high flow rate especially when it's only serving the hot water cylinder.
Perhaps I am out of touch with the latest Vaillant boiler models but have not yet seen a 728 model.

Typically an auto bypass should start to conduct when 2/3 of the rads are off.

Or alternately when the head exceeds about 3 m.

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