CornishCrofter

So, the problem occurs when the system switches itself off I assume?

When that happens the water in the pipes cool down and the pressure drops to approaching 1 bar, whereas it was probably running at between 1.2 and 1.5 bars.

Clearly you're managing to get the flow to and from the rads in question whilst the system is running, but all your work is undone when flow ceases.

Are you sure the downstairs rads were working properly in the first place before you fitted the new boiler?

I'm not a qualified plumber, but I do have a multi discipline engineering honours degree. We did a smidgeon of fluid mechanics that helped me to understand how and why fluids going into a split in a circuit may not behave in the way one would expect. If the water is flowing too fast in one direction and there is no momentum at the branch to draw some off this can be for a number of reasons ranging from a lack of differential pressures at flow and return connections to restrictions and blockages.

Whilst thinking on this. is it possible that comparatively high return temperatures from the main system circuit compared with those from the rads downstairs are inhibiting flow? Higher temperature would give rise to higher pressure and if you've got a significant local pressure differential - high pressure flowing past a tee that serves the return on a rad that isn't circulating at all or very little (and hence is cold) then the pressure differential could be such that the return from that rad won't get a look in, and hence you'll get no flow from it either.

Suppose you could heat a downstairs radiator return externally, you would find that the return may start to flow back into the main return for the circuit (as pressure and temperature increase and the pressure differential would decrease) and you would introduce flow in the radiator in question. I'm not suggesting you do this, but the point helps to explain my thinking.

I suspect your actions work because you manage to get the fluid mechanics working together in an operating system. However once the system ceases to operate your settings are not conducive for the whole system working from startup.

So where does that leave you?

Can you adjust the temperature that the water is heated to so that these temperature and pressure differentials that cause the problem can't get too great, or occur at a time when stalling the circulation in the downstairs rads is possible?

I know I've waffled on, but there is methodology in my thinking. Essentially I think the dynamics of the system in terms of changes in pressure and temperature at certain points as it fires up, you adjust the rads and get them working, then as it shuts down are your problem here.

If I am right, you need the fluid mechanics to be right on start up. At the moment they are not.

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