Fully pumped open vented heating system and position of cylinder return

[harry]

I have read

that on a fully pumped open vented system the cylinder return must be the last conection before the boiler. Why is this? I have this system but the cylinder return is 3 radiators with trv's into the heating return side. Is this going to work or should I extend the return back to the boiler? (which will mean taking the floors up again). Or is there an easier way around it?

Thanks for any help

Harry

[Ed Sirett]

There is a possibility that if some of the radiators return before the cylinder return _and_ some after the cylinder a problem will occur. When doing HW only then some of the return water will flow backwards through the 'upstream' rads to point where all the radiators have a common connection on the flow side. Then through the remaining rads to join the 'downstream' return connection. This will cause the house to heat when not required. A confirmation that this is happening is that some radiators will get warm starting from the side that's normally cooler.

[harry]

Thanks Ed Is there an easier way round this? Whats stoppping an upstream flow if the cylinder return is first by the boiler?

[Ed Sirett]

Nothing. Either the cylinder should return after the radiators or the radiators returns should join together before joining with the cylinder return.

If the return pipework is not easy to modify then zoning the two sets of the radiators (if that's a reasonable idea) would also fix the problem.