would an automatic bypass fix my problem?

I have recently made two changes to my existing CH system and have a problem. The boiler has been upgraded to a Glow-worm 18 hxi as the old potterton really was on it's last legs (about 20yrs old) this appears to be working very well.

At around the same time we had an extension build with the addition of two small radiators. It would appear the hydrostatic pressure of the system is now generating problems. The radiators at the ends of the system don't get hot. I've spent weeks trying to balance the system to no avail.

As a test I closed the manual bypass valve and lo and behold all the radiators now get VERY hot. It would appear that the static pressure in the system means that the pump is sending the water through the bypass shunt, rather than to the ends of the system.

The question is, would an automatic bypass valve solve my problem. The CH is controlled from a room stat and closes a motorised valve to the CH and turns off the boiler, however the pump continues to run for about 5 minutes. Needless to say, until I get this sorted the bypass is OPEN.

SHort answer, YES!

Longer answer -the boiler requires a flow of 774 litres per hour, = .21 litres/sec. Look at a graph for your pump which is problably a 5m head one like a 15 - 50 and you'll see it's almost flat out to provide that. If your system when on allows for all circuits to be shut, the bypass valve has to take all the flow. So set your bypass to a head which on the pump graph means it would pass that much.
That's almost 5 metres. If you aren't getting the farthest rads hot at that pressure with all rad valces open then you need a bigger pump or the balancing is wrong.

Its a condensing boiler which adds the complication of a flow control valve. You have the added requirement for the differential temp across the system: - Turn off all radiators that can be shut off by the user and check to see if less than the maximum differential allowed of 20ºC can be achieved across flow and return.

All pretty difficult to achieve - the boiler only modulates down to about 5kW, a heck of a lot of radiators at 20 degrees differential.