On 12/05/2018 21:37, Roger Mills wrote:

I'm almost tempted to suggest converting the primary circuit to
unvented, which should make it easier to eliminate the air. But it's
worked ok as a vented system for decades - so shouldn't be necessary.

Am I missing anything obvious?

Are all the radiators getting hot? If not you may have a airlock in the
pipes supplying them which may be the source of the some air getting
back to the to the pump or boiler. If so run the pump at full speed and
just turn on one radiator at a time to try and force the water through
the problem pipe length.

Since the tank ran dry have the radiators been bled? While the tank was
dry air could have been sucked in and distributed to the radiators.

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