In article .com,
"Honeymonster" writes:
I reluctantly have to agree that there *must* be a leak in the system
somewhere. I guess what I really need is for it to get worse. For the
record the pressure gauge is at the same level as the highest radiator.
Since air gets in only in CH mode (not DHW mode) I guess the problem
might be close to the suck side of the pump?

You can't make any such assumption.

When the system is at its hottest, the water holds the minimum
of dissolved air and the excess is forced out into top of the
radiators (most commonly just one or two, depending on pipe
layout). When the system cools down, the water is ready to
dissolve more air. An air molecule just next to the leak will
see a 50% vacuum in the water in the heating system, even if
the water is still pressurised, and will be sucked into
solution in the water. Next time the water is heated, half the
air in solution will again be forced out into bubbles. Air,
being much less viscous than water, can leak in faster than
the water leaks out in some cases, although its likely to be
limited to the speed it can diffuse through the water in the
leak in most cases.

--
Andrew Gabriel