Longish post - please do read to the end if you are at all experienced
in this area. I'm becoming desperate!

I installed a wet central heating system & oil combi boiler from
scratch last year. The only problem with it last autumn/winter was the
loud noise of air in the pipes and the fact that I needed to bleed one
rad every few weeks.

The air was evident from the start & I gave the system a double dose
of Fernox in the hope that this would prevent corrosion while the
system hopefully 'settled down' and ceased to introduce air.

Needless to say, the problem still exists and is extremely noisy (a
loud, intermittent gurgling noise in the four radiators nearest the
boiler) now that the CH has been fired up a couple of times this
autumn.

There is no visible sign of water loss. Pressure is just over 1Bar
cold (although I did temporarily raise it to nearly 2Bar in an attempt
to prevent air entering). I have searched previous forum threads for
similar problems and followed all relevant advice such as sealing rad
valve gland nuts with wax etc.

As you will imagine, this situation is somewhat annoying and worrying
from the corrosion angle. I have spoken to a CH installer and a boiler
service chap. Neither could suggest what can be at fault, with the
system under good pressure.

One thing I have noticed this year, but not last, is that there seems
to be a noticeable amount of air getting into the domestic hot water
side. I can hear it in the hot pipe when a bath is being run. There is
no such noise in the cold.

Can there possibly be any connection between the CH & DHW sides?
Diverter valve? There is certainly nothing visually amiss inside the
combi boiler cabinet. I have even had a listen with a stethoscope to
various parts of the plumbing (see, told you I was becoming
desperate!)

Any advice gratefully received.

Many thanks, in advance.