External pipework

[becboodle]

We have an external condencing, combi boiler, serving the heating and hot water.
We have a problem with air in the circulating pump, which we have to bleed daily before the boiler will reset and come on.
We have isolated the boiler before christmas (before the cold weather) and pressure tested the system, and the pressure held constant for 3 hours.
We have had Worchester out 4 times and they have change the pump every time, the AAV, the PCB, the diverter valve, and checked the preasure release valve and expansion vessel.
1. has anyone got any ideas what could be causing this?
2. the pipework to the boiler externally is in copper (insulated in a boxing stuffed full of more insulation). A soldered joint failed two winters ago, causing a similar problem. If we change this to Hep2O is it less likely to fail than copper, ie. is Hep2o better suited to external pipework than copper?

[stoneyboy]

becboodle,
You do not say whether you are having to regularly top up the system from the mains
if yes - you have a leak in the system somewhere.
If no - is the pump turned up too high and has corrosion inhibitor been added to the system each time it has been drained down?
Personally I would not consider using plastic pipe directly to the boiler.
end

[becboodle]

Just encase anyone gets a similar problem to this.
We finally had Worcester out to fix the problem (took 4 visits).
There wasn't a leak, the pipe to the expansion vessel was blocked, thus when the pressure got to high (as the expansion vessel was effectively not working), the boiler dumped out a load of water, the heating would go off on the timer, cool down and lock out on low pressure. We also believe there may have been a fault with the air intake valve.
We now have a magnetic system filter installed - which should have been done when the new boiler was installed.