Rushing water sound when C/H starts

[sharkey]

I have just had a complete new central heating system installed.
Vaillant eco plus 831
6 x radiators (3 up & 3 down)
2 x down stairs are tall radiators
Boiler located in loft with 15 mm supply cold, 22 flow & return with 15 ends

when central heating starts i get a loud rush of water flowing through the pipes and especially in the 2 x taller radiators.
There is a noticeable temperature difference between each radiator.
The boiler is dead silent.
All rads have been vented.

The plumber said i will get this sound in the 2 taller radiators anyway.

please help.

 

[waterworks]

Water rushing sounds in pipework usually means air.
Do you mean all flow & returns are 15mm or do you mean gas supply is 15mm?.

 

[gas4you]

As the boiler is in the loft it will be the highest point. I would suspect the auto air vent cap in the boiler has not been loosened. They are supplied done up tight from the factory.

 

[sharkey]

do i leave it fully loosened?
why do i have to loosen it? whats the purpose of the vent?
cheers

 

[jhgasman]

you leave the cap fully loosened ( about 2 turns ) at all times , the purpose of the auto air vent is ....erm ...well as it says its an auto air vent ,,,,,, it will purge any air from that point in the system , check also if additional AAV's have been fitted to the high points on the flow and return .
Hass

 

[sharkey]

thanx mate ill try that