large amounts of air entering CH system

[saviller]

I've had a problem with frequent venting of the radiators being required.
The setup I have is open vented with a Potterton Prima f60, Grundfoss 15-60, 3 port valve, on a Y plan, with bypass loop fitted. Only 2 rads have TRVs - bathrooms, and bypass valve is closed.
There are seperate feed (15mm)and expansion(22mm) pipes.Whole system has been successfully powerflushed. I also reversed flushed down the expansion pipe and up the feed pipe with fresh water.
The bedroom radiator at the highest point is getting air bubbling into it every 10 seconds or so.
The pump is running at 1.
Boiler is at number 4
The temperature of the radiators is around 76 degrees (on all of them).
The feed pipe tees into the flow to the pump with inverted loop, expansion pipe is teed into 28mm flow from boiler, before feed - feed and expansion pipes approx 150mm apart.
Feed pipe is getting hot, vent pipe stays coolish.
I have tightened all the gland nuts on the radiators up, and have checked the compression joints on the pump flanges for leaks.
I have checked that air is not being drawn down the expansion pipe using glass of water - no fall in level.
There are no obvious leaks on any connection on the rads.

I'm running out of ideas as to how the air is getting in, and I'm having to vent a lot of air!!
I did get overflow from the F&E tank with the boiler on Max a couple of days ago, so turned the boiler down.

Could this be simply that the thermostat on the boiler is just set too high. ?

I'll drop it to 1 and see what happens

 

[Softus]

Are you sure that the vent on the highest rad is shut?

Also, are you venting with the pump switched off? If not then you should be.

 

[corgiman]

might not be air mate may be hydrogen, which is produced in huge quanties by iron magnetic (sludge)

 

[saviller]

Hi, yes the vent on the highest rad is fully shut off.

Gas is air - whole system was power flushed - all sludge removed, status of water in system before inhibitor added was equivalent to tap water in both PH and TDS. But I will do a check to make sure.

System has been running on number 2 on boiler for the afternoon and rad not full of air as yet - we'll see what happens when it fires up first thing in the morning