My heating's been pumping over for a while now, spilling over the overflow from my header tank. I can't seem to get to the bottom of the problem.

I've extended the height of the open vent which is now two feed above the top of the header tank. I have also bent the arm of the float valve so that the water level in the header tank is about five inches high, but it still pumps over and out of the overflow.

I recently put fernox F3 through the system, flushed it all out and re-treated. I also descaled the boiler using fernox DS40 as part of the same job.

I have read that the aerjec air separator things can get blocked and cause pumping over, but I'm not sure how I would re-pipe it if I removed it, or perhaps it isn't a problem?!

Here's a photo I took of my pipe setup in the airing cupboard (ignore the leaky hot tank, that was quickly replaced):

http://i.imgur.com/iF75oHt.jpg

The pump flows from top to bottom.

Any ideas guys?

Thanks,

Tom