Mystery fizzing boiler

[Simple Simon]

Someone PLEASE help me solve this boiler mystery!

1. My boiler was kettling (it's only six months old) so I added some Sentinel scale remover. No result.

2. So, I drained the whole system down, changed the pump (it had been ticking), added sludge remover (Putrimachos Flushex), left for 4 weeks. Kettling disappeared immediately, but towards the end of the 4 weeks, I started to get 'fizzing'.

3. I drained out the sludge remover, flushed through till the water was clear, and refilled with Putrimachos Scalex and Protex.

4. Now I get moderate kettling when starting the boiler from cold, but real 'fizzing' whenever the boiler's heating.

I'm at a loss. It can't surely still be a scale problem? Did I leave the Flushex in too long, and it's damaged the boiler? Help!

 

[oilman]

I bet you have 22mm pipes coming out of the boiler. It's likely the boiler is quite a high output BUT YOU HAVEN'T TOLD US WHAT THE BOILER IS!!! In this case it may be quite likely to make a noise. It gets less noticable when the fan bearings wear out and their noise drowns out the fizzing noise.

 

[Simple Simon]

It's a Worcester Bosch 24 CBi (80,000 BTU) wall hung boiler with 22mm pipes out the top. It's not a 'normal' noise because it never used to make it - and it doesn't make it when the heat comes off but the pump's still on. It sounds like sand being washed around in the water. Also, the boiler's less than a year old.

 

[Simple Simon]

...oh, and I should have added that's it's an open, vented system with two 2-way valves for the central heating/hot water. The pump speed's at 2 and there's a Tower Airsep component at the base of the vent pipe.