Air gurgling noises only when hot

[friendlyaviator]

Dear all,

In short I am diagnosing terrible trapped air noises in my CH system.

I've discovered it only happens when my boiler has been heating the central heating water. The main sound is in the horizontal pipes in the ceiling above the boiler.

If I leave the boiler off overnight and let cold water only circulate then it's very quiet.

Is this a sign of scale in boiler? If so, will some noise reducer like X200 fix it. I think I'm getting kettling of the boiler as it pops every now and then while on.

Can I just add X200 to my system? It's currently running with a new dose of Screwfix own inhibitor.

Thanks in advance

 

[friendlyaviator]

Furthermore:-

This happens even if the boiler is on it's lowest thermostat setting.
Does not happen with hot water only
The noise stops as soon as the boiler is off (but pump kept on)

It's an open vented system, nothing is blocked as I opened a drain cock with supply still on and the whole system passed fresh water through it. The vent pipe is 500mm above the waterline in the tank and all radiators get hot and have no air in them.

Both the Feed pipe and Vent pipe have a short 1.5M horizontal section in the loft but have a slight rise going to the tank.

I'm running ouit of ideas.