back boiler boiling but rads only warm

[biker dave]

Hi everyone this is my first time on this site, i hope someone can help me. I have a baxi burmuda with 6 rads and the boiler even turned right down is bubbling as it burns. I can only guess this is a case of bad circulation, i can fear the pump running and the water running through the pipes, so the only thing i can think is a sludge problem. I read another post about sludge problems and sentinel x300 was used should i go this way? your thoughts would be appreciated. cheers

[plumbbob]

If you can "hear" the water running through the pipes suggests that air is trapped in the system. Normally circulation is silent and any noise is actually water trickling past airlocks.