My central heating system is Pumped Open Vented with 3 way diverter valve for heating water tank. I was working in the boiler area and could hear what sounded like boiling water coming from the boiler, wooshing and gushing, when I checked I found that hot water was being pushed up the vent to the header tank with a slight amount coming from the overflow pipe, the pump appeard to be running and both heating and hotwater stats were calling.
No I don't believe water was boiling it was too hot to touch the vent pipe though, it was as if the pump had stopped for some reason, it was set on speed 11 where it has been for long time, so I moved it to speed 111 and although it is a bit more noisy, especially when heating is off and only hot water is calling, it has settled down.
Pumps generally only have three speed settings 1,2 and 3. Marked I, II, and III respectively. So by altering it from II to III, you have increased the speed to max. Now this is not an issue as nearly all systems use speed three anyway - apart from the water circulating faster, it just means the pump is noisier.
Check the water in the header tank is not hot and the vent is not over-pumping water.
Quite why the noise occurred originally is a little unclear.
ok "plumbbob" I understand but I really suspect the Grundfos Pump, I have talked to Grundfos tech guys and they tell me that a pump performance can deteriorate over time, this pump is 11 years old and always been on speed 11 until now and was in need of a boost but speed 111 is too high so it falls between setting and Grundfos have said that it could need replacing, so I have got a new pump and will fit it this week-end and I will let you know the result.
This is a common problem on open vented systems. You have a hardened sludge build-up in the return pipe to the pump, because this build-up does not move around the magnetic filter you installed will not solve any problems as the sludge / problem is not mobile. The build-up is where the downfeed and vent pipe meets the return pipe. The system water is over-pumping because there is less resistance in going up the vent pipe rather than having to go through the narrowing in the return pipe, just after the vent pipe. You will have to cut out and clean this section or re-pipe that bit.
Thanks "power flush man" you are absolutely right, I have cut out the area in question and it sure was blocked, it had been totally blocked but I managed to cut a 5mm hole through yesterday so I cut get top-up water in the system.
When I cut out the TOWER air separator and all the pipe around it, could not believe the build-up in the air separator and pipework, the tee at the point where cold feed meets flow pipe from boiler had been totally blocked but now I have replaced all pipework.
System is now working good without any noise and over flow problems.