DIY Doctor UK DIY Forums

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.

Anyone got any ideas???

Water should never be coming out of the expansion vent pipe if the heating is working correctly. The question is though; was the water boiling or not?

How hot is the header tank?

Normally boiling water would be associated with loud knocking and banging noises.

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.

Thanks "plumbbob" for your replies, as an engineer myself I find them a bit academic, my hope was that an heating engineer who had experienced a simular problem and could offer advise.

Thanks for tying to help.

Finding an engineer with experience won't be a problem, but what will is trying to narrow down the possibilities of what could be happening.

If I wrote every possibility of what might be wrong it would end up being a tome. What we need to do is try and identify more symptoms so the search can be narrowed down.

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.

Thanks for your help.

dbwoody wrote:Grundfos have said that it could need replacing, so I have got a new pump and will fit it this week.

True,if the pump is momentarily seizing or slowing then the water trapped in the boiler will begin to boil and might make the noises you describe.

Grunfos are also right in saying pumps can deteriorate over time, but then, many are working fine after 30 years service.

When you drain down, use a strainer on the hose end to collect, for inspection, any large particles that might be responsible for jamming the pump.

I used a strainer last week when I flushed the system and found some small debris but nothing significant but I have now installed a SPIROTRAP filter.

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.

Thank you for your comments, I will certainly investigate, I have no problem with this area when I refilled the system after flushing though, but as I say I will cut out the pipe and have a look.

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.

Thanks once again.