Venting into header tank

I have an intermittent boiler venting problem which has proved difficult to diagnose.

Background: I have lived in the same house for some thirty years, and the core pipes (I believe they are cast iron, and quite thick) for the heating system must be at least 10 years older than that. Various radiators have been replaced over the years. Anti corrosion liquid has been added whenever the system had to be drained. The oil fired system serves 17 radiators in four bedrooms and three reception rooms (though never are they all heated at once).

A conventional boiler was replaced by a condensing boiler (with a magnetic filter) with a rating of 90-120 (the same as the previous boiler) in November 2018. I believe that prior to that replacement the hot water system was gravity fed. Now there are motorized valves (but no thermostatic room controls). The hot water tank was replaced about four years ago following a leak. The pump was replaced earlier this year, and the system has been much more silent since the new pump was fitted.

For just over a year the new boiler functioned just as well as the old one. It was fitted by someone who had done work for me before and whom I regarded as trustworthy; but on this occasion I found his general performance less satisfactory – delays, postponements, undue haste etc. He has been difficult to contact since then, and I have lost confidence in him.

The day after it received its first annual service the pump sprang a leak, and the joints had to be re-fitted. The following day the boiler vented hot water into the header tank. I presumed that this was a coincidence (my wife was less confident!). I was advised to keep the pump running slow, and to ensure that not too many radiators were turned off. I did this, with no further problem. Except that on the only two occasions when I ran the system on hot water only (by accident: - this is not something I normally need to do) the system vented into the header tank, which overflowed.

Yesterday the heating was off for a few hours while we were out. We switched the heating on again, and about 20 minutes afterwards with much gurgling and banging the system vented again. The boiler was obviously very hot. When it cooled we put the heating on again – nervously! – and it has functioned normally since then (I took the precaution of turning on some extra radiators, though when the venting happened there were no fewer radiators on than had been the case earlier, when all functioned normally).

I am at a loss to understand why it should have vented after functioning normally for some time, with no difference in settings or circumstances. But I am left feeling nervous!

I have sought advice from two local plumbers, but they cannot offer an explanation.

Could it be that the boiler is too powerful? I was told that a lower output would be adequate, but the new boiler is the same rating as the old one.

Another thought: there was a gateway valve fitted to the hot water tank when the new boiler was put in. Is it possible that this is set too low, and not enough flow is getting through to the hot water tank? Might this account for the problem? (I could always open the valve a little, but I am reluctant to adjust setting which I do not properly understand; and anyway, why should this be a problem so intermittently?

For completeness: there is one radiator (upstairs) which I find I have to vent very frequently – which may indicate a leak somewhere?

It has been suggested to me that a full power flush of the system might help. Is this worth trying? Any downsides to doing that?

1. Have you checked / cleaned the magnetic filter since it was installed?
2. Next port of call would be to investigate a full or partial blockage of the feed from the small feed and expansion tank in the loft into the system. Can get sludged up with magnetite / general detritus over time.
3. A full power flush might be useful, but with 17 radiators I'd expect a proper powerflush to take 1 1/2 days and cost in the region of £600.

You could also simply have an air lock that's causing the problem.

Put some cleaner in it for a time, flush it out, full drain down, open all the bleed screws, close them when drained, don't bleed any rads, closing all the rad bleed screws go round opening just one rad at a time as you do this see what's coming out of the drain hose. Close that rad and move on to the next.

You might be suprised at what comes out ! When you fill it open one rad at a time to fill and bleed starting downstairs and work your way up.
Once filled and working if you can hear the sound of rushing water you've got air in the system. Fully filled you can't hear air or water movement.

I am grateful to both of you for these comments. I can confirm that the magnetic filter has beeen regularly cleaned.


What puzzles me most is the intermittent nature of the problem: it is only now and again that it occurs, and I would have expected it to be fairly constant if there was an obvious blockage. Also the fact that it seems to be linked ot he Hot Water Only setting.

Your problems do sound like an airlock- quite common on the hot water side (is there a bleed valve on the primary pipework top of the coil)?
The valve on the hot water primary side- determine where it is now (by winding it all the way anticlockwise- open- and counting the turns needed, then close it back to where it was, then close it all the way (clockwise) again counting the turns needed. Finally, set it to 1 turn open from where it was, write the numbers down somewhere.
EDIT The valve is there to prevent too much primary water going throughthe cylinder & starving the radiators.

Thank you very much for that helpful suggestion.

AN airlock is interesting! Would that be consistent with the intermittent/ hot water only nature of my problems?

Are your suggestions about pening the valve intended to address the air lock, or (as I think is more likely) to facilitate the flow of water to the cylinder?

When you say:"The valve on the hot water primary side" do you refer to what I have described as the gateway valve, ie a valve on a pipe that (presumably coming from the boiler) branches off to go into the hot water cylinder about one third of the way up the cylinder and which pipe then continues to rise towards the loft (I presume as some sort of venting system)? I have wondered whether this valve might be set too closed and thus restricting the flow of water from the boiler into the hot water cylinder and thus accounting for the venting into the loft header tank;but I have been reluctant to interfere with something that I do not fully understand!

If I have interpreted what you are saying correctly then I will certainly try what you suggest. Presumably it could do no harm since I could always re-set the valve to its current setting if necessary?

You ask:
is there a bleed valve on the primary pipework top of the coil? I can only say that I do not see anything that lookslike a bleeder valve on any of hte pipes that rise up out of the hot water cylinder.

Read more: https://www.diynot.com/diy/threads/venting-into-header-tank.565804/#ixzz6okMndurr

Read more: https://www.diynot.com/diy/threads/venting-into-header-tank.565804/#ixzz6okKTufSn"

Yes the gateway valve is the one i'm referring to.
It would do no harm to run full system with that valve wide open for an hour or so- if there is air trapped in the coil that might shift it to where it can be bled out.