hot water venting into expansion tank

can anyone help with this problem please? I have hot water venting into the expansion tank in the loft whenever the central heating boiler comes on. I think it has been doing this for a while as I have heard water trickling into a tank say early in the morning when still in bed and the heating and/or hot water comes on. I have onle just been up to investigate when putting xmas decs away. The system is 13 yrs. old and has been serviced yearly, the house was new when we moved in. the boiler is a Glow Worm economy plus. the only other problems we have had is a pupm replacement about 9 or 10 years ago (can't remember why) and some small part on the boiler replaced a few years back. I used to check the loft tanks 3 or 4 times a year as the expansion tank had a leak when we moved in (it was fixed) but decided to keep an eye on it, but have not checked it since early in 2008. I checked the measurements of the pipework on a website and they are all ok. Could the pump be at fault? I think it is set on the lowest setting but not clear as to the markings on it.
Has anyone any ideas why it is doing this?

blockage in/near cold feed/open vent is quite likely.

Thanks mick, not sure where you mean I'm not too good with plumbing, could you expand a bit please. Also someone told me another reason could be a hole in the coil inside the hot water tank, is this possible? I hope it's not that! Sounds expensive.

pandaman said:

Thanks mick, not sure where you mean I'm not too good with plumbing, could you expand a bit please. Also someone told me another reason could be a hole in the coil inside the hot water tank, is this possible? I hope it's not that! Sounds expensive.

Click to expand...

Have you checked to see if your pump is running. This can lead to the water overheating and rising up thriugh the vent

a very common cause is that there is black sediment (iron oxide) causing a partial blockage. This most often occurs at the tee where the 15mm feed and expansion tank in the loft joins onto the larger circulating pipe, probably near the pump in the airing cupboard or by the boiler. If this is the case, hold a strong magnet to the pipe, and you will find it is attracted if there is a lump of sediment inside.

You might be able to loosen this with a mild chemical cleaner like Sentinel X400 which will cost you £15. Otherwise you will have to drain down, cut out the blocked section and relace it. (after this you ought to do a chemical clean anyway to remove some of the remaining sediment in the system) so it is certainly with a try.

You will know the X400 is starting to work when the circulating water goes inky black with loosened sediment.

after 4 weeks circulating, drain it out, rinse and refill using a corrosion inhibitor such as Sentinel X100.

this problem is very common with older, open vented systems.

If you can afford £100 and do basic DIY plumbing, fit a Magnaclean as well. I guarantee you will be amazed and delighted at how much sediment it traps, and it will prevent future blockages.

note that a chemical clean can only work while the water is still circulating. Once any pipe becomes completely blocked the chemical cannot flow through it.

Thanks dan2 pump is running ok, I can hear it whirring away, especially on the over run.
Also thanks JohnD will try to investigate your solution, thanks to all for trying to help. If anyone else has any other suggestions they will be welcome.

Thanks JohnD have tried the magnet on the T join where you said and it sticks to it well all round that area. You say this can cause a partial blockage. If the pipe is only partially blocked would this cause the pipe to vent the hot water into the tank? The heating seems to be working as well as it has done before. Also can I just pour the X400 straight into the expansion tank and leave it to try and clear the debris. Thanks.

a partial blockage can cause the flow to go up the vent as the pump can't get it round the right way. It is worth a try with X400 as it will be easier for you than cutting out and remaking the section (but that would be the better fix).


tie up the ball valve and bale out any mud and sediment from the F&E first (you don't want to add to the muck in your pipes). Then draw off a couple of gallons of water from the drain cock downstairs. Then tip in your X400 into the clean F&E and untie the ball so it refills, stir it well to mix it and it will be drawn down into the system to replace the water you drained out.

Observe the colour of the water, it should quickly go inky black with loosened sediment.

X400 works for about 4 weeks before it loses its power, so drain down at the end of that time and rinse before refilling with an inhibitor such as X100. If the pumping over has not stopped before then, you will have to cut out that tee. You will find it easier to remake with compression joints as it is very difficult to solder wet pipes.

Thanks JohnD for rapid reply, I have cleaned out tank already, all brown sludge in bottom of tank, that stuck to magnet too. Let it refill for now hoping to go and get some X400 tomorrow. Thanks again.

JohnD, can I draw on your knowledge again please. I have put a 1 litre bottle of x400 into the CH system as you advised. I drained off about 2 gallons of water from the small rad in the downstairs loo as it has the drain valve on it. Afterwards that rad is cold, so assumed an air lock. Tried turning off all the other rads downstairs and turning them on in various combinations to no effect. The pipe on the other side of the rad away from the on/off valve is hot as far as the rad. drain vave on that side too. This may seem a silly question but does the boiler have to be on or off to try and sort this out. Have you any ideas?

if you mean all the other rads are hot and this one is cold, sounds like a blockage.

You have tried turning off all the others to try to force the flow to this one, and it has not worked Make sure that the flow is not escaping through the hot water cylinder feed, and turn up the pump for a bit.

You might be able to trace the blockage with your magnet.

however you say the Flow pipe is hot up to that rad, so there must be a bit of flow through it for the hot water to have got that far. Check both the valves are open on the cold rad. If it has a TRV, take the top off and see if the pin is moving freely. If there is any flow through it, there is a chance the X400 will soften it.

However the problem you were trying to resolve was the partial blockage at the feed, so while that is resolved, you might have to put up with one blocked rad for a bit and cut out the blockage later.

Consider closing the valves at each end of the cold rad, taking the rad outside and squirting it through with a hosepipe.

Thanks again JohnD turned all rads off this time, didn't turn them all off before, this did the trick, all rads hot again. Will now wait to see if X400 does the trick. Many thanks again for your advice.