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Discuss Hot water venting into header tank, this issue not covered on previous posts in the Plumbing Forum | Plumbing Advice area at PlumbersForums.net

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Hi all, I'm new to this forum, I apologise if I miss any protocols

My issue on the face of it has been covered many times, but specifically in this instance it makes no sense and I have not yet found a reasonable answer, if you can help it would be very much appreciated.

I have an open vented CH/HW system. For some time there has been a reasonable trickle of hot water being emptied into the header tank via the vent. The pipework is installed correctly, the vent and the cold feed are less than 150mm apart, and in the right sequence from the boiler outlet.

What makes no sense to me as that this ONLY happens when the HW is demanded of the boiler and the Y valve is set to provide both CH and HW. if there is NO demand for HW then the vent remains dry (CH on full tilt)

Given that when there is a demand for both HW and CH the water has more pipework to flow around and therefore the pressure should be reduced, therefore making the overflow LESS likely, So why is this happening?

I have carried out many power flushes, treatments of all kinds - today I have bled the system right down after 2 days of sentinel 800 and still the problem persists.

It happens on pump speed of 2 or 3, and my vent pipe is the correct height above the header tank.

Thanks.
 
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Yes it is - the vent, feed, and pump inlet are all in line and not more than 200mm apart start to finish
 

townfanjon

Esteemed
Plumber
Gas Engineer
Advent Win
It being close coupled should mean the vent and feed should be sludge-less lol . Blocked feed and vents usually cause your symptoms. Did the system fill quickly when you refilled ?
 
I think they are sludgeless, no attraction when I put a magnet anywhere near the junctions or on the pipes. Yes it re-filled fairly quickly - My house is a but of a nightmare hybrid between 15mm and microbore which does affect filling times, but I really don't think there is a blockage on either pipe. Took half an hour to fill and bleed the whole house? (9 rads).

Just doesn't make sense to me that turning the HW on can have a negative affect, when it should reduce system pressure. Pump runs at the same speed regardless, is the boiler clever enough to wack a load of extra heat into the system when there is additional demand from the HW? enough overcompensation to send water up the vent? I doubt that too as I don't see a temp spike on the boiler output readings.
 
You might have sludge/blockage of a type that is not magnetite and hence won't be detectable with a magnet.

If not, it's a long shot, but are you sure you have pump pumping in the correct direction? People do sometimes fit them incorrectly.

If that's not the answer, perhaps a sketch of you pipework would help us figure out what's going on.
 
Pump is definitely in the right direction. You are right there may be other sludge in the cold feed, but it does drain down fairly well, so it's not blocked.

Here is a pic of the area leading to the pump, the vent is the 22mm pipe with a 90 deg in it, just above that in 15mm pipe is the cold feed, both of these join the 22mm feed into the bottom of the pump. The pipe in the foreground is the return from the hot water cylinder.
Post automatically merged:

More testing this morning:

Turn on the HW on it's own, immediate dribble through vent into header tank.

Turn on CH as well, dribble gets no worse but is still there

Turn off HW, leaving CH on, no dribble at all - immediately disappears.

So if the cold feed was blocked in any way surely I would have the same issue with either of the CH or HW on?

Suggests to me that there is blockage or restriction in the pipes inside the hot water tank?? (my hot water does not take long to get warm so it's not very occluded)

Could there be an issue with the y valve? only partially opening when the HW is switched on?
 

Attachments

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Its a strange one.
I take it from post #1 that there is no problem on pump speed 1?, if that is a UPS 2 pump with proportional pressure (PP) control then fixed speed 1 is 4M, 2 is 5M and 3 is 6M, one would think that speed 1 should be sufficient, also is the cylinder return combined with other returns ?, it may be acting as a unrestricted by pass, if that red gate valve isn't a balancing valve on the cylinder return then you might consider fitting one and see if that has any effect. You could also try the pump on PP3 setting but if a fixed speed1 setting of 4M isn,t sufficient then IMO PP3 will be too weak also at < 3M.
 
It is the same on pump speed 1, but I don't use that setting as the boiler doesn't like it so much, it keeps going into anticycle mode as the flow isn't quick enough (I blame the old microbore)

Yes the cylinder return goes straight down past that red valve you can see in the picture and joins into the 22mm CH return to the boiler.

I have never adjusted the red gate valve, I suspect that was fitted as an isolation rather than a balancing valve, It is just set to full open currently.

So two things I am considering now - is it possible to just flush the coil in the cylinder in isolation somehow? in case this is gunked up internally and causing the restriction? (when I turn the HW on it takes a few seconds for the return pipe at the bottom of the cylinder to get hot, not sure what would feel normal in a clean system). Secondly - I am thinking my 3 way valve may not be perfect, I can't latch it into manual open mode - the lever seems floppy and not connected to anything, also as I switch CH-HW etc. the range of motion of the valve looks limited to me. I might replace this, allowing me to see what the pipework looks like inside that junction and ruling this out as a restriction when HW is selected - what do you think? if you agree - what is your opinion of Neomitis 3 way valves i.l.o. the honeywell I have? they are about a third of the price and apparently are like for like in terms of dimensions, I have no experience of them at all though...

Thanks all for your help so far.
Post automatically merged:

contrary to the above - I have just retested on pump speed 1 and all is fine (having messed around with the boiler settings a bit) no water coming out of the vent in any scenario. Change pump speed to 2 and the water comes out of the vent instantly, back to pump speed 1 all is ok.

Problem is pump speed 1 means it takes an age for the rads to heat around the house.. - so I think my two previous questions still stand...
 
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Can't comment on the 3 way valve as I have no experience of them.
I would firstly ensure that the CH&HW are both circulating then just shut that balancing valve fully (takes 4 to 5 full turns) and see if that dribbling has stopped, if it has then re open the balancing valve say 1 full turn, if still no dribbling shut off the CH and observe it again. I just think that the cylinder coil is acting as a 3/4 ins full by pass which is also causing the cycling, at any rate if it does stop the dribbling it will have proved something for nothing. Normally, a 1/2 to 3/4 of a full turn from closed should give quite acceptable cylinder heat up.
Post automatically merged:

It is the same on pump speed 1, but I don't use that setting as the boiler doesn't like it so much, it keeps going into anticycle mode as the flow isn't quick enough (I blame the old microbore)

Yes the cylinder return goes straight down past that red valve you can see in the picture and joins into the 22mm CH return to the boiler.

I have never adjusted the red gate valve, I suspect that was fitted as an isolation rather than a balancing valve, It is just set to full open currently.

So two things I am considering now - is it possible to just flush the coil in the cylinder in isolation somehow? in case this is gunked up internally and causing the restriction? (when I turn the HW on it takes a few seconds for the return pipe at the bottom of the cylinder to get hot, not sure what would feel normal in a clean system). Secondly - I am thinking my 3 way valve may not be perfect, I can't latch it into manual open mode - the lever seems floppy and not connected to anything, also as I switch CH-HW etc. the range of motion of the valve looks limited to me. I might replace this, allowing me to see what the pipework looks like inside that junction and ruling this out as a restriction when HW is selected - what do you think? if you agree - what is your opinion of Neomitis 3 way valves i.l.o. the honeywell I have? they are about a third of the price and apparently are like for like in terms of dimensions, I have no experience of them at all though...

Thanks all for your help so far.
Post automatically merged:

contrary to the above - I have just retested on pump speed 1 and all is fine (having messed around with the boiler settings a bit) no water coming out of the vent in any scenario. Change pump speed to 2 and the water comes out of the vent instantly, back to pump speed 1 all is ok.

Problem is pump speed 1 means it takes an age for the rads to heat around the house.. - so I think my two previous questions still stand...
IMO because as stated above you are re circulating too much water through the "by pass" so leave it on speed 1 and throttle in the balance valve to ~ 1/2 to 3/4 open and that will increase the circulating flow through the rads and still give sufficient flow through the cylinder coil without any pump over.

You can also try it then on speed2.
 
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EvilDrPorkChop

Gas Engineer
Advent Win
I would agree with @John.g too, my first thought on reading the thread was that it's going around the cylinder coil too quickly. If there isn't one, you need to fit a gate valve to the cylinder flow or return to balance down the cylinder coil. This should solve your issue. I would then set the pump on speed 2.
 
Thanks guys.

Yes there is an immersion fitted, but it's never used.

JohnG and EvilDr, ok I could try that, you think the water is travelling too fast through the coil whereas I was thinking it was flowing too slow! I could try to use the red gate valve as a balancer and restrict the flow with that one, if that fails I will drain down and fit a different valve there.

Thanks.
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Just tried setting pump speed to 2, and adjusting the red gate valve further and further closed in increments, I'm afraid the water never stops coming out of the vent unless the gate valve is closed fully, which of course knackers the boiler.. back to pump speed 1 and all is ok again, so it doesn't seem that throttling the cylinder coil makes any difference.
 
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Last Plumber

Esteemed
Plumber
Gas Engineer
Advent Win
Pump is definitely in the right direction. You are right there may be other sludge in the cold feed, but it does drain down fairly well, so it's not blocked.

Here is a pic of the area leading to the pump, the vent is the 22mm pipe with a 90 deg in it, just above that in 15mm pipe is the cold feed, both of these join the 22mm feed into the bottom of the pump. The pipe in the foreground is the return from the hot water cylinder.
Post automatically merged:

More testing this morning:

Turn on the HW on it's own, immediate dribble through vent into header tank.

Turn on CH as well, dribble gets no worse but is still there

Turn off HW, leaving CH on, no dribble at all - immediately disappears.

So if the cold feed was blocked in any way surely I would have the same issue with either of the CH or HW on?

Suggests to me that there is blockage or restriction in the pipes inside the hot water tank?? (my hot water does not take long to get warm so it's not very occluded)

Could there be an issue with the y valve? only partially opening when the HW is switched on?
You must currently have a positive pressure on the vent and negative on the cold feed and a similar (slightly less) resistance to that of the DHW circuit, if that makes sense?

I would alter the pipework to suit the following diagram. This is how we always piped open vented systems. You should have a neutral point in the system where the cold feed enters. If the vent connection is within the right distance and the height of the F&E tank and the vent are good, then there must be a reason why there is a pressure difference between those two points which is enough to cause circulation.

If you don't cure it by doing this, then there is obviously another cause but in my opinion you are better to rule out the possibility of it being something as simple as pipework configuration.

Note: Cold feed is inverted.


1573138892736.png
 

EvilDrPorkChop

Gas Engineer
Advent Win
You must currently have a positive pressure on the vent and negative on the cold feed and a similar (slightly less) resistance to that of the DHW circuit, if that makes sense?

I would alter the pipework to suit the following diagram. This is how we always piped open vented systems. You should have a neutral point in the system where the cold feed enters. If the vent connection is within the right distance and the height of the F&E tank and the vent are good, then there must be a reason why there is a pressure difference between those two points which is enough to cause circulation.

If you don't cure it by doing this, then there is obviously another cause but in my opinion you are better to rule out the possibility of it being something as simple as pipework configuration.

Note: Cold feed is inverted.


View attachment 41360
If you was going to start messing with pipework though I'd just change it to a combined Feed and Vent. That'll sort the issue.
 

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