Hot water venting into header tank, this issue not covered on previous posts

[karlrich1]

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.

 

[townfanjon]

Wheres the pump ? it should be with the vent and feed .

 

[karlrich1]

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

 

[townfanjon]

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 ?

 

[karlrich1]

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.

 

[Chuck]

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.

 

[karlrich1]

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.
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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?