Heating overflowing from vent

Morrispd · May 29, 2023

Hi, I'm after some help regarding my Father in laws heating system.
He has had a local engineer around but hasn't had a fix and is very remote so I'm looking at the issue before arranging someone to try to fix. He is on oil and has a modern raeburn boiler/range. He has a vented system with wented hot water tank, s plan wiring and the CH vent pipe goes out of the roof possible 1m above the highest rad (rooms in roof). When the CH is on he is losing water out of the vent pipe and has to bleed the highest radiator daily. I'm assuming this is expansion causing the overflow but why hasn't this always happened? He is obviously diluting any inhibitor daily and wasting a lot of water, where is the problem likely to be? If you need any more info about the system please let me know, I'm an engineer and quite handy. I just can't get my head around what the fault may be and as nothing has changed I can't see how there is an inherent problem with the install. TIA for any insight.

ShaunCorbs · May 29, 2023

Anything been changed ?

Morrispd · May 29, 2023

ShaunCorbs said:
Anything been changed ?

I don't think so

exedon2 · May 29, 2023

Some clear pictures of pump motorised valves and how open vent/cold feed is connected would be useful or a sketch of pipework layout?
Which model Rayburn? from memory on the central heating cookers Rayburn ask for a close coupled open vent/cold feed (max 150mm between)
You can also run then as a sealed system the prv needs to be 2 bar.

John.g · May 30, 2023

As above, photo handy, if the system has a control stat and a overetemp stat then can be converted to a combined cold feed and vent like my 50 year old system which will never pump over or draw any air in.

Morrispd · May 30, 2023

exedon2 said:
Some clear pictures of pump motorised valves and how open vent/cold feed is connected would be useful or a sketch of pipework layout?
Which model Rayburn? from memory on the central heating cookers Rayburn ask for a close coupled open vent/cold feed (max 150mm between)
You can also run then as a sealed system the prv needs to be 2 bar.

Morrispd · May 30, 2023

There is the stat on the raeburn and on the cylinder. Cold feed is to the 2 tanks, CH and DHW tanks. Dribbling from the vent in the roof is only when the central heating is on and is greatly reduced if the ch temp Is reduced on the boiler

Morrispd · May 30, 2023

John.g said:
As above, photo handy, if the system has a control stat and a overetemp stat then can be converted to a combined cold feed and vent like my 50 year old system which will never pump over or draw any air in.

I don't know why it hasn't always been like this as nothing has changed so am tring to identify a fault that may not be there, what is involved with combined feed and vent?

exedon2 · May 30, 2023

On that model of Rayburn if open vented the manufacturers instructions ask for a 22mm open vent and a 22mm cold feed in a close coupled arrangement.
You can also use sealed system that model is ok with standard 3 bar prv.
System should have had system auto bypass fitted as there is overheat stat that runs the pump .

Morrispd · May 30, 2023

How do I identify this
"System should have had system auto bypass fitted as there is overheat stat that runs the pump"
Pump does seem to run unnecessarily but current setup has been in 12 years without requiring excessive bleeding and without losing water from the vent.

John.g · May 30, 2023

Morrispd said:
I don't know why it hasn't always been like this as nothing has changed so am tring to identify a fault that may not be there, what is involved with combined feed and vent?

Look up "Vent Pipe Blanked Off" in Search.

Morrispd · May 31, 2023

Thanks for your help so far, I've looked into this a bit now and water pours out of the vent onto the roof when the CH is on. I've turned the pump speed down to 1 and it still does it, I've sketched the layout and I don't think it's anything unusual (but i may be wrong) and I've attached a pic of the feed and vent, both in 22mm. I'm wondering if a blockage could be causing this and how you would begin to clean a vented system (I have flushed sealed systems many times) TIA (again).

John.g · May 31, 2023

It's essential correct but the vent should really be installed closer to the cold feed, or visa versa, the closer the better but no further than 150mm apart, but you can't beat the combined cold feed and vent IMO.

Morrispd · May 31, 2023

John.g said:
It's essential correct but the vent should really be installed closer to the cold feed, or visa versa, the closer the better but no further than 150mm apart, but you can't beat the combined cold feed and vent IMO.

Sorry for my ignorance, the feed from the CH header tank joins the return to the boiler in the same place (2 adjacent tees) as the vent out the roof joins the return to the boiler. These connections are maybe 2m from the boiler and 1m max from the pump. Where should they be and what pipework would change to make combined feed and vent?

John.g · May 31, 2023

Will reply in more detail tomorrow but the vent is on the flow from the boiler? The pump is pumping from the flowside of the boiler through the mot.valves?

Morrispd · Jun 1, 2023

John.g said:
Will reply in more detail tomorrow but the vent is on the flow from the boiler? The pump is pumping from the flowside of the boiler through the mot.valves?

Yes, based on the arrow of flow throught the pump the feed and vent are on the pipework from the boiler, to the pump and then through the motorised valves

John.g · Jun 1, 2023

Morrispd said:
Yes, based on the arrow of flow throught the pump the feed and vent are on the pipework from the boiler, to the pump and then through the motorised valves

OK, but from the boiler it should be Vent then Cold Feed then the Pump, (VCP), not cold feed ,vent, then pump.

To convert it cut the feed (on the horizontal) close to where it joins main line and cap it, maybe cut in the middle of the short horizontal section and cap it with a compression cap & olive and leave in case you need to reverse the mod, then at the F&E tank, T in the feed to the vent, like mine, below. You will obviously have to tie up the ball cock and partially drain down the system, when finished fill the system very slowly to avoid trapping any air or better again install a isolating gate or lever valve on the mains feed to the F&E tank, you can then use this to refill the system very slowly.

Or T it in like the one in post#11.

Heating overflowing from vent

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