Hot water flowing from vent pipe into loft tank

[JohnD]

It's totally normal.

No it isn't. Under normal circumstances nothing should come out of the vent pipe. Normal expansion is accomodated by the F&E pipe. But this is too slow to mix in chemicals.

Venting aerates the water and accelerates corrosion.

 

[Madrab]

To be clear ....

Your system shouldn't ever pump over into the F&E tank, is it a safety design feature of an open vented CH system. If it is pumping over then there is either a design fault or the system is restricted somewhere - it probably is in the pipe in between and around the feed and vent as that is a well known typical choke point.
Given you have already tested with a magnet and there is an attraction at that area, that points towards that area being clogged with magnetite then some of the pumped water is taking the path of least resistance, which is up and over the vent and then being drawn back down the feed, that section needs to be replaced in the first instance.

You then need someone with experience of setting up open vented systems. That includes checking that it is piped correctly, the pump is flowing the correct way and then set to what the system requires and the ABV needs to be setup correctly to ensure suitable flow through the boiler on valve closure and overrun.

Unfortunately bouncing back and forward on a forum where, quite frankly, you are now receiving conflicting and incorrect advice is not helping you sort this out.

 

[HERTS P&D]

Unfortunately bouncing back and forward on a forum where, quite frankly, you are now receiving conflicting and incorrect advice is not helping you sort this out.

From other Diyers having a guess at what the problem might be...........this seems to be happening more often now!

Andy

 

[JohnD]

or anywhere else where the water can go.

with the result that in a well-designed and constructed system, water does not come out of the vent pipe.

I have a feeling that in your house, it does. And you have convinced yourself that it is normal.

In your house, do you have valves that can shut off circulation when the pump is running? That's wrong.

 

[DP]

There's no proof of the over pump. Proof is easily achieved with a picture of vented water falling into tank.

The physics would not allow an over pump given the huge column of water inside the vent pipe. I would expect some intermittent overflows when valves shut off. I would expect some dribbles.

Sorry, you lost me there. The F&E cistern water level determines where the water in the vent pipe is ie at the same level as water finds its own level. Air column is NOT infinite or massive or large- it is from water level to open end of vent pipe

Any competent heating engineer know that suction side of pump will be at negative pressure on pumped dude at positive pressure. If you look closely, you would appreciate F&E is configured as close coupled, So, both feed and vent pipes will be at negative pressure UNLESS there is a blockage in the 150mm or less section which makes the vent at higher pressure resulting in pumping over through the vent and sucking down on the body of water in the cistern. Bear in mind the pump is at lowest speed

It has to be further emphasised pump speed is set to overcome system resistance and achieve correct temperature differential and NO pump should be taking place

Of course I could be talking garbage do will be open to education on why the OP is having issues.

OP has said pump is correctly oriented

 

[BlueLoo]

Sorry, you lost me there. The F&E cistern water level determines where the water in the vent pipe is ie at the same level as water finds its own level. Air column is NOT infinite or massive or large- it is from water level to open end of vent pipe

Any competent heating engineer know that suction side of pump will be at negative pressure on pumped dude at positive pressure. If you look closely, you would appreciate F&E is configured as close coupled, So, both feed and vent pipes will be at negative pressure UNLESS there is a blockage in the 150mm or less section which makes the vent at higher pressure resulting in pumping over through the vent and sucking down on the body of water in the cistern. Bear in mind the pump is at lowest speed

It has to be further emphasised pump speed is set to overcome system resistance and achieve correct temperature differential and NO pump should be taking place

Of course I could be talking garbage do will be open to education on why the OP is having issues.

OP has said pump is correctly oriented

Your comment about pumping pressure being strictly positive or negative is not quite right.

You have a head to the system you have to take into consideration.

 

[JohnD]

the 3 way valve suddenly stop feeding into the tank loop.

you have a 3-port valve that you think can shut both sides at the same time?

that's interesting.

 

[Madrab]

have a guess where the water momentum goes when the pump stops

Either through the ABV or rises up the feed into the cistern.

Yes, I do agree that an open vented system will breathe, with expansion of the hot system water rising up through the feed into the F&E cistern. The vent on the other hand is only designed to be there as a safety feature to allow the system to vent air/gas or water if there are problems and the system starts to over pressurise, it then provides an open path to air, that's why there should never a valve on the vent pipe and why the vent rises a min distance up and over the F&E cistern. It also why a combined feed and vent isn't a desired fix as it can mask incorrect circulation through the F&E cistern.

As @DP suggests -

It has to be further emphasised pump speed is set to overcome system resistance and achieve correct temperature differential and NO pump should be taking place

the pump in an open vent is not there to act as a pump per se, providing any real pressure (head) as such, it is there as a circulator and should only provide enough 'pressure' to overcome the system resistance and excess pressure would then need to find an outlet and the only place there could be would be the open vent and were back to pumping over.

 

[DiyNutJob]

you have a 3-port valve that you think can shut both sides at the same time?

that's interesting.

If one flow out of two shuts, the doubled momentum has to go somewhere. So up the vent it goes.

 

[JohnD]

If one flow out of two shuts, the doubled momentum has to go somewhere. So up the vent it goes.

No, up the feed and expansion pipe. And there is very little. You have not created extra water.

 

[denso13]

Also it's under a greater pressure from a tank full of water

No, same pressure in both the feed and vent pipe. It doesn't matter how big the cistern is, it is the height of the water which creates pressure and will be the same for vent and feed.

 

[Madrab]

It doesn't.

Regardless of what you think, yes it does

the water momentum is dissipated into the vent

Regardless of what you think no it isn't, the water level in the vent may rise by the same amount in the feed as their pressure is the same but it is not designed to vent water over the top unless it's under fault conditions

 

[BlueLoo]

No, same pressure in both the feed and vent pipe. It doesn't matter how big the cistern is, it is the height of the water which creates pressure and will be the same for vent and feed.

Well, not quite.

If they are close together, yes (hence the 150mm rule of thumb).
If they are far apart, they are the same as the DP in that particular part of the system.

 

[Madrab]

Having a valve on the vent pipe would allow control of how much venting is done. In this case, it isn't necessary. You incorrectly assume a valve would be there to close the vent off

there should never be a valve on the vent pipe

Open vent CH system design 101, hence why it's called open vent

 

[denso13]

If they are close together, yes (hence the 150mm rule of thumb).
If they are far apart, they are the same as the DP in that particular part of the system.

With the pump off the water level in the F&E cistern and vent pipe will be same, therefore having the same pressure. It doesn't matter how far apart they are.