Central Heating F @ E tank feed pipe gets too hot

[Stuck1]

I have an "S" plan open vent system newly flushed and new boiler, pump, etc etc replaced.
Normal pipe arrangement, one zone for central heating and one for the hot water.
All radiators and water HW tank heating fine BUT.....
Hot water is migrating up the F and E tank feed pipe. By this I mean the pipe which allows water from the tank to enter the system just before the pump.
This gradually makes and the water in the tank hot.
In turn this makes masses of condensation which finds its way into the overflow pipe and drips outside.
(Please note the system overflow pipe is fitted in the right place and is not blocked and hot water is not going up this into the tank. )
QUESTION : Can I safely put a non-return valve in the F and E tank feed pipe which allows water from the tank to enter the system so that cold tank water can run down from the tank but hot water cannot migrate up into the tank.

 

[Madrab]

Can I safely put a non-return valve in the F and E tank feed pipe which allows water from the tank to enter the system so that cold tank water can run down from the tank but hot water cannot migrate up into the tank.

Absolutely not - an open vent system needs to 'breathe' - it's part of its normal function, some expansion is always expected and required, if it becomes excessive then usually that's down to the free flow of the feed and vent water being restricted.

1st thing to test for would be a magnetite restriction where the feed and vent (H) connects to the system pipework. Time for the magic magnet test. Find those connections on the system and test with a magnet on the pipe, if the magnet sticks then that's the issue.

 

[fixitflav]

Hot water is migrating up the F and E tank feed pipe.

The cold feed pipe should enter the circuit from underneath via a U-bend, to reduce convection in the pipe warming the F/E tank.

 

[Nige F]

Absolutely not - an open vent system needs to 'breathe' - it's part of its normal function, some expansion is always expected and required, if it becomes excessive then usually that's down to the free flow of the feed and vent water being restricted.

1st thing to test for would be a magnetite restriction where the feed and vent (H) connects to the system pipework. Time for the magic magnet test. Find those connections on the system and test with a magnet on the pipe, if the magnet sticks then that's the issue.

I had the same problem - the magnet test showed it up . Cutting out the pipe showed a solid part blockage that required poking out with a metal rod. Flushing/chemicals wouldn't touch it.

 

[fixitflav]

(Please note the system overflow pipe is fitted in the right place and is not blocked and hot water is not going up this into the tank. )

Just to be clear, do you mean by this that you are not getting over-pumping (water flowing out of the open vent pipe above the F/E tank)?

 

[Stuck1]

I am not getting over-pumping and it is definitely hot water migrating slowly up the feed pipe
Thanks for your U Bend suggestion though, I think that might help a lot. I haven't had a chance to do this yet but will try.

 

[Johntheo5]

I am not getting over-pumping and it is definitely hot water migrating slowly up the feed pipe
Thanks for your U Bend suggestion though, I think that might help a lot. I haven't had a chance to do this yet but will try.

Can you post the pump make/model/mode/setting, also is your system something like this, where, from the boiler (flow) you have, Vent, Cold feed, Pump, post a photo of your set up.

 

[ChrisJP]

I am not getting over-pumping and it is definitely hot water migrating slowly up the feed pipe
Thanks for your U Bend suggestion though, I think that might help a lot. I haven't had a chance to do this yet but will try.

"This gradually makes and the water in the tank hot.
In turn this makes masses of condensation which finds its way into the overflow pipe and drips outside."

If the water in the feed and expansion tank is getting hot, this is indeed evidence of over-pumping because, to become hot, hot water from the central heating circuit must be entering the tank. Mere convection up the F&E tank connecting pipe will not make the F&E tank water hot.
The normal first thing to try to stop over-pumping, is to turn the circulating pump down a notch.

 

[fixitflav]

"This gradually makes and the water in the tank hot.
In turn this makes masses of condensation which finds its way into the overflow pipe and drips outside."

If the water in the feed and expansion tank is getting hot, this is indeed evidence of over-pumping because, to become hot, hot water from the central heating circuit must be entering the tank. Mere convection up the F&E tank connecting pipe will not make the F&E tank water hot.
The normal first thing to try to stop over-pumping, is to turn the circulating pump down a notch.

I assume the OP knows whether he's getting overpumping or not.

 

[fixitflav]

Can you post the pump make/model/mode/setting, also is your system something like this, where, from the boiler (flow) you have, Vent, Cold feed, Pump, post a photo of your set up.

Is that a pic of your system? I thought you had a combined feed and vent.

3 things I'm not keen on
1. Pumping downwards is non-preferred.
2. The cold feed pipe should enter the circuit from underneath via a U-bend.
3. I wouldn't have open vent across the top of a tee, as it gives a head start for flow up the open vent, increasing the risk of overpumping. Better to come off a horizontal section.

 

[Dereekoo]

Is that a pic of your system? I thought you had a combined feed and vent.

3 things I'm not keen on
1. Pumping downwards is non-preferred.
2. The cold feed pipe should enter the circuit from underneath via a U-bend.
3. I wouldn't have open vent across the top of a tee, as it gives a head start for flow up the open vent, increasing the risk of overpumping. Better to come off a horizontal section.

Interesting my feed system is more or less the same (except for air separator) and no such issues

 

[fixitflav]

Interesting my feed system is more or less the same (except for air separator) and no such issues

I wasn't trying to say it would cause immediate problems, just that if starting from scratch I wouldn't do it that way, unless unavoidable.
Does your F/E tank get warm? The horizontal run might help reduce convection.
Can't see your pump, is it downwards?

 

[Madrab]

You may also have an 'H' pipe setup like the one in @Johntheo5 's pic or have an air separator, as per @Dereekoo 's setup, they tend to be more typical in an OV

With a normal unrestricted system, especially a larger system, the F&E cistern can warm up a bit, especially where the feed and vent are shorter. It's all down to whether the level in cistern doesn't rise too much and when it's cold it drops back down. It can also be down to pump placement if the feed and vent are downstream of the pump. Can you post a pic of how your feed and vent are connected to the system?

Can we confirm that there is no water entering the cistern through the larger vent pipe when the CH is on?

Is this new behaviour? How warm is the system set at?

 

[fixitflav]

the F&E cistern can warm up a bit, especially where the feed and vent are shorter. It's all down to whether the level in cistern doesn't rise too much and when it's cold it drops back down.

The rise in level is due to expansion in the whole system, mainly the rads, not just the F/E tank (if it warms, which ideally it shouldn't!).

Can we confirm that there is no water entering the cistern through the larger vent pipe when the CH is on?

Do you mean coming out of the vent pipe i.e. overpumping? OP said it isn't, but good to confirm.

 

[ChrisJP]

I assume the OP knows whether he's getting overpumping or not.

Well, I was responding to the OP's comment that the F&E tank gets hot enough to produce "masses of condensation" which drips outside via the overflow. I don't think mere heat convection up the F&E pipe would cause this. It has to be hot water coming up the vent pipe and discharging into the F&E tank - ie overpumping.