My open vented central heating system suffered from air ingress so I
had it converted to a sealed system. However, this has not cured the
problem, air still collects in one of the upstairs radiators. I've
done the hydrogen test and it really is air. Any idea how it can be
getting in? The at cold system pressure is constant, so I presume
there are no leaks. As the air collects the pressure doesn't rise but
when I bleed it out I then have to replenish the system via the filling
loop to restore the normal "at cold" pressure. It makes no sense to
me!

Honeymonster wrote:
My open vented central heating system suffered from air ingress so I
had it converted to a sealed system. However, this has not cured the
problem, air still collects in one of the upstairs radiators. I've
done the hydrogen test and it really is air. Any idea how it can be
getting in? The at cold system pressure is constant, so I presume
there are no leaks. As the air collects the pressure doesn't rise but
when I bleed it out I then have to replenish the system via the filling
loop to restore the normal "at cold" pressure. It makes no sense to
me!

It means you have a leak somewhere. Even with a pressurised
system, air will leak in through leaks. Such leaks can be hard
to find as the moisture can dry on hot pipework before it gets
noticed.

If the leak is slow, I suggest waiting until you switch the
heating off for summer, and then use the filling loop to increase
the cold pressure to match what the highest hot pressure is.
Then go round looking for leaks which will hopefully become
more obvious. Also bear in mind the leak uses up your
inhibitor more quickly than normal and you might want to
schedule replenishing that at the same time, when you've
fixed the leak.

One other remote possibility -- what is your cold pressure, and
how high above the pressure guage are the highest heating pipes
or radiators? If you have the guage in the basement of a 3 or
more story house such that there's more than 30' head of water
above it, you might need to increase the cold pressure of the
system to ensure it's all pressurised all the time.

--
Andrew Gabriel

I reluctantly have to agree that there *must* be a leak in the system
somewhere. I guess what I really need is for it to get worse. For the
record the pressure gauge is at the same level as the highest radiator.
Since air gets in only in CH mode (not DHW mode) I guess the problem
might be close to the suck side of the pump?

Honeymonster wrote:

My open vented central heating system suffered from air ingress so I
had it converted to a sealed system. However, this has not cured the
problem, air still collects in one of the upstairs radiators. I've
done the hydrogen test and it really is air. Any idea how it can be
getting in? The at cold system pressure is constant, so I presume
there are no leaks. As the air collects the pressure doesn't rise but
when I bleed it out I then have to replenish the system via the filling
loop to restore the normal "at cold" pressure. It makes no sense to
me!

Are you sure that it is air? If you do not have enough inhibitor you
could be getting a reaction that is releasing hydrogen and this would
have the same effect in the radiators.

I don't know how safe it would be to test with a match as you bleed a
radiator.

Andrew

I know the system has plenty of inhibitor in it. Maybe I should squirt
in some Fernox leak sealer and see what happens.

Honeymonster wrote:
My open vented central heating system suffered from air ingress so I
had it converted to a sealed system.

Check static pressure is adequate for the system. What is the height,
where is the fill point and pressure gauge?

Any automatic air vents, where are they?

Check system layout. You should be pumping away from the expansion
tank.
The pump generates a pressure difference, PD.

The expansion tank connection is the point of no pressure change, the
pressure change is negligible with the pump on or off. There is a
constant static pressure, SP, applied at the expansion tank.

If pumping away from the expansion tank, the pressure at the pump inlet
will be SP, pressure at pump outlet will be SP+PD.

If pumping towards the expansion tank pressure at pump outlet will be
SP. Pressure at pump inlet will be SP-PD. NB this could be negative at
the pump inlet and for some way upstream, especially at high level. Air
will leak into the system.

In article .com,
"Honeymonster" writes:
I reluctantly have to agree that there *must* be a leak in the system
somewhere. I guess what I really need is for it to get worse. For the
record the pressure gauge is at the same level as the highest radiator.
Since air gets in only in CH mode (not DHW mode) I guess the problem
might be close to the suck side of the pump?

You can't make any such assumption.

When the system is at its hottest, the water holds the minimum
of dissolved air and the excess is forced out into top of the
radiators (most commonly just one or two, depending on pipe
layout). When the system cools down, the water is ready to
dissolve more air. An air molecule just next to the leak will
see a 50% vacuum in the water in the heating system, even if
the water is still pressurised, and will be sucked into
solution in the water. Next time the water is heated, half the
air in solution will again be forced out into bubbles. Air,
being much less viscous than water, can leak in faster than
the water leaks out in some cases, although its likely to be
limited to the speed it can diffuse through the water in the
leak in most cases.

--
Andrew Gabriel

On Thu, 02 Feb 2006 03:55:55 -0800, Honeymonster wrote:

My open vented central heating system suffered from air ingress so I
had it converted to a sealed system. However, this has not cured the
problem, air still collects in one of the upstairs radiators. I've
done the hydrogen test and it really is air. Any idea how it can be
getting in? The at cold system pressure is constant, so I presume
there are no leaks. As the air collects the pressure doesn't rise but
when I bleed it out I then have to replenish the system via the filling
loop to restore the normal "at cold" pressure. It makes no sense to
me!

plenty of background info in the FAQ below.

--
Ed Sirett - Property maintainer and registered gas fitter.
The FAQ for uk.diy is at http://www.diyfaq.org.uk
Gas fitting FAQ http://www.makewrite.demon.co.uk/GasFitting.html
Sealed CH FAQ http://www.makewrite.demon.co.uk/SealedCH.html
Choosing a Boiler FAQ http://www.makewrite.demon.co.uk/BoilerChoice.html

On Thu, 02 Feb 2006 06:21:03 -0800, Honeymonster wrote:

I reluctantly have to agree that there *must* be a leak in the system
somewhere. I guess what I really need is for it to get worse. For the
record the pressure gauge is at the same level as the highest radiator.
Since air gets in only in CH mode (not DHW mode) I guess the problem
might be close to the suck side of the pump?

Where is the expansion vessel in relation to the pump?
It might just be that on its highest setting with the expansion vessel
near the outlet that the pressure on the inlet side can get seriously
reduced.

Running the system with a higher than normal starting pressure (provided
the highest pressure is not too high) might help a lot.

--
Ed Sirett - Property maintainer and registered gas fitter.
The FAQ for uk.diy is at http://www.diyfaq.org.uk
Gas fitting FAQ http://www.makewrite.demon.co.uk/GasFitting.html
Sealed CH FAQ http://www.makewrite.demon.co.uk/SealedCH.html
Choosing a Boiler FAQ http://www.makewrite.demon.co.uk/BoilerChoice.html

replying to Aidan, Rob Wright wrote:
Honeymonster wrote:
My open vented central heating system suffered from air ingress so I
had it converted to a sealed system.
Check static pressure is adequate for the system. What is the height, where is
the fill point and pressure gauge?
Any automatic air vents, where are they?
Check system layout. You should be pumping away from the expansion
tank.
Â*Â*The pump generates a pressure difference, PD.
The expansion tank connection is the point of no pressure change, the
pressure change is negligible with the pump on or off. There is a
constant static pressure, SP,Â*Â*applied at the expansion tank.
If pumping away from the expansion tank, the pressure at the pump inlet
will be SP, pressure at pump outlet will be SP+PD.
If pumping towards the expansion tank pressure at pump outlet will be
SP. Pressure at pump inlet will be SP-PD. NB this could be negative at
the pump inlet and for some way upstream, especially at high level. Air
will leak into the system.


I think the above answer is a very good answer. Many people cannot understand
that a "pressurized" system can draw in air due to the suction effect upstream
of the pump.

--
for full context, visit https://www.homeownershub.com/uk-diy...em-238935-.htm

Wow that's a blast from the past to see Ed Sirett's name pop up, mind you it was a thread from 2006. Anyone know what happened to him, I do miss his sage wisdom.

Richard

"Rob Wright" m wrote in
message ...
replying to Aidan, Rob Wright wrote:
Honeymonster wrote:
My open vented central heating system suffered from air ingress so I
had it converted to a sealed system.
Check static pressure is adequate for the system. What is the height,
where is
the fill point and pressure gauge?
Any automatic air vents, where are they?
Check system layout. You should be pumping away from the expansion
tank.
The pump generates a pressure difference, PD.
The expansion tank connection is the point of no pressure change, the
pressure change is negligible with the pump on or off. There is a
constant static pressure, SP, applied at the expansion tank.
If pumping away from the expansion tank, the pressure at the pump inlet
will be SP, pressure at pump outlet will be SP+PD.
If pumping towards the expansion tank pressure at pump outlet will be
SP. Pressure at pump inlet will be SP-PD. NB this could be negative at
the pump inlet and for some way upstream, especially at high level. Air
will leak into the system.


I think the above answer is a very good answer. Many people cannot
understand
that a "pressurized" system can draw in air due to the suction effect
upstream
of the pump.

Always vent radiators with the pump turned *off* in case some of the hiss
that you hear is air being drawn *in* rather than escaping.

On 25/09/2018 00:14, Rob Wright wrote:
replying to Aidan, Rob Wright wrote:
Honeymonster wrote:
My open vented central heating system suffered from air ingress so I
had it converted to a sealed system.
Check static pressure is adequate for the system. What is the height,
where is
the fill point and pressure gauge?
Any automatic air vents, where are they?
Check system layout. You should be pumping away from the expansion
tank.
Â*Â*The pump generates a pressure difference, PD.
The expansion tank connection is the point of no pressure change, the
pressure change is negligible with the pump on or off. There is a
constant static pressure, SP,Â*Â*applied at the expansion tank.
If pumping away from the expansion tank, the pressure at the pump inlet
will be SP, pressure at pump outlet will be SP+PD.
If pumping towards the expansion tank pressure at pump outlet will be
SP. Pressure at pump inlet will be SP-PD. NB this could be negative at
the pump inlet and for some way upstream, especially at high level. Air
will leak into the system.


I think the above answer is a very good answer.

Its not particularly since it goes to much effort describing the effect
around the header tank - something a sealed system does not actually have!

Many people cannot
understand
that a "pressurized" system can draw in air due to the suction effect
upstream
of the pump.


--
Cheers,

John.

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