specs: - Potterton Profile Boiler - Grunfos Selectine pump - Hercal h/w
tank

I have a 3 bed house, my central heating system used to be fine, bar
the banging of an old boiler.

I got the boiler and the hot water tank changed. After this a small
amount of air everynow and then used to get into the radiator in my
small bedroom. So I'd bleed it once a month and everything was fine.

This year I got 2 radiators changed (a large one in the lounge and a
small one in the bathroom). Now more air is entering the small bedroom
radiator. The frequancy of the air is un-acceptable, not only making
bubbling noises that wake people up but also requiring to be bled once
a week or every other week.

What I want to know is 'why' is this air getting in ?

I have not seen any leaks in the system, my plumber says that it's
through the vent and the pipework needs to be changed to 'fully pumped
system'. BUT the question in my mind, is why is this happening? The
pump hasn't changed, so surly the vacumn in the system hasn't changed,
so why whould the water in the vent pipe now be getting sucked lower
for air to get into the system? Also why would this be made worse once
again by 2 new radiators.... it doesn't add up in my mind, any ideas?

- also tried to knock the speed of the pump down to test, by this has
made no difference.

If you've no leaks, the only open place is the vent pipe. This is
probably on the suction side of the pump with the make up pipe on the
delivery side. When the pump runs, There's enough pressure differential
to pull the water down the vent and then air follows.
Can you raise the header tank?

On Sun, 07 May 2006 01:36:34 -0700, paul_murphy50 wrote:

specs: - Potterton Profile Boiler - Grunfos Selectine pump - Hercal h/w
tank

I have a 3 bed house, my central heating system used to be fine, bar
the banging of an old boiler.

I got the boiler and the hot water tank changed. After this a small
amount of air everynow and then used to get into the radiator in my
small bedroom. So I'd bleed it once a month and everything was fine.

This year I got 2 radiators changed (a large one in the lounge and a
small one in the bathroom). Now more air is entering the small bedroom
radiator. The frequancy of the air is un-acceptable, not only making
bubbling noises that wake people up but also requiring to be bled once
a week or every other week.

What I want to know is 'why' is this air getting in ?

I have not seen any leaks in the system, my plumber says that it's
through the vent and the pipework needs to be changed to 'fully pumped
system'. BUT the question in my mind, is why is this happening? The
pump hasn't changed, so surly the vacumn in the system hasn't changed,
so why whould the water in the vent pipe now be getting sucked lower
for air to get into the system? Also why would this be made worse once
again by 2 new radiators.... it doesn't add up in my mind, any ideas?

- also tried to knock the speed of the pump down to test, by this has
made no difference.

Make your self a diagram of where all the pipes go. It sounds like the
system was not right from the outset but operated tolerably for you,
but now it doesn't.

Especially take note where the vent pipe goes and the feed pipe,
especially if they are on opposite sides of the pump.


--
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

wrote:

This year I got 2 radiators changed (a large one in the lounge and a
small one in the bathroom). Now more air is entering the small bedroom
radiator. The frequancy of the air is un-acceptable, not only making
bubbling noises that wake people up but also requiring to be bled once
a week or every other week.

Did this radiator change require a drain down? If so, was new corrosian
inhibitor added as the system was refilled?

If there is not enough inhibitor in the system then one possiblity is
that they gas you are collecting is not air, but being generated by
corrosian.

What I want to know is 'why' is this air getting in ?

I have not seen any leaks in the system, my plumber says that it's
through the vent and the pipework needs to be changed to 'fully pumped
system'. BUT the question in my mind, is why is this happening? The
pump hasn't changed, so surly the vacumn in the system hasn't changed,

so why whould the water in the vent pipe now be getting sucked lower
for air to get into the system? Also why would this be made worse once
again by 2 new radiators.... it doesn't add up in my mind, any ideas?

The new rads may have changed the flow resistance of the whole circuit...

When I had a (badly designed) vented system it suffered a similar
problem due to the vent being on very close to the suction side of the
pump. Everytime the pump kicked it it would suck in a gob of air via the
vent. I fixed it by cutting into the vent pipe about a foot above the
pump and inserting a tee, then connecting the feed from the header tank
to the tee (blanking off the connection to the other side of the pump
that it used to connect to).


--
Cheers,

John.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/