Hi Andy,

Can't really offer any advice, but I have a similar problem with my
heating system. I believe air is being sucked in either down the vent
pipe, or through the radiator unions when the heating zone valve
closes. I can hear sharp spurts of hissing in radiators when the
heating is on. These spurts increase when pump overrun kicks in, and
when the overrun stops.

My system has pump overrun and is S-plan, with an automatic bypass
valve to provide a pump overrun circuit.

I originally had the bypass valve set quite high. This meant the pump
had to work to open the valve. Putting a balloon on the vent pipe saw
it blow up as the pump ramped up, and dissapear down the vent as the
bypass opened (obviously held onto the balloon!).

Setting the bypass to a low pressure improved this no end with hardly
any deflection on the balloon.

Still getting air in the system, and still trying to solve the problem.

Best Regards,
Ryan East.


Andy Twose wrote:
Hi,

Having done my heating the wrong way with pumped return etc I think I
now understand pump positioning, neutral point etc.
I have re-worked the plumbing so that I have ...

1. Pumped flow
2. Vent pipe immediately on back of pump at outlet of boiler
3. Make up pipe connected via lazy U to same position as vent pipe
4. Header tank in loft approx 8' above upstairs rads, 14' or so above
pump
5. Vent is 22 mm, make up is 15 mm both longish run

Looks like this...


Vent
| Make Up
| |
| |
| |
| |
| |
---------- | |
| | | | -------- ---------
| |-----------| Pump |------| Valve |----- To rads
| | ^__| -------- | ---------
| | |
| | | ---------
| | ----| Valve |----- To DHW
| | ---------
| Boiler |
| |
| |
| |
| |-------------- Returns
| |
----------

I now get less air in the system than before (it was bad), but still
find
I am bleeding radiators. I can sometimes hear air in the system.

To my mind, lowest pressure in system would be at connection point of
vent and make up,
and that this would be approx 14' water pressure (height of header to
downstairs).
All other points in system would be at pressures greater then this
when
pump is on.

I believe (from watching an open bleed valve on an upstairs rad) that
system goes briefly negative when heating valve closes.

Any thoughts, observations, opinions appreciated.

Andy