• Thread starter packetvoodoo
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Sealed system (cold pressure +1bar at highest point in system).
22mm pipework throughout - heating and H/W circuit, smaller tails to radiators. All pipework for the cylinder is brand new - from three-way-valve to common return under floor.
Indirect cylinder for H/W (upstairs), system boiler with integral pump (garage).
30KW Vaillant boiler, rates down to 10KW minimum.
Pump running adds about 0.2bar to system pressure at cold.
To avoid any chance of sucking in air through the pump AAV, it was closed after the system was purged of air. The only other AAV is on the cylinder flow (at the highest point in the system) and is left open.


System works well when just heating, but seems to suck in air (gurgles loudly) when the boiler fires when in H/W mode.

After firing, the air dissipates through the AAV on the cylinder flow over several minutes and gurgling stops. Eventually, the boiler will stop heating due to high water return temperature as the cylinder coil cannot absorb 10KW that the boiler generates at minimum burn.

When the return water temperature drops and the boiler fires again, more air gets sucked in and the process starts again.

Process has continued over several months now. No water has been added as no pressure has been lost. The system is well dosed with inhibitor, and the air is definitely not hydrogen (tested).

Have tried closing the cylinder AAV (in theory making the whole system airtight) before the boiler fires again, but this makes no difference.

The boiler is plumbed correctly - flow and return correct way round.

Can't for the life of me figure out what would be causing this. In theory the whole system is at least +1bar relative to atmosphere and therefore air *cannot* be sucked in.

Seriously grateful for any pointers. Damn thing is going to end up in the skip at this rate.

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Deleted member 105431

Add some fernox F4 to the system and open the boiler pump aav, closing it doesn't help.
26 Feb 2009
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United Kingdom

You have answered many of your own queries, it is not 'sucking air' as you have not re-pressurised system. i have had this siuation many times on my systems, but i usually get over it.
My own conclusion would be:-

1. Cavitation, i know it's a sealed system and the pump will have been set on No2, but simply try turning this down to No1, see if this has any effect.
2. Auto Bypass in boiler, have you tried adjusting this,or have you also fitted an external one in the pipework,this may also be worth looking at.
3.Flush the system again,thoroughly and infuse with a good inhibitor.
4. I'm not sure if there is a filter in this boiler,but while it is drained down check this.
5. Is your regulating valve fully open??

Hope this helps
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1. The pump is a two speed affair, set to lowest speed. Agree that cavitation is the most likely scenario, but confused as the entire system is in 22mm - would expect to see cavitation on the heating side too. Having said that, the system resistance will be higher when just on H/W. Would that cause cavitation?
2. The auto bypass in the boiler hasn't been adjusted - will look into this as this presumably will guarantee a minimum flow and avoid cavitation.
3. The system has already been thoroughly flushed and the water is visibly and has been chemically tested clean.
4. There isn't a filter in this boiler.
5. Not sure what a regulating valve is - again will look into this.


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