Air keeps getting into CH system?

[b0ng0]

I'm a bit of a noob when it comes to all this, so please bear with me

Recently had the pump replaced in the CH system (Grundfos 15-90N I think) and since then, there is a decent amount of air trapped in the system - you can hear it swooshing through the boiler and especially at the pump next to the hot water tank.

The plumber who fitted the pump came round and drained the system and that seemed to remove the air temporarily, but the problem came back the next day.

His theory is that the air separator (small grey thing behind the Grunfos in the pic below) could be filled up with rust (quite a bit of black water came out the old pump) and it's exacerbating the problem by not really separating the air.

Any ideas how to troubleshoot this before taking the step to remove the air separator? I've bled the radiators off frequently, and it seems like the air that's bled off just keeps getting replenished.

It's an open-vented system, with a header tank and expansion tank supplying 9 radiators. Pics below of the tanks pipework:

Tanks in the loft
View media item 82962Hot water tank
View media item 82963 [/u][/b]

 

[Armo74]

Hold a magnet on the pipework around the air separator if it attracts you have a blockage usually in the cold feed pipe (the 15mm one).
Or you can fill a jug of water go in the loft and submerge the vent pipe to see if it sucks the water.
Sounds like a good chemical flush and rad flush wouldn't do it any harm.
You also need to confirm what pump it is and what speed it's set at.

 

[b0ng0]

Just away to leave work, but I think the pump it set to speed III - i.e. the highest. Might be worth lowering it I suppose?

 

[Armo74]

It's still worth doing the other things suggested but if it was ok before the pump was swapped it is worth looking at that aswell.

 

[b0ng0]

The pump model is a Grundfos UPS 15-60 130, set at II out of III.

Checked the expansion pipe going into the header tank and it wasn't sucking water when I put a cup under it (the pipe was also stone cold at the top, but hot near the hot water tank, but I assume that's normal).

Had a look in the expansion tank and there's a constant stream of bubbles coming up out of it (see below). I assume this is the pipe leading up from the air separator which is why there is air constantly coming out of it.

Bubbles in the expansion tank (far right of the picture)
View media item 82965
Put a magnet on the air separator and it stuck fairly tightly, but not sure if this is due to the coating on the separator or the magnetic gunk inside it?

I'm still a tad confused as to why the air isn't eventually just bleeding off though - even if the air separator isn't functioning optimally, there is clearly air escaping the system via the expansion tank - so air must be being drawn in from somewhere??

 

[Armo74]

Air being drawn in is usually due to a blocked cold feed pipe.
You could try isolating the incoming cold main which tops up the tank and partially draining the system to see if the tank empties but my money's on the blocked seperator.
You must remember if it is blocked everytime you bleed the rads the system isn't being topped up properly allowing room for air in the system.

 

[b0ng0]

Thanks for the feedback.

Sorry if I'm being a bit dense, but if air is leaving the system through the separator (the stream of bubbles from the expansion tank), how does a blocked separator bring in more air?

Is it actually sucking air down the expansion pipe due to turbulence or something like that?

Would it make more sense just to remove the separator entirely rather than put another one in, since there's already a vent valve coming off the hot water tank?