Replaced a rattling Grundfos central heating pump three weeks ago with the modern equivalent (Alpha 2L). According to the packaging, it’s self-bleeding and has no bleed screw.

But two weeks later I can still hear air gurgling around the system when the pump is running. It’s spent most of the two weeks on the fastest speed setting with no improvement.

Have I got a faulty one? It’s an unvented system so one would assume any leak would let water out as opposed to letting air in.

Leak somewhere in the system or corrosion in the rads creating gas?

Thanks sharkbait. The pressure has remained bang on one bar ever since moving in a year ago which suggests to me that nothing’s leaking out.

Hadn’t considered corrosion. Entire system is four years old. How much would you have to dilute the inhibitor before it stopped working properly?

Do you think it’s worth draining the system down and refilling with fresh inhibitor?

Presumably there’s a automatic bleed valve somewhere else in the system – normally in or around the boiler? That needs to be loose or it may be gunked up?

Hi Martin – yeah, there’s a Flexvent near the pump. I checked that the cap was backed off a couple of turns when I moved in. There’s a small amount of limescale around the cap so it looks like there’s some mileage in your suspicion that it’s gunked up.

Irritatingly it’s been installed without the shut-off valve so no way to replace without draining anyway.

Fixed. Possibly.

Increased the pressure to 2 bar briefly and the Flexvent burped out a load of air. Perhaps a bit gummy inside. Reduced it back to 1 bar now and totally silent.