Grundfos Alpha2 Pump Noise Traced to Trapped Air and Silenced with a Spirotech Deaerator

An Alpha2 running in constant-pressure mode at head setting CP1 should be close to silent. When the noise starts within five to ten seconds of the pump energising and fades if you leave the system running for an hour, the cause is dissolved and entrained air passing through the rotor, not a mechanical fault. Grundfos rates the Alpha2 for a minimum inlet pressure that assumes the water is deaerated. Feed it aerated water and the low-pressure zone behind the impeller drops below vapour point, bubbles collapse, and you hear it as a fine hiss or a rattle like gravel.

The fix that holds is a deaerator on the flow, and a Spirotech SpiroVent is the common choice on domestic systems. Before fitting one, it is worth confirming the air is actually the problem, because a genuine dry-run or a seized bearing produces a different, steadier noise that does not respond to bleeding.

Where the air is coming from before you fit anything

Air enters a sealed heating system through three routes, and the SpiroVent only deals with the consequence, not the source. The first is a slightly negative pressure somewhere in the circuit, which happens when the pump head is set higher than the static fill pressure can support. On a two-storey house with radiators on the first floor, a cold fill pressure of 1.0 bar leaves very little margin at the top of the system once the pump is pulling. Raise the cold fill to 1.2 to 1.5 bar and the negative zone often disappears.

The second route is a tired expansion vessel. When the air charge behind the vessel diaphragm has leaked away, the system pressure swings widely between cold and hot, and every time it dips toward atmospheric at a high point, air is drawn in through an automatic air vent that is meant to only let air out. A vessel with a failed charge reads near-zero on a tyre gauge at the Schrader valve when the system is drained to zero.

The third is fresh fill water itself, which carries dissolved oxygen and nitrogen. Every time you top up a system that keeps losing pressure, you introduce a fresh load of gas that comes out of solution as the water heats past 60 degrees C. That is why a system with a slow leak is also a system that is always gurgling. The gurgle is a symptom of the top-up, and chasing the leak matters more than any air separator.

Fitting the SpiroVent so it actually works

A SpiroVent removes air by slowing the flow through a chamber packed with a wire mesh element, giving microbubbles a surface to coalesce on until they are large enough to rise and vent through the float head at the top. That mechanism only works where the water is hottest and the dissolved gas is most likely to come out of solution, which is the flow pipe within about 300 mm of the boiler outlet, before the water has given up any heat to the system.

Fit it vertically with the vent head uppermost. Mounting it on the return, where the water is cooler, cuts its effectiveness by more than half because cool water holds gas in solution and never releases it into the chamber. On a combi where space at the boiler is tight, the next best position is the flow pipe as it leaves the airing cupboard, still on a vertical run.

The SpiroVent Air unit for 22 mm pipe fits inline with compression ends and needs no servicing beyond checking the float cap occasionally. Expect the loudest bubbling in the first two to four days as the accumulated air in the system clears, then progressively quieter running. If the Alpha2 is still rattling after a week of continuous running, the air separator is doing its job and the noise is coming from somewhere the separator cannot reach, which points back to a negative-pressure zone or an ongoing water top-up.

Balance the radiators before blaming the pump

An unbalanced system pushes almost all the flow through the two or three radiators nearest the pump and starves the far ones, which forces you to wind the pump head up to CP3 or the fixed-speed III setting to get heat to the far end. That higher head is exactly what creates the low-pressure cavitation zone behind the impeller. Balance the radiators and you can often drop the Alpha2 back to CP1 or the lowest proportional-pressure setting, and the noise stops on its own.

Balancing means setting the lockshield valve on each radiator so that the temperature drop across it is roughly 11 degrees C between flow and return under design conditions. Start with every lockshield fully open, run the system hot, then close down the lockshields on the nearest radiators until the far ones warm up at the same rate. A clip-on pipe thermometer or two digital probes on the tails give you the flow and return readings. The nearest radiator to the pump usually ends up with its lockshield barely a quarter turn open; the furthest stays fully open.

This is slow work across an evening, going round twice, because adjusting one radiator changes the flow to all the others. On a system with 10 radiators it is normal to make two full circuits before the return temperatures settle. Once balanced, the pump moves less water at lower head, draws less electricity, and the whole circuit runs quieter.

The dosing that stops the air coming back chemically

A freshly deaerated system will slowly regenerate gas if the inhibitor has been diluted by top-ups or was never dosed. Sentinel X100 is dosed at one litre per 100 litres of system water, which for a typical eight-radiator house with a 100 litre content means a single one-litre bottle. The inhibitor suppresses the corrosion reactions between the steel radiators and any dissolved oxygen, and it is those reactions that generate hydrogen gas, the black sludge, and the accompanying pockets of air that a pump chops into noise.

Inject it through a radiator using a dosing gun, or into the filling loop, then run the pump for an hour to circulate it. On a system that has been running dry of inhibitor for years, the water drawn off will be black. Flushing that out and refilling with a correct X100 dose is often what finally quiets a system that has been dosed with air separators and higher fill pressures without lasting effect.

A quick pressure sanity check

With the system cold, the pressure gauge should sit at the fill pressure, commonly 1.0 to 1.5 bar, and the expansion vessel Schrader charge should match that within 0.2 bar. If the cold reading has drifted below 0.8 bar, the system will pull air at every high point.

What the numbers look like on a real repressurise

Take a two-storey house with a total pipe run reaching 5.5 metres above the boiler. Static head to the top radiator is about 0.54 bar. If the cold fill is set at 1.0 bar, the pressure at the top radiator when cold is only 0.46 bar, and once the Alpha2 at CP2 pulls its suction side down, the top of the circuit dips close to atmospheric and draws air through the automatic vent every heating cycle.

Repressurise the cold fill to 1.4 bar and the top radiator now sits at 0.86 bar cold. Recharge the expansion vessel to 1.2 bar with a foot pump at its Schrader valve while the system is drained, so it does not sit hard against the diaphragm. Refit the SpiroVent on the flow, dose one litre of X100, and run continuously. The audible bubbling through the pump typically fades over three to five days as the trapped air is vented and no fresh air is drawn in.

What none of this addresses is a system that keeps losing pressure between top-ups. If the gauge still creeps down over a fortnight after all of the above, the air is a passenger on a slow leak, and no separator will keep pace with water that is being replaced faster than it can be cleaned. The open question then is where the water is going, and whether it is weeping at a joint you can see or evaporating invisibly through a pinhole under a floor.