Pump tops change performance of the pump by changing the geometry of the injection chamber. As a rule, top replacement can alter hoses outlet and inlet position as well as a way to connect them – from plastic non-replaceable taps to threaded ones connected by barbs.
Attention: If you hear the crackle / humming of the motor, try to release the screws tightening the top, by 1/4—1/2 turns.
Here are some details on how the impeller of Laing DDC pumps works.
The impeller has no rotation shaft, but stands (or rather soars) on ball bearing (ceramic ball acts as a sleeve bearing) together with the electromagnetic rotor. If you look at the pump without top, it can be clearly seen that the impeller stands out above the base of the pump.
After pump starts to rotate, the rotor together with impeller rises even a bit higher while remaining in the hovering state the entire time pump operates. There should be a sufficient gap between the impeller and the top to be filled with water (water also acts as a lubricant). If the gap is too large, there can be some negative points. For example, formation of bubbles (cavitation effect) that reduces the efficiency of the pump. In case of a tiny gap, there is a risk of stopping the impeller.
So what happens if the gap is not large enough? When the pump starts rotating, the rotor and impeller will not be able to rise high enough for its free hovering. Even if the pump continues to operate, it does not always mean that there are no problems. Electronic pump control tends to increase its speed in case of a low flow rate to compensate the loss of performance. But in this case, performance loss is not cause by high flow resistance of the blocks in the loop but by the friction force arising due to insufficient gap. The motor generates more heat while trying rotating the rotor at a higher rate, but performance is not increasing. Laing DDC pumps are designed so that the stator gets cooled by water. If the flow rate is insufficient, then sooner or later, the pump will burn.
So what are the normal speed rates for the pump? My Laing DDC 3.2 runs at speeds of 4200-4300 RPM, the temperature the motor compartment is about of 35-37°С. Note that the pump rotates at speeds lower than ones stated by the manufacturer. This means that there are no problems with the gap between the top and the impeller. Electronic pump control system will increase rotation speed of the rotor automatically if there is a drop in performance.