Grundfos alpha fitted on boiler return

AIUI, it's rotor speed (relates to flow) and motor load (relates to head).

Regarding the TRVs... have you checked that they are installed the right way round? Tyoically if this is wrong, you will get a vibrating or chattering sound.

Is it possible that you have most or all of the lockshield valves turned down too far? This could also be because of one or two radiators being too large for the pipework feeding them and the others having to be turned down to compensate. Try rebalancing the system according to the group FAQ. The noise with restricted flow sounds like a rushing of water.

As far as the pump mounting is concerned, you can have the pipe runs vertical or horizontal. WHat you can't do is have the shaft vertical.

Although it's considered undesirable to pump downwards as it can make it more difficult to get all the air out of the system when filling.

The pump isn't running then, though....

It is sensing 'resistance to flow' and is attempting (within limits) to maintain a constant head (pressure) regardless of the amount of flow. Whether it's in the flow or return pipe shouldn't make any difference to that.

It is distinctly possible that your TRVs are uni-directional, and that the flow is gowing through them in the *wrong* direction. When water is flowing through a radiator, you can tell which is the flow pipe and which is the return by feeling them - the flow pipe is hotter. The TRVs will probably have an arrow on them to indicate the required flow direction. Check whether they are the right way round.

As far as I know, the only stipulation is that the motor shaft must be horizontal. You can rotate the pump body around that horizontal axis to make the pipes point in any direction you like (within the same vertical plane).

No, but (IME[1]) you can never completely fill a system in one go. In fact it usually takes about a month's running to get _all_ the air out, during which time a downward pumping pump is not an advantage.

OK, I see what you're saying.

It's much the same for sealed systems. Although you can get 90% of the air out as you fill and most of the rest a few minutes after starting the pump it still take a long time for the _all_ the small air bubbles to settle out.

Thanks Guys... The pump is now working fine, seems it was me not understanding how it operates, and being impatient! I wired an ampmeter into the pump circuit and did some 'long term' monitoring under a range of conditions, it seems it takes the pump (or my system) between 10 -