We have a five bedroom farmhouse with about 15 radiators. The boiler is in
an outbuilding whulst the pump is in the house. There is about thirty-five
metres (times two) of pipe between the two. It isn't possible to move the
boiler nearer.

The old pump and LPG boiler were having problems so I have installed an
90,000 BTU oil boiler with a Grundfos Alpha pump. However I'm still not
sure the water is being circulated fast enough due to the resistance of the
piping. The oil boiler isn't commisioned yet but the LPG certainly
struggled big time.

Is there any definition of what the minimum acceptable water velocity is ?

And if it is too low, is it possible to install a second pump by the boiler
just to pump water through the long legs, presumably with an automatic
bypass valve in the house, and then use the Alpha just to pump the water
around the house ?