"G&M" wrote in message
...

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 ?

When calculating the rad/pipe sizes of a system the pump size is also
determined. You may have is far too many elbows giving too much resistance
fooling the Alpha which then drops its speed.

Two practical ways around this:

1. Fix a high head fixed speed pump at least the same head as the replaced
pump.

2. Instead of installing loops with one pump to assist the other, just split
the system into two zones with a time clock on each. Then you could have
say the upstairs off during mid day. Divide and rule and save fuel too,
which is a good thing with a oil boiler in price and storage longevity.