"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 ?


If the water velocity is too high, you'll get noise in the pipes. If it's
too low, you'll have too big a temperature drop across your radiators.

Start by getting yourself a decent non-contact thermometer (like the 30 quid
IR one from CPC) and balance your system to get an equal drop across all
radiators. Ideally, this should be about 11 degC. If it's substantially more
than this, water isn't getting to your radiators fast enough to balance the
heat loss to the room - and you need to increase the velocity. If your pump
is running flat out, but is still not enough, I'd be tempted to install
another one very close to the existing one but in the other pipe - so that
one is in the flow and the other in the return. [Make sure they're the right
way way round so as not to fight each other!] Connect them together
electrically so that they both go on and off at the same time. In effect,
one will be circulating water between the house and the boiler and the other
will be circulating it round the radiators - but they will just be working
in series, so there's no need for any additional by-pass circuits.

[I hope that the long pipes between the house and out-house are well
lagged!]

HTH.

Cheers,
Set Square