Do any of the clever folk on here know how to correctly size and select a waterpump for a domestic central heating/hotwater system?

The house is very spread out with long pipe runs and im convinced the existing pump is struggling monumentally!

There are several suppliers, but I dont have any idea how best to choose a decent pump...

Anyone have any cunning ideas?

have a look at the existing pump and see if there is any markings on it if it is grundfos it will either say 15/50 or 15/60 other manufacturers mark them differently if you can post a pic for me this would help. and any more symptons rob.

Or if its 1 particular leg or a zone that you believe to be strugeling, put a pump on that to boost it.

Then if you ever wanted you could put a thrmostat on that leg/zone and control it independantly.

Sometimes bigger houses have the upstairs and downstairs zoned for example, As the upstairs can be switched off sooner due to heat gain from the floor below.

It gets a bit complicated zoning the house without tearing the house to bits...

I need a pump powerfull enough to pump water through two plate heat exchangers (one in a gas boiler and one in an airsource heatpump), then circulate the water around quite a spread out house...

The only information I can find on the current pump is that it delivers 15L/m

By my calculations, for the house to abosrb 20Kw and to keep the flow rate below 90Cm/sec I need to circulate 1400 litres of water per hour...

The existing pump only does 900 by my calculations...

[Edited on 11/12/07 by tegwin]

Have you any more details on the system or how it works, maybe a diagram or a description?.

Whenever i've seen a plate heat exchangers they've been to generate Domestic Hot Water (DHW) instantaniously from Heating system boilers.

These would be fed from their own leg from the boiler (or a low loss header) with their own pumps so this circuit can run all day and all night and during the summer when the rest of the Heating system is off on a time clock or satisfied (or even modulated down when the system is coming upto temperature).

This link may be of help for finding a pump of the correct flow rate you require:
http://net.grundfos.com/Appl/WebCAPS/custom?userid=GMA&appl=1

And click on sizing on the banner at the top.

Hope that helps of feed us some more info on the setup.