Line Sizing

A reducer would be good to connect 25mm to 32mm.

All fittings, bends and line sizes have an effect on the system performance. The pump is only another part of it. The operating point is where the pump characteristic and the system characteristic curves intersect.

As has been said if you have a pump flange at 25mm you will need a reducer to connect it to a 32mm header. (A reducer has a much lower pressure drop than a sudden step change)

Note that your pump only delivers 1.5cmh at a certain head. Pumps have a performance curve and if you only require 0.5cmh then the pump will deliver that flow at a higher head. Consequently we would expect the pump to cope with the pressure drop of the reducer. Normally OI would expect there to be some sort of control device (either a fully automatic control valve or a manually adjusted globe vlave). If you find that you do not get the pump performance you need open the globe vlave a little or adjust the CV control algorithm so it is more open. These will then take less pressure drop allowing more pressure drop in the fittings