Secondary return on cylinder

[dan_the_plumber]

Should be a bronze pump on the return. I’d suspect the pump has been fitted backwards.
However as a different idea, what about if the ‘flow’ and return have got different flow rates. We’re talking now about pipe sizing and number of bends ‘equivalent pipe length’.
If say we are pumping out in 15mm and the return is in 22mm then the volume of water in the DHW pipework will decrease and so the pressure will drop

 

[leiadad]

The hot coming off the cylinder is in 22 and the pump is 100 percent feeding in the correct direction. It's a bronze pump

 

[dan_the_plumber]

The hot coming off the cylinder is in 22 and the pump is 100 percent feeding in the correct direction. It's a bronze pump

Ok fair enough, maybe there’s a valve closed/part closed on the secondary run?