I think flow rate is the answer here , i used to run 250 and a 175 linked up so 425 in total .
With these i worked a full day and went home with naff all left every day .
I had my flow set at 80.
I then took my second van off the road and had a 350 spare , i put this in my van with the 175 linked together which gave me 525 i then decided on a higher flow and upped the flow to 95 , i now work shorter days and earn the same and most weird of all never have to open up the 175 !
Rich