lets say he chooses low pressure membrane which operates optimum 80-100psi, his tap pressure is 30-40psi during demand hours, membrane will not produce good hourly quantity of water with low enough tds. more resin will be used.
the disappointment may and will occur.
with booster pump 200l/hr + is achievable with single digit tds prolonging resin life.
hf5 type will work with tap pressure of 40 psi above without booster pump but tds will be slightly higher and will produce more water than 4040.