Are low pressure hot water systems a "fault"?
No. Low pressure hot water systems are not an actual “fault”.
Unless they have Dux Qest, leaky water supply pipes.
But a low pressure system is something you should be made aware of in a building report.
Many Nelson homes built prior to 1990 have low pressure hot water systems.
With this system the town water supply, which arrives at the house about 14º C, slowly flows into a concrete header tank in the ceiling space above kitchen/bathroom area. In later houses header tanks were plastic tanks.
Gravity provides pressure to force cold water from the header tank into the hot water cylinder, that in turn forces the hot water from the hot water cylinder through the pipes to the hot water outlet.
A low pressure hot water cylinder is built to accommodate the relatively low pressure of the gravity of the system.
Copper or modern polybutylene water supply pipes should be able to take the additional water pressure, but these should be tested by the plumber doing any replacement hot water cylinder to check that there are no leaks.
Low pressure cylinders were often well-built with copper storage tanks, and it is possible to find 50-year-old hot water cylinders that can be still operating satisfactorily.
The problem is that hot water pressure in a shower can be very low. The pressure provided by gravity from a header tank up in the ceiling space can be limited. Water just dribbles out, particularly for showers where the height differential between header tank and shower outlet is minimal.
And if someone turns on a hot tap elsewhere, in the kitchen or laundry, water flow in the shower can come to an abrupt halt.
Yikes!! That shower is suddenly all cold water!!
There are two alternative solutions to overcome this.
Train the house inhabitants to not use hot water when the shower is in use. Good luck with that.
A more long-term remedy is the replacement of the hot water cylinder with a mains pressure unit, maybe a solar power system to drastically reduce energy costs.
The replacement of old low pressure hot water systems with a mains pressure system should be factored into the long term cost of the house.