My partner has an apartment with an unvented hot water cylinder that’s heated by a Heatrae C600 electric heater.

We have a cyxlinder full of hot water and zero pressure coming out of taps. Pressure vessel is showing 1.5 bar but it seems to lose pressure extremely quick.

Today it lost pressure and I’ve re-pressurised it but barely any water is coming through the hot tap. Cold water pressure is fine.

Any ideas?? I used to sell heating controls for Honeywell so know my way around a heating system but this has baffled me

I assume the gauge you are reading is on the sealed heating system, if so it has no relevance to the hot water coming from your taps.

First thing is to check all valves are open, look for the obvious first, easy to snag clothing on a lever valve in an airing cupboard.

Cylinders work on displacement. Is the water flowing in? Check strainers filters, PRV’s etc. If so there is not a lot to stop it on the outlet side other than a valve potentially.

Does it affect all the hot water taps or just one? And where is the cylinder located below or above the outlets?

As bear has pointed out above it sounds like a scale blockage on strainer/filter or an air lock.

Air lock unlikely on unvented system.

Does it affect all the hot water taps or just one? And where is the cylinder located below or above the outlets?

All taps and cylinder is on same level as it’s an apartment.

Where are the strainers?

In the control group fitted to the cylinder inlet, usually consists of strainer, PRV and check valve