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My son has just moved into a new flat and this is my first experience of an unvented system!

His problem is that the water is initially very hot but then runs cold quickly - he has to be very fast in the shower.

The cylinder is heated by a conventional sealed system boiler (also does the rads) and this appears to be working fine. Selecting hot water causes the 3 way valve to divert hot water into the cylinder, but after about 10 mins the boiler begins to cycle against it's water temperature thermostat (which is set to 4 out of 5) so clearly the primary loop temperature is high but the water in the lower half of the tank is still cold.

The tank is a bubble top type but I can't see a maker's name. The case is octagonal and the connections are a bit puzzling to a novice. The primary heating coil is clearly marked but the in /out connections have been reversed - output from 3 way valve goes to the connection marked heating coil outlet (the upper connection). Don't think will make huge difference.

The cold inlet and the hot outlet are in the middle of the tank and about 12 inches apart. I was expecting cold at the bottom and hot at the top! Of course they may go anywhere inside the case. There is also some sort of 'relief valve' (has a twist knob marked + -) between the cold in and hot out -what is this?

There are also two more pipes exiting at the top of the boiler and I can't work out what these are.

As this is a rented flat I will be keeping my tools in the bag and getting the landlord to fix it, but my curiosity is getting the better of me. I've seen similar problems when a vented system tank gets completely clogged up with calcium deposits and the tank does look quite old.

Any ideas or explanations would add to my education.
 

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