Central heating 3port valve?

[Kev2632]

I was at a job today where there was a central heating comprising of 3port valve, what i couldn't understand was why is there HW off connection to be used in the programmer and also a third connection to be used on the tank stat, also this system also wanted a 2port valve incorporated into the HW pipe feading the coil to the tank ?? Why is that all? Could anybody help me understand this?

 

[Stroppy]

I've seen the result of a cylinder overflow being run in plastic pipe ("well its designed to carry hot water!") when the cylinder overheated and the safety valve opened to discharge the pressurised well-over-boiling-point steam the plastic pipe melted and jetted across the loft to hit the opposite wall over 30 feet away. When we got there 20 minutes after it happened the wall was still steaming...

Exploding Unvented Hot Water Cylinder.wmv - YouTube

Impressive slow-motion at 1:25 Exploding Unvented Hot Water Cylinder.wmv - YouTube

 

[snowhead]

Oh ive learnt some thing there. Don't Mega flows have pressure release valves then?

Yes, but that's the last safety feature.
Relying only on the Temp and Pressure Relief would be like having bare live cables and saying it's O.k it's RCD protected.

Ive not seen these two port cable set up before on unvented cylinders. Is it a new addition?

Can't name the date, but they've been around quite a while.