Combi as System - overflow

[Crumpet9162]

Hi,

I have a Worcester Greenstar 30cdi Combi, with a vented cylinder. The DHW feeds only the kitchen tap. CH is a Y plan.

When the mains water is off, the hot tap runs hot, presumably draining the cylinder. The level in the header tank drops.

When the mains is on, but the cyclinder is isolated, the hot tap runs hot, the boiler starts, presumably triggered by the hot tap running.

I’m taking this to assume my initial statement is correct. The boiler is operating as a system boiler, with the kitchen taps on the combi DHW.

The kitchen tap must be connected to both the boiler DHW feed, and the vented cylinder as gets water when both are isolated.

Here’s the problem - the overflow on the header tank has started leaking, it’s filling with hot water constantly. I discovered this because the chap who installed it didn’t connect the overflow.

There’s low pressure on the hot tap. I assume this is because the DHW from the boiler is making its way to the cylinder through the kitchen tap and overflowing it into the loft.

Q1) based on this description is my assessment of the setup plausible?

Q2) should there be a non return valve between cylinder and kitchen tap to prevent the back feeding and could this have failed open?

Many thanks!

 

[MagicDoughnut]

Could it be your boiler is being fed from the tank? Isolate the tank then run your kitchen hot tap. Or if you have an isolation on the hot supply from the cylinder or the cold supply feeding the cylinder, you can use that too.

There’s low pressure on the hot tap. I assume this is because the DHW from the boiler is making its way to the cylinder through the kitchen tap and overflowing it into the loft.

Kitchen tap is a mixer tap, when open in the mid position it's possible the cold is backflowing through the hot. It's also the posibility of your cylinder stat tripped and it's overheating.