Why is the CH flow temperature controlling HW temperature

[neil0]

Hi all, hoping for some guidance, please!

A bit about my system (note, I'm not a plumber!)
Worcester Greenstar 18i System boiler MK3
Main HW Cylinder and expansion vessel
S-plan (has 2 motorised valves near the HW cylinder and HW pipework)
Wireless thermostat
Potterton boiler controls
HW cylinder set to 4/5 (65degrees?)
Boiler CH temperature set to - 3/6 = (61degrees)

So... I previously had my boiler CH temperature set to 4/6 (68degrees). Note, in the manual it specifically says this is the CH temperature only. There is no other knob or setting and has a Radiator image next to it too. I thought I would experiment with turning this down to 61degrees to see if it's still comfortable which it is.

However, the issue is that the HW isn't as hot as it used to be, and having played around with it and the controls, the HW is being controlled by the CH temperature. I was expecting it to fire up on max. output for HW to match the cylinder thermostat level of, say, 60-65 degrees and then stop firing once it had reached that temperature.

The outcome I would like is:
When the HW is requested from controls, it gives 100% output and temp to the cylinder until it hits the internal thermostat, so the water is hot
and
When CH is called upon it simply follows the CH temperature on the boiler - i.e. 55-60 degrees, as per the instruction manual for the boiler.

As I have 2 motorised valves and S-plan, I would think this is possible and I'm not sure why it's behaving like that considering the instruction manual.

Can anyone help?

Thanks in advance, Neil

 

[muggles]

The functionality you describe can be achieved but you need to ditch the S-Plan and fit the integral diverter kit instead

Worcester Greenstar i System ErP Internal Diverter Valve Kit 7733600089

Worcester Greenstar i System ErP Internal Diverter Valve Kit 7733600089. Lowest price guaranteed. Paypal Accepted. Trading Online Since 1999.

www.plumbnation.co.uk

 

[Mister Banks]

The boiler doesn't know if it is firing for CH or DHW because with an s plan the request for heat is on the same terminal within the boiler. The thermostat on the HWC is just to tell the DHW 2 port valve to close when its set temperature is reached, if the set temperature on the thermostat is at 60c and the boiler flow temp is at say 50c the cylinder will never get up to temperature. As stated by @muggles you need to install the Diverter Valve Kit to do what you want.