Vaillant ecoTec plus 418 cycling

dave654321

Hi, I have just had a Vaillant 418 ecotech plus installed. A new castle grade 3 cylinder, variable speed interpart pump and new 3 port interpart valve. When on water only the boiler is cycling. Top water dial is set to full, but I understand this doesn't do anything without external weather controls fitted. Bottom dial is set to 69 degrees. External controls are Salus IT500. When heating is running the cycling doesn't occur. From what I've read it's to do with the temperature of return too close to that of flow and the heat loss in the radiators drops the return temperature sufficiently. I did see S.53 - appliance is in within the waiting period of the operation block function due to water shortage (VL-RL spread too big). The installer adjusted the gate valve between the flow and return pipes near the cyclinder. The pipework is per the install diagram except the gate valve is in place of the automatic bypass. I'm told correctly adjusted this is the same function.
The house is 5 bed detached with 11 radiators.
This appears to be a common problem. Any help is much appreciated.