pump speed

Hi, I have a Worcester Bosch 35cdi System boiler with optional diverter valve (diverter valve enables it to be used with a hot water cylinder). The control package is via Tado and integrates via the Worcester EMS bus system. My issue is that when the system is heating the hot water tank, it...

We have an unvented Vaillant ecotec 630 system boiler with a Gledhill 200 litre slim tank on the first floor and the boiler in the utility room on ground floor. Can anyone advise why pump speed (d.019) 2 would be used over speed 1 & what setting would be best for minimum gas usage? Thanks