Radiators get hot when I turn on the hot water

I am assuming that the ecoTec 618 *isn't* a combi boiler - and that it is supposed to provide *stored* hot water plus central heating. Is that right?

The 6400 is just a 2-channel time clock - so there must also be one or more motorised valves - and probably a room and cylinder thermostat - making up the overall control system. It would help to know whether you have a Y-Plan (one 3-port valve) or S-Plan (two 2-port valves) system. See

If it's a Y-Plan, there will be a single 3-port valve which has 3 positions - HW only, CH only or both together (mid-position).

For whatever reason, your valve appears to be in the mid position when it should be in the HW-only position. This could be faulty wiring - or the valve could be sticking and not letting the return spring move it to the HW position. It could *even* be incorrect plumbing - assuming this is a brand new system which has *never* worked properly.

You need to get the installer back to sort it.