Filling hot water tank on 2013 Dorset

I bought my Dorset a few weeks ago but it has been sitting at the dealer’s site since last June so I wanted to sanitise the tanks.  I emptied fresh and waste water tanks but forgot the boiler under the seat. My question is, I have now filled it with Puri Clean but the boiler could have still been full with the original water from a few weeks ago at handover. If I open the yellow valve to the boiler now and empty it, will it automatically take water from fresh water tank to refill or would I need to put the pump on? In short, if I open the boiler valve will it also drain my fresh water and waste all the Puri Clean I’ve just out in?

The dump valve will empty the hot water tank without the pump on. With the pump on it will empty the fresh water tank eventually by trying to fill the hot water tank.

So open the dump valve without the pump on, open a tap too. When empty, close dump valve, switch on pump and it will fill with the puri clean solution.

Brilliant, thank you so much Peter.