You can do it with the pump running. Mandatory to open valve 8 all the way first. It shouldn’t technically matter the sequence of the other 2, but closing 9 first will stop flow through the heater before closing the outlet side. You won’t put un-necssary pressure on the heat exchanger vs closing 10 first which would allow pressurized water to get trapped in the heat exchanger. Always close and open the valves slowly.
Reverse the sequence when opening. Open 10, then 9, then tweak 8. Although depending on what happens to flow with the heater bypassed, valve positions may need to be tweaked to get the desired flow rate.
