Dhw is integrated into main phex.
When hot water demanded , if phex temp is above dwh temp pump will run for 4 secs to dissapate then stop then allowing all energy to heat dhw.
If phex is below dhw temp pump wont run until dhw demand stopped then resume for heating side.
So say heatings been off and dhw demand only. The whole phex is heated with no pump action enabling it to transfer all heat into dhw.
If heating been on say max, and dhw demanded. The pump will run for 4 sec to stop temp spiking in phex.