Need a little more info. Do you have an indirect WH, or an internal tankless, or what?
The unit I have has an outside reset so it adjusts the desired temp based on that and the return water sensor. I have a tempering valve on my hydronic heating circuit so it never gets the full output of the boiler. So, on mine, it adjusts based on the load, but, if the WH requires heat, it goes to max temp until the demand is over. The WH (an indirect) is on a priority circuit, so the pumps for the radiant won't come on even if the thermostats call for it while it is heating domestic hot water.