Steam comes up while uing hot water?

Hi, we have a weil mclain eg65 with a hot water coil belive it's 15gpm, along with a 40g storage tank, recently noticed that the steam / heat comes up while using the shower for log periods of time, is that something normal? or should i be looking for service, any ideas what's causing this to happen? This is a relatively new furnace less than 4 years old, had the damper motor replaced recently...

Thanks for your help