I think you must mean 15°C. Keep in mind that if the return temperature is at the dew point the amount of condensation will be negligible. It needs to be significantly lower to actually have a significant impact.
For example, at a dew point of 57°C the partial pressure of H2O in the combustion products will be 131 mmHg. A return temperature of 45°C will reduce the H2O SVP to 72 mmHg, which means that only about 45% of the H20 will be condensed.
According to my calculation, a boiler that runs on pure H2 with lambda = 1.1 will have a pp(H2O) = 240 mmHg. A return temperature of 55°C, which corresponds to SVP of 118 mmHg, will allow 50% of the H2O to condense.
I'm liking your maths and clearly clarifies what I said above in my comment.