boiler(oil)

I take it this is the domestic hot water? And is this a hot water heat system?

How does the domestic hot water get heated? Is it a separate loop (I hope) to a storage tank?

I ask these questions because if it is a separate loop with it's own controls heating a storage tank, there should be a thermostat -- called an aquastat -- for that storage tank which controls the temperature in the tank. That may have failed, or it may be set incorrectly.

In any event, the system which I would prefer would be that the domestic hot water supply come through a mixing valve, which should be adjusted so that the temperature of the hot water supplied to the house is not over 120 F -- and could be less, if preferred.