Homeowners policies cover your "dwelling" and that would include any service equipment which is physically attached to the dwelling including boilers used to provide heat. Assuming you have a standard Homeowner Policy Form #3 or its' equivalent or better, the boiler would be covered for "risk of direct loss" unless specifically limited or excluded.
You will find an exclusion for any loss "caused by" wear and tear, mechanical breakdown and such but note this exclusion doesn't exclude any loss "resulting from" mechanical breakdown or wear and tear to your boiler.
So I would conclude your homeowner's policy should provide coverage for your boiler should it be damaged from some external cause and also for damage "resulting from" any internal cause.
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You will find an exclusion for any loss "caused by" wear and tear, mechanical breakdown and such but note this exclusion doesn't exclude any loss "resulting from" mechanical breakdown or wear and tear to your boiler.
So I would conclude your homeowner's policy should provide coverage for your boiler should it be damaged from some external cause and also for damage "resulting from" any internal cause.