Very few types of bit coal are prone to any kind of spontaneous combustion. Of the few coals that are they require just the right amount of moisture, very fine sizes, and large amounts of coal stored. NONE of this will be encountered in any residential or commercial heating situation.
The only coals that are prone to this on any significant scale are subbit coals due to their moisture content and propensity to break down while in storage into fine sizes. Even these coals would require a most extraordinary circumstance to produce spontaneous combustion in any type of small commercial or residential setting; you're probably more likely to be hit by lightning.