Whereas maintenance is concerned with maintaining the original condition (to counter wear and tear, ageing and the effects of the weather), repair is concerned with remedying damage and defects that have occurred to a property. Maintenance is the process of keeping a property in its original condition. In repair, the original condition (impaired by damage, destruction, etc.) is restored.
If a heating system is serviced regularly or if wearing parts are replaced, this is maintenance. If a defective heating system is completely or partially replaced, this is called maintenance.