In the case of a boiler or steam reservoir not in continuous service, and where satisfactory proof of this is made, an extension of time may be granted for the removal of flues and lagging, removal of caps from flexible staybolts, for giving hydrostatic tests, and for washing the boiler or steam reservoir and cleaning the water glass and gauge cocks. The said extension of time is not to exceed the number of days the boiler or steam reservoir is out of service. No extension of time will be granted when the boiler or steam reservoir is out of service less than 10 consecutive days.