Coker heater operation is improved by on-line spalling of coker heater pipes. In one embodiment an off-line pipe is added to the on-line coker heater pipes. When an on-line pipe is to be spalled, flow is diverted to the off-line pipe allowing for full operation of the coker heater. In another embodiment, a thermal transfer resistant zone plate is movably mounted in the radiant section of the coker heater. By moving the zone plate from an operating position to a spalling position and adjusting the temperature of the plurality of burners, the temperature of the pipes in the zone of the heater radiant section to be spalled can be lowered, while the temperature in the remaining zones of the heater radiant section are fully operational.