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This is the manual for a different model heater but maybe it operates the same way....
http://www.rural-energy.com/publications/catalog/documents/document-498.pdf
deactivate the LOCK press and hold the
button for 3 seconds until the indicator
goes off.
During normal operation the LOCK may be
activated and all controls, other than the
OFF switch, will be locked. Deactivating
the LOCK releases the controls.
If the LOCK is activated while the unit is
turned OFF, then all functions will be locked
including the ON/OFF function. If the unit
is turned OFF while the LOCK is activated,
it cannot be turned ON again until the
LOCK is deactivated.