John:
Solid fuel burning appliances are particularly susceptible to heavy grease accumulations. These accumulations can contain a high concentration of ash, which bulks up the residue and makes ignition easier.
Service every 6 months is unlikely to be sufficient.
You should get a copy of Phil Ackland's book, Fire Investigation in Commercial Kitchen Systems. Also,review Chapter 14 of NFPA 96.
Happy New Year.
John Lentini, CFI, D-ABC
Fire Investigation Consultant
Florida Keys
[
www.firescientist.com]