Why did I have a Flue Fire?

Your flue fire has been caused by a build-up of unburnt gases that have cooled and solidified in your flue pipes that have then collected more unburnt solids slowly building up until you have then had a high fire situation that has ignited this very combustible fuel deposits which can then burn at temps up to 2000 degrees potentially causing structural damage to the flue pipes themselves. This build up has most likely occurred due to either poor fuel quality resulting in poor combustion and/or due to continuous low heat fires which don’t allow complete secondary combustion thus allowing these particles to exit the fire without being converted to heat.