Some Fun Time gas fagging here.
LEL - Lower Explosive Limit (in % air concentration )
UEL - Upper Explosive Limit (in % air concentration )
When concentrations are withing these limits a gas is considered flammable and therefore explosive.
Your furnace is a nozzle and can be thought of an area controlled continuous explosion
a house filling with gas when ignited will exploded when the LEL is reached within the house. otherwise the gas will not ignite,
Natty Gas Super Funtime Facts (CH(little4)) - also called Methane
LEL - 5.0 %
UEL - 15.0 %
Maximum super fun devastation is the midpoint of 10%
Bonus fun time - concentrations above the UEL will not ignite because there is not enough oxygen for combustion.
Natty gas explodes unless flared off in a burner element within the LEL and UEL