After purge, the burner drives to low fire and a low fire switch must close before ignition is allowed. Lighting at minimum fuel keeps the initial flame small, so any small delay in ignition does not become a violent light-off. After main flame is proven, many boilers also hold at low fire for a warm-up period to protect refractory and limit thermal shock.
It's trying to light at 40 percent because the low-fire start switch is out of adjustment, that's a hard light-off waiting to happen.
Also heard as
- low fire start
- low-fire light-off
Related terms
Purge
Blowing air through the furnace and flue before ignition or after shutdown to clear out unburned fuel.
Mod motor
The modulating motor that drives a burner's air damper and fuel valve through their range on command.
Light-off
The moment a burner ignites, and also the verb for lighting it.
Thermal shock
Damaging stress from a sudden temperature change in boiler metal, such as cold water hitting hot iron.