Set above the operating control and below the relief valve setting, the high limit is the second line of defense against overpressure or overheating. On steam boilers it is typically manual reset, so someone has to find out why it tripped before firing again. Hot water high limits must stop the burner at or below 250 degrees F on heating boilers.
Boiler's locked out on the manual reset high limit, before you hit the button, find out why the operating control didn't stop it.
Also heard as
- high limit control
- limit control
- HL
Related terms
Operating control
The automatic-reset control that cycles or modulates the burner to keep steam pressure or water temperature in range.
Safety valve
The spring-loaded valve on a steam boiler that pops fully open to vent steam before pressure exceeds the boiler's rating.
MAWP
Maximum allowable working pressure: the highest pressure a boiler is approved to operate at, stamped on its nameplate.
Lockout
A safety shutdown that holds the burner off until someone presses a manual reset.