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Safety Control Operation
Temperature Limit Control (Water Boilers) – The temperature limit control interrupts burner operation when the supply
water temperature exceeds the set point. Maximum allowable water temperature in a water boiler is 250F. The L4006A
will reset itself when the water temperature drops below the set point less the differential (typically 15F). Manual reset
water limit controls, such as the Honeywell L4006E, must be manually reset. For this reason, they should be set well
above the operating limit control to avoid nuisance boiler shut-downs.
Pressure Limit Control (Steam Boilers) – The pressure limit control interrupts burner operation when the steam pressure
exceeds the set point. Maximum allowable steam pressure is 15psi, however the pressure limits should be set below this
to prevent the safety valve from opening unnecessarily. Manual reset pressure limit controls, such as the Honeywell
L404C, must be manually reset. For this reason, they should be set well above the operating pressure limit control to
avoid nuisance boiler shut-downs.
Low Water Cut-off (Steam and Water Boilers) – Low water cut-offs interrupt burner operation if the water level in the
boiler drops below a safe level. Steam boilers may be equipped with two low water cutoffs so that there is a back up low
water cut-off if the first one fails to shut down the boiler in a low water condition. In this case, the back up low water
cutoff must be installed below the “operating” low water cut-off and is usually a manual reset type.
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