A.O. Smith SBD 30 150 Installation And Operation Manual page 16

TWO TEMPERATURE - TWO PRE-HEATERS WITH MIXING VALVE OR BOOSTER
HEATER WITH OR WITHOUT BUILING RECIRCULATION
TWO PRE-HEATERS WITH MIXING VALVE
DANGER
TEMPERATURE SETTING SHOULD NOT
EXCEED SAFE USE TEMPERATURE AT
FIXTURES. SEE WATER TEMPERATURE
CONTROL WARNING ON PAGE 20. IF HIGHER
PREHEAT TEMPERATURES ARE NECESSARY
TO OBTAIN ADEQUATE BOOSTER OUTPUT,
ADD AN ANTI-SCALD VALVE FOR HOT WATER
SUPPLIED TO FIXTURES.
TWO PRE-HEATERS WITH BOOSTER HEATER
16