A.O. Smith 400A - Manual (Page 32)

3
TWO TEMPERATURE - TWO HEATERS HIGH TEMPERATURE
WITH OR WITHOUT BUILDING RECIRCULATION
TWO TEMPERATURE - THREE HEATERS (TWO PRE-HEATERS/ONE BOOSTER HEATER)
WITH OR WITHOUT BUILDING RECIRCULATION
MUST BE IDENTICAL HEATERS
FOR MULTIPLE HEATER INSTALLATION SEE MANIFOLD KIT
SPECIFICATIONS, PAGE 35.
CAUTION: IF BUILDING COLD WATER SUPPLY HAS A BACK-
FLOW PREVENTER, CHECK VALVE OR WATER METER WITH
CHECK VALVE PROVISIONS FOR THERMAL EXPANSION OF
WATER IN THE HOT WATER SYSTEM MUST BE PROVIDED.
DANGER
TEMPERATURE SETTING SHOULD NOT EXCEED SAFE
TEMPERATURE AT FIXTURES. SEE WATER TEMPERATURE
CONTROL WARNING ON PAGE 3. IF HIGHER PREHEAT
TEMPERATURES ARE NECESSARY TO OBTAIN ADEQUATE
BOOSTER OUTPUT, ADD AN ANTI-SCALD VALVE FOR HOT
WATER SUPPLIED TO FIXTURES.