A.O. Smith BTH 120 - 250 - Manual (Page 25)

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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 28.
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 4.
IF HIGHER PREHEAT TEMPERATURES ARE NECESSARY TO OBTAIN
ADEQUATE BOOSTER OUTPUT, ADD AN ANTI-SCALD VALVE FOR HOT
WATER SUPPLIED TO FIXTURES.