A.O. Smith BTN 120 THRU 400/A Series Installation And Operaion Manual page 15

INSTALLATION DIAGRAMS-SIDE INLET/OUTLET USAGE
A listed temperature and pressure relief valve of adequate
capacity is installed on the heater. The locations shown in the
installation diagrams on the following pages are typical.
TWO TEMPERATURE - TWO HEATERS, ONE PRE-HEATER/ONE - BOOSTER HEATER
TEMPERATURE SETTING SHOULD NOT EXCEED SAFE USE
TEMPERATURE AT FIXTURES. SEE WATER TEMPERATURE CONTROL
WARNING ON PAGE 25. IF HIGHER PREHEAT TEMPERATURES ARE
NECESSARY TO OBTAIN ADEQUATE BOOSTER OUTPUT, ADD AN ANTI-
SCALD VALVE FOR HOT WATER SUPPLIED TO FIXTURES.
CIRCULATING RETURN LINE CONNECTION
- FROM FIXTURE LOOP, IF USED, CONNECT
TO PRE-HEATER INLET.
- FROM BOOSTED TEMPERATURE LOOP, IF
USED CONNECT TO BOOSTER INLET.
† AVAILABLE FROM A.O. SMITH.
* PIPE RELIEF VALVE TO OPEN DRAIN.
INSTALL IN ACCORDANCE WITH LOCAL CODES.
WITH OR WITHOUT BUILDING RECIRCULATION
DANGER
The discharge opening of the temperature and pressure relief
valve must be piped to an open drain and should not be subject
to freezing conditions. DO NOT REDUCE, BLOCK OR PLUG
THE DISCHARGE OPENING OF THE VALVE.
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