25
Maximum Equivalent Pipe Length (ft)
Natural Gas 1000 BTU/FT
3
0.60 Specific Gravity @ 0.5 in. WC Pressure Drop
Propane Gas 2500 BTU/FT
3
1.53 Specific Gravity @ 0.5 in. WC Pressure Drop
Input 3/4” 1” 1-1/4” 1-1/2”
Model (KBTU) N P N P N P N P
259 250.0 15 35 50 125 210 480 445
409 399.0 * 15 20 55 95 225 215 480
*A 3/4” gas line can be used for up to 5 ft maximum length from the
gas valve in addition to the sediment trap.
PIPE SIZING FOR GAS CONNECTIONS
MODEL PIPE SIZE MIN. GPM MAX. GPM*
259 1-1/4”–1-1/2” - 2” 25 125
409 1-1/4”–1-1/2” - 2” 40 125
*When flow rates exceed maximum GPM an external auxil-
iary bypass valve is required. See external bypass valve
section for details.
FLOW RATES
HEAT EXCHANGER PRESSURE DROP TABLES
BRASS HEADER
FLOW
GPM
PRESSURE DROP (FT OF HEAD)
259 409
20
25 4.4
30 5.2
35 5.3
40 5.4 9.9
50 5.8 10.2
60 6.4 10.8
70 7.6 11.6
80 8.6 12.5
90 10.6 13.6
100 11.8 15.0
110 21.0 23.5
120 26.0 28.5
125 28.0 30.0
INTERNAL AUTOMATIC BYPASS VALVE
A built-in automatic bypass valve is provided in the
in/out header. The internal bypass valve automatical-
ly responds to changes in water flow in the piping sys-
tem. Proper amount of water is maintained through
the heater under varying flows dictated by the condi-
tions of the pump and filter.
An auxiliary bypass valve should be used when flow
rates exceed 125 GPM. Usually a high-performance
pump size larger than two horsepower will exceed this
flow rate. This valve is required to complement the
function of the automatic bypass valve, particularly
when starting the heater in winter or early spring when
the spa or pool temperature is below 50°F. It also
serves to eliminate needless pressure drop through
the heater and accompanying reduction in the flow
rate to the spa jets, etc.
EXTERNAL AUXILIARY BYPASS VALVE
(Where Required)
GAS PRESSURE ADJUSTMENT LOCATIONS
GAS INLET
GAS CONTROL
SWITCH SHOWN IN
“OFF” POSITION
From Heater To Heater
From
Pool/Spa
To
Pool/Spa
AUXILIARY BYPASS VALVE
(DO NOT USE GATE VALVE)
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