17
Gas Pressure Adjustment
Figure 16. Robertshaw 7000 DERHC (Heater Model 408)
Gas Pressure Adjustment
Figure 17. Honeywell VR8304 (Heater Models 268 and 408)
5.6.4. Pipe Sizing for Gas Connection
These capacities shown below based on using SCH 40
black iron pipe. For capacities using other materials,
consult local code.
Model
Input
3/4"
1"
1-1/4"
1-1/2"
(KBTU) N
P
N
P
N
P
N
P
268
266.0 15
(5)
35
(11)
50
(15)
125
(38)
210
(64)
480
(146)
445
(136)
408
399.0
*
15
(5)
20
(6)
55
(17)
95
(29)
225
(69)
215
(66)
480
(146)
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
* A 3/4" gas line can be used for up to 5' (1.5 m) maximum length from the gas valve
in addition to the sediment trap.
Table H. Maximum Equivalent Pipe Length (ft/m)Flow
Rates
Model
Pipe Size
Min. GPM
(lpm)
Max. GPM*
(lpm)
268
1-1/4"–1-1/2" - 2"
25 (95)
125 (473)
408
1-1/4"–1-1/2" - 2"
40 (151)
125 (473)
* When flow rates exceed maximum GPM (lpm) an external auxiliary bypass valve
is required. See external bypass valve section for details.
Table I.
Flow Rates
Figure 18. External Bypass Valve
Flow
GPM (lpm)
Pressure Drop (Ft of Head)
268
408
30 (114)
8.0
40 (151)
9.0
9.0
50 (189)
9.8
9.8
60 (227)
10.5
10.5
70 (265)
11.0
11.0
80 (302)
11.5
11.5
90 (341)
14.0
14.0
100 (378)
17.0
17.0
Table J.
Heat Exchanger Pressure Drop
