Sizing a Black Iron Gas System from the Plan

Total connected load: 40 + 35 + 80 + 65 + 40 = 260 MBH. Column used for every section: the 70 ft column, because that is the longest length from the meter to the farthest outlet. Every section is sized against that same column even if the section itself is short.

Trunk sections (downstream load determines the size):

• Meter to first tee: 260 MBH. 1-1/4 in. = 398, adequate; 1 in. = 212, not adequate. Use 1-1/4 in.
• After the water heater tee: 260 - 40 = 220 MBH. 1 in. = 212 is just short (220 > 212). Use 1-1/4 in.
• After the dryer tee: 220 - 35 = 185 MBH. 1 in. = 212, adequate. Use 1 in.
• After the furnace tee: 185 - 80 = 105 MBH. 1 in. = 212 and 3/4 in. = 112, both adequate. Use 3/4 in.
• Final leg to the grill: 40 MBH. 1/2 in. = 46, adequate. Use 1/2 in.

Branches (sized on the same 70 ft column):

• Water heater 40 MBH: 1/2 in. = 46, adequate. 1/2 in.
• Dryer 35 MBH: 1/2 in.
• Furnace 80 MBH: 1/2 in. no, 3/4 in. = 112 yes. 3/4 in.
• Range 65 MBH: 3/4 in. = 112, adequate; 1/2 in. = 46, not. 3/4 in.
• Grill 40 MBH: 1/2 in.

Sanity check: the smallest branch is 1/2 in. and every appliance gets a listed shutoff and a sediment trap.

The reason:

• The table entries represent the total pressure drop along the entire run from the meter to the outlet, at a fixed allowable drop (here 0.5 in. w.c.). The pressure available at the farthest outlet is what determines whether that appliance fires properly, so the whole system is designed around that outlet's length.
• Sizing a short section off a short length column assumes pressure that has already been spent upstream of it.

The shortcut, demonstrated:

• The section after the dryer carries 185 MBH and is only 30 ft from the meter. On the 30 ft column, 3/4 in. would appear to carry about 170 MBH - close enough that a crew might upsize to 1 in. and call it done.
• On the 70 ft column, 3/4 in. carries only 112 MBH. The section needs 1 in.
• If the shortcut is used throughout, the last appliance - the grill - gets a trunk that has already spent its pressure drop, and it shows up as a lazy flame, delayed ignition, or a unit that will not stay lit at low fire.

Rule to state on the exam: size by the total load downstream and the longest length from the meter to the farthest outlet, not by the length of the individual section.

New total load: 260 + 200 = 460 MBH. New longest length: 85 ft, so the 70 ft column is no longer valid and the 100 ft column is the safe next step up in the table.

Trunk sizing on the 100 ft column:

• 1-1/2 in. = 500 MBH, adequate for 460 MBH. 1-1/4 in. = 332, not adequate.
• The trunk from the meter to the first tee must be 1-1/2 in., and recheck each downstream section against its new downstream load on the 100 ft column - the runs to the furnace and range may also have to grow.

Beyond pipe size:

• Meter capacity. A standard residential meter flowing roughly 250-400 MBH at 0.5 in. w.c. will not deliver 460 MBH. Call the utility - a larger meter, or delivery at elevated pressure with a pound-to-inches regulator at the house, may be required. Pipe sizing alone cannot fix an undersized meter.
• The pool heater needs its own listed shutoff, sediment trap, and connector, and a union at the appliance.
• Elevated-pressure systems change the whole exercise: the tables, the regulator location, and the required regulator venting all change. Do not upsize pipe and assume an elevated-pressure system is a simple substitute.
• Recheck the longest-length column after the reroute; the new pool run may push the farthest outlet past 85 ft and force another size increase.