Added Load on an Existing 1/2 in Gas Line and CSST Routing
A 1950s house has a 1/2 in black-iron gas line from the meter to an 80,000 BTU/h furnace. The owner is adding a 199,000 BTU/h tankless water heater and a 60,000 BTU/h gas range. The contractor proposes to keep the existing 1/2 in line, install a 1/2 in tee at the furnace, and run 3/8 in corrugated stainless steel tubing (CSST) 45 ft with six 90° elbows to the range. The CSST will pass through a 10 ft section of return-air plenum and be bonded with a #12 AWG wire to a junction box on the furnace. The tankless heater will be connected with a 1/2 in flexible connector. The regulator vent on the new meter regulator terminates 18 in below a bedroom window and 2 ft from the dryer vent. No sediment trap is shown on the drawing. The local utility supplies natural gas at 7 in w.c. at the meter.
1. Identify the code and installation defects in the proposed piping layout, and explain the hazard of each.
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Defects: (1) The existing 1/2 in black-iron line was sized for 80,000 BTU/h; adding 259,000 BTU/h of load without a sizing calculation is likely undersized. (2) 3/8 in CSST, 45 ft with six 90° elbows, is probably beyond its capacity for 60,000 BTU/h; use the manufacturer's table with equivalent length. (3) CSST through a return-air plenum is not permitted unless the tubing is listed for plenum use; ordinary CSST cannot be run in ducts. (4) Bonding must be per the CSST manufacturer, typically a minimum #6 AWG copper conductor to the grounding electrode or panel, not #12 to a junction box. (5) The regulator vent is too close to the bedroom window and dryer vent and is below an opening; it must terminate outdoors where gas can disperse, away from openings and air intakes per B149.1 and the regulator manufacturer. (6) A sediment trap is required at each appliance; a flexible connector through a wall is prohibited. (7) The tankless heater may need a larger connector and a sediment trap, and its minimum inlet pressure must be checked.
2. Describe the sizing check you would perform before approving the installation, including the data you would collect.
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Collect the total connected input, the pressure at the meter, the allowable pressure drop, and the appliance minimum inlet pressures. Measure the actual route and convert each fitting to equivalent length: a 90° elbow may add several feet depending on diameter and table. Then use the manufacturer's CSST capacity table for 3/8 in at the total equivalent length and the available pressure drop. Check the 1/2 in black-iron trunk for the new total load using the appropriate table and the longest run. Check the meter and regulator capacity. If the 3/8 in CSST cannot carry 60,000 BTU/h at 45 ft plus fittings, upsize to 1/2 in or 3/4 in. Do not guess from pipe diameter alone; CSST capacity varies by brand and liner.
3. State the sequence for correcting and testing the system.
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Shut off gas, obtain the required permit, and revise the drawing. Install a properly sized trunk and branch, sediment traps at the furnace, range, and tankless heater, and a listed connector. Reroute the CSST out of the plenum or use listed plenum-rated conduit. Bond the CSST with the manufacturer's required conductor to the grounding electrode. Relocate the regulator vent outdoors, above grade, away from the window, dryer vent, and any air intake. Pressure-test the new piping, purge, and leak-test with a combustible gas detector and soap solution. Set the appliance pressures, clock the input where possible, and perform combustion analysis. Document all corrections and notify the owner.