Gas Piping for a Duplex Tankless Conversion
A duplex is being converted from two 40-gallon natural gas storage water heaters to one 199,000 Btu/h condensing tankless heater and a new 40,000 Btu/h range in each unit. The existing service is 2 psig at the meter, with a 250-cubic-foot-per-hour meter and a 2-psi-to-7-inch water column regulator at the building. The proposed gas header will run 70 feet from the meter to the farthest appliance, with branches of 25 feet to the tankless, 30 feet to the Unit A range, and 45 feet to the Unit B range. The contractor wants to reuse the existing 1/2-inch black iron branch to the Unit A range and run 1/2-inch to the new tankless. The tankless manufacturer requires a sediment trap and a 3/4-inch gas connection. The building has corrugated stainless steel tubing inside a chase.
1. Determine the total connected gas load and whether the existing meter is adequate.
Read the worked answer
Add the connected loads: 199,000 + 40,000 + 40,000 = 279,000 Btu/h. At roughly 1,000 Btu per cubic foot for natural gas, that is about 279 cubic feet per hour. The existing meter is rated 250 cubic feet per hour, so the meter is undersized before any pipe sizing is considered. The 2-psi service and building regulator may also need review, but the meter is the first bottleneck. The code sizing method uses the connected load unless the authority approves a diversity factor; a tankless heater at 199,000 Btu/h plus two ranges is a realistic simultaneous load. The correction is to have the gas utility upgrade the meter and service regulator, then size the header for the new load.
2. Evaluate the proposed 1/2-inch branches and explain the correct sizing approach.
Read the worked answer
Pipe size is selected by the longest-length method. The farthest appliance is the Unit B range at 70 feet from the meter, so the header must carry 279 cubic feet per hour at that length, with allowances for fittings. A 1/2-inch black iron pipe at 70 feet carries only a small fraction of that - in the range of a few dozen cubic feet per hour - so 1/2-inch is not even close for the header. The 1/2-inch branch to the 199,000 Btu/h tankless is also wrong: the manufacturer requires a 3/4-inch connection, and 1/2-inch at 25 feet cannot deliver about 199 cubic feet per hour. The Unit A range at 40,000 Btu/h and 30 feet might appear to pass on some 1/2-inch tables, but it should not be reused without verifying the exact table, fitting allowance, and pressure drop. Use the gas sizing table for the available pressure, add the tankless at its full rating, and increase the header to at least the size the table requires, likely 1-1/4 inch or larger.
3. What is required for the sediment trap, appliance connector, and CSST bonding?
Read the worked answer
A sediment trap is required at the tankless and is good practice at each appliance. It is installed downstream of the appliance shutoff valve and upstream of the controls, with a capped nipple at least 3 inches long at the bottom of a tee, so dirt and pipe scale drop out before reaching the burner. The connector must be listed for gas, not concealed in a wall or ceiling, and not pass through a floor unless protected. The tankless needs its 3/4-inch connection; reducing to 1/2-inch at the appliance is a code and warranty problem. If CSST is used in the chase, it must be bonded per the manufacturer and local code, commonly with a No. 6 copper bonding jumper from the gas piping to the grounding electrode system, and the bond must be accessible. The gas piping must also be tested and purged.