Adding a Tankless Water Heater to an Undersized 1/2 in. Gas Line
A single-family house has a natural gas meter with a 7 in. w.c. outlet pressure. The existing furnace is rated 80,000 BTU/h and is fed by 35 ft of 1/2 in. Schedule 40 black iron measured from the meter, including four 90° elbows and a 6 ft connector. Static pressure at the furnace inlet is 6.4 in. w.c. with only the furnace running. The homeowner is adding a 40,000 BTU/h tankless water heater on a 15 ft branch taken off the same 1/2 in. trunk, making the longest developed length from the meter 50 ft. The adopted code is the longest-length method with a 0.5 in. w.c. allowable drop and 0.60 specific gravity. The installing technician proposes to tee directly into the existing 1/2 in. line and add a shutoff valve and sediment trap for the new appliance, arguing that the meter's 7 in. w.c. outlet leaves plenty of margin.
1. Evaluate the existing 1/2 in. run for the furnace alone. Is it adequate, and what does the measured 6.4 in. w.c. tell you?
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Start with the load in cubic feet per hour: 80,000 ÷ 1,030 ≈ 78 cfh for the furnace alone.
On the 0.5 in. w.c. drop table for 1/2 in. Schedule 40 at roughly 40 ft equivalent length, capacity is only about 63 cfh - below the 78 cfh required. The run is already undersized before anything is added.
The measured 6.4 in. w.c. inlet confirms it: 7.0 − 6.4 = 0.6 in. w.c. drop, which exceeds the 0.5 in. w.c. allowance with only one appliance running. The 7 in. w.c. meter outlet is not margin to spend - the code allowance is a pressure drop budget, not an absolute pressure target, and every appliance must still see its rated pressure (typically 7 in. w.c. or the nameplate minimum) at full load.
The proposed tee would make it worse: with the tankless firing, total load becomes about 117 cfh through the same 1/2 in. trunk, driving the drop well past 1 in. w.c. The furnace would then be starved, causing weak flames, delayed ignition, and nuisance pressure-switch or flame-sense faults. Do not tee into this line.
2. Size the trunk and the new branch for a total load of 120,000 BTU/h at 50 ft developed length. Show the numbers you used.
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Convert the load: 120,000 ÷ 1,030 ≈ 117 cfh, rounded to 120 cfh for sizing.
Longest length from the meter is 50 ft (40 ft of trunk to the farthest appliance plus branch and fittings). Using the 0.5 in. w.c. drop, 0.60 SG table:
- 1/2 in. at 50 ft ≈ 56 cfh - no
- 3/4 in. at 50 ft ≈ 115 cfh - just under 120, no
- 1 in. at 50 ft ≈ 215 cfh - yes
So the trunk from the meter must be 1 in. black iron, reduced to 3/4 in. only after the last branch if the remaining load allows it. The 15 ft tankless branch carrying 40,000 BTU/h ≈ 39 cfh is served by 1/2 in. at that length (capacity well over 100 cfh), but many tankless manufacturers require 3/4 in. at the unit connection - check the installation manual, which overrides the table.
Confirm with the adopted table rather than these approximations, include fitting allowance in the developed length, and verify the result by clocking the meter with all appliances at full fire. Confirm the meter itself is rated for 120 cfh plus the existing connected load.
3. What installation details must the new branch include, and what would you check before putting the appliances back in service?
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Mechanical details for the branch:
- Drip leg/sediment trap at the appliance connection, with the tee oriented so debris falls into the capped leg, not into the valve.
- Listed shutoff valve ahead of the union, in the same room and within the code-specified distance of the appliance, accessible and clearly identified.
- Union or flanged connection for service, plus proper support and protection where the pipe passes through framing.
- No concealed or prohibited locations - no running through return-air plenums or concealed spaces without the permitted methods; sleeve and seal penetrations.
- If any CSST is used, it must be listed, bonded per the manufacturer's instructions and local code, and not used where prohibited.
Before returning to service: pressure-test the new piping (typically 3 psi air for a specified duration on a closed system, or with the appliances isolated), purge and leak-test every joint with soap solution or an electronic detector, verify no cross-connections, then operate all appliances simultaneously and confirm inlet pressure at each is at or above nameplate minimum with the others firing. Finally, verify the furnace's manifold pressure and temperature rise after the trunk change, since the corrected pressure can alter firing characteristics.