Adding a 199,000 BTU/h Tankless Heater to an Existing Gas System
A ranch house is served by a 7 in. w.c. (1/2 psi) natural gas system. The meter outlet is 1-inch black iron, and the developed length from the meter to the mechanical closet is 45 ft. Existing loads are an 80,000 BTU/h furnace, a 40,000 BTU/h range, and a 35,000 BTU/h dryer on a 1/2-inch branch. The owner adds a 199,000 BTU/h condensing tankless water heater in the same closet, and the installer plans to tee off the existing 1/2-inch dryer branch with 6 ft of 1/2-inch pipe and a 3/8-inch flexible connector. The closet measures 8 ft x 6 ft x 8 ft and has only a louvered door to the hallway.
1. Total the connected load. Explain why the proposed 1/2-inch branch cannot serve the tankless heater, and what piping you would specify instead.
Read the worked answer
Load: 80,000 + 40,000 + 35,000 + 199,000 = 354,000 BTU/h, which is about 354 CFH of natural gas at 1,000 BTU/ft3 (correct upward for altitude).
Why 1/2-inch fails: the longest run to the tankless is roughly 45 + 6 = 51 ft. At that length with the code's customary 0.5 in. w.c. total pressure drop, 1/2-inch schedule 40 steel carries on the order of 60-75 CFH - well under a quarter of the tankless demand. The appliance cannot fire at rating; expect nuisance ignition faults, error codes, and a call-back.
What to specify:
- Main: check 1-inch at 45 ft for 354 CFH. It is typically in the 230-260 CFH range - already short before the tankless. Plan on 1-1/4-inch, or convert to a 2 psi meter/regulator system with a line regulator at each appliance.
- Meter: 354 CFH is at or above many standard residential meter ratings. Call the utility before you pipe anything - a meter upgrade may be required.
- Branch to the heater: 3/4-inch minimum at 51 ft (1-inch is often required by the manufacturer's instructions for 199,000 BTU/h). Eliminate the 3/8-inch connector; use a listed connector sized for the full load.
- Always confirm against the manufacturer's required minimum inlet pressure and allowable drop, which can be stricter than the general table.
2. Evaluate the closet for combustion air. Show the calculation and state the options.
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Volume: 8 x 6 x 8 = 384 ft3.
Unconfined threshold: the usual rule is 50 ft3 of volume per 1,000 BTU/h of input in the space. 199,000 / 1,000 x 50 = 9,950 ft3 required (worse if the furnace shares the space). The closet is confined by a wide margin.
A louvered door alone does not fix it unless the combined space is genuinely unconfined and the louver free area is documented - inspectors will not take it on faith.
Options:
- Two permanent openings, one within 12 in of the ceiling and one within 12 in of the floor, each with 1 in2 free area per 1,000 BTU/h = 199 in2 when they connect directly to outdoors or to a properly sized space. If the openings feed through horizontal ducts, double it to about 398 in2 each.
- Direct-vent (two-pipe) installation - a condensing tankless that draws its own combustion air from outdoors eliminates most of the combustion-air opening requirement. Follow the listing and the manufacturer's clearances and termination rules.
Also verify the closet is not a bedroom or bathroom and that all listed clearances are maintained.
3. Name three other items the inspection will check on this installation.
Read the worked answer
Pick any three, with the reasoning:
- Sediment trap / drip leg at the appliance, plus a full-port manual shutoff in the same room and a union. The trap keeps scale and pipe dope out of the gas valve; the shutoff is needed for service and emergency.
- Appliance connector: listed, no longer than about 6 ft, not concealed in or through a wall, floor, or ceiling, and not used to connect two sections of permanent piping.
- Venting: a condensing appliance must use the vent material named in its listing (PVC, CPVC, or polypropylene as specified), with correct slope back to the appliance, proper termination clearances, and a condensate drain with a neutralizer where required. A single-wall galvanized vent here would be a serious error.
- Bonding: if corrugated stainless steel tubing is used anywhere, the system must be bonded to the grounding electrode per the manufacturer's instructions (typically 6 AWG), and the metallic gas piping system must be bonded.
- Support and protection: hangers at code intervals, and protection where the pipe passes through framing or is subject to damage.