Tankless Heater: Undersized CSST and a Combustion-Air Assumption
A 199,000 Btu/h condensing tankless water heater replaces a 40-gallon tank in a 5 ft x 7 ft closet opening off a hallway. It is installed as a two-pipe, sealed-combustion appliance: 2 in. PVC intake and exhaust, each terminating outdoors, with the intake 6 in. above grade and 8 in. from the clothes dryer's exhaust hood. Gas comes from 45 ft (developed length) of 1/2 in. CSST tapped off a 1 in. black iron manifold at the meter that also feeds a 100,000 Btu/h furnace and a range. Static pressure at the tankless inlet is 7.4 in. w.c.; at maximum fire it falls to 4.5 in. w.c., and to 3.9 in. w.c. when the furnace also runs, at which point the heater derates and displays a low-gas-pressure fault. The closet has a 24 x 12 in. louvered door, and the installer states that this supplies all the combustion air the unit needs. The nameplate requires 3.5-10.5 in. w.c. inlet pressure and a maximum of 60 equivalent ft of vent; five 2 in. elbows are installed.
1. Is the 1/2 in. CSST adequate? Use the measured pressures and the piping method to justify the answer, and state the correct fix.
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No. Using the CSST manufacturer's capacity table at 0.60 specific gravity with a 45 ft developed length (fittings converted at their published equivalent lengths), 1/2 in. CSST carries roughly 60,000-80,000 Btu/h - a third of what a 199,000 Btu/h appliance needs. The field data confirm it: 7.4 in. w.c. static to 4.5 in. w.c. under load is a 2.9 in. w.c. drop, and 3.5 in. w.c. with the furnace running. The manufacturer's fault code and derate are the appliance protecting itself from a starved gas train, not a defective heater.
The proper correction is a dedicated 3/4 in. or 1 in. run from the meter manifold, sized with the longest-length method for the full connected load including the furnace and range, with a full-size sediment trap, a listed shutoff within reach, and a leak test. Then measure again at maximum fire with every appliance running; the dynamic inlet pressure must stay comfortably above the 3.5 in. w.c. minimum. Do not compensate by raising the meter regulator: appliance maximum inlets are typically 13.8-14 in. w.c., the other appliances would be pushed out of range, and the tankless's modulating valve needs a stable supply, not a higher one.
2. The installer says the 24 x 12 in. louvered door supplies the combustion air. Evaluate that claim, and explain what would change if this were a single-pipe installation.
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For this appliance the claim is irrelevant: with a two-pipe sealed-combustion installation, combustion air comes down the outdoor intake and the closet free area is not the air source. The louvers may matter for the manufacturer's clearance and ventilation requirements and they mean the closet is not a tight combustion appliance zone, but they are not combustion air and their size is not a code issue here.
What is a real issue is the intake termination 8 in. from the dryer hood. Most manufacturers require 3 ft from any exhaust outlet, and drawing lint, moisture and detergent residue into the intake can foul the burner and the flame sensor. If this were a single-pipe indoor-air installation, NFPA 54's two-opening method would require one opening near the top and one near the bottom, each sized at 1 in² per 1,000 Btu/h of input - 199 in² each for this appliance. The 24 x 12 in. door is 288 in² gross, but louver free area is typically 50-75%, it is a single opening with no high/low arrangement, and it would be drawing from the hallway and bedrooms. On that basis the door would fail.
3. Before sign-off, what must be corrected, verified, or documented beyond the gas line?
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Vent length: five 2 in. elbows at roughly 5-8 equivalent ft each, plus the straight run, must be totaled against the 60 ft maximum; if it is over, upsize to 3 in. or re-route rather than ignore it. Slope: condensate must drain back to the unit at the manufacturer's slope, with a neutralizer where required and freeze protection on the condensate line.
Terminations: relocate the intake for the required clearance from the dryer hood, confirm clearances to operable windows, doors and the gas meter/regulator (typically 4 ft from the regulator vent), and confirm the exhaust plume does not hit the wall or a walkway.
Appliance: relief valve piped to a safe discharge, clearance to combustibles per the plate, correct vent material and primer, and the manufacturer's required clearances inside the closet.
Records: a leak test on the new run, a pressure reading at maximum fire with all appliances operating, and a combustion analysis at both minimum and maximum fire - air-free CO, O2/CO2, and a supply-air CO check against ambient. Photograph the terminations and the data plate and note the corrections in the invoice.