Condensing Boiler Lockouts From Vent Recirculation Under a Deck
A newly installed 100,000 Btu/h input condensing boiler fires and runs for 8 to 12 minutes, then locks out on flame failure on cold, windy days. The boiler is direct-vented with 2-inch PVC. The exhaust and intake terminate side-by-side under a 10-foot by 12-foot wooden deck; the exhaust is 12 inches above grade and the intake is 12 inches to the side. The installer used 18 feet of pipe and four long-radius elbows on each pipe. The manufacturer permits 2-inch pipe up to 60 equivalent feet with two elbows; each additional long-radius elbow counts as 5 equivalent feet. Combustion analysis at high fire shows O2 4.2%, CO2 10.1%, CO 210 ppm air-free, flue temperature 135°F. At low fire: O2 5.1%, CO2 9.4%, CO 45 ppm. The boiler combustion air intake CO reading is 18 ppm. Gas inlet pressure is 7.0 in w.c. static and 6.6 in w.c. while firing; manifold pressure is per rating. The condensate trap is full and the vent slopes back to the boiler.
1. Calculate the equivalent vent length for the exhaust and intake. Is the installation within the manufacturer's limit?
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Each pipe has 18 ft of straight pipe and four long-radius elbows. The first two elbows are included in the 60 ft allowance; the two additional elbows count as 5 equivalent feet each. Equivalent length = 18 + (2 × 0) + (2 × 5) = 28 ft per pipe. That is below the 60 ft maximum, so the vent length itself is acceptable. However, the calculation does not make the termination location acceptable. The exhaust and intake are under a deck, where flue gas can collect and be drawn back into the intake. Check that termination clearances, snow line, and manufacturer location rules are met. If the manufacturer prohibits termination under a deck or within a certain distance of a corner, the installation is wrong even though the equivalent length is within limits.
2. Interpret the combustion and intake CO readings. What is the most likely cause of the lockouts?
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At high fire the O2 is 4.2%, CO2 is 10.1%, and CO is 210 ppm air-free. At low fire the O2 is 5.1% and CO is 45 ppm. The intake CO of 18 ppm is above normal outdoor ambient and is the key clue: the boiler is drawing some of its own flue gas. Recirculation lowers O2 and raises CO2, and it can drive CO up as the flame becomes unstable or incomplete. The high-fire CO of 210 ppm is not acceptable for a new condensing boiler and should be investigated, but changing the gas valve to cover it would be wrong. The lockouts on cold, windy days fit a termination under a deck where a pressure eddy holds flue gas. Check gas inlet pressure while firing, manifold pressure, vent slope, and condensate trap, but correct the termination first.
3. What permanent correction and verification steps would you require?
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Relocate the intake and exhaust so they are not under the deck and so the intake cannot draw flue gas. Follow the manufacturer's clearance diagram for roof or sidewall termination, maintain separation between intake and exhaust, and keep both above snow and away from windows, doors, and corners. Do not simply lengthen the exhaust only or add a makeshift shield. After the correction, perform combustion analysis at low and high fire, recording O2, CO2, CO air-free, flue temperature, and stack pressure. Measure CO in the combustion air intake; it should be near ambient. Verify stable ignition, no lockouts over several cycles, correct condensate drainage, and proper vent slope. If CO remains high after termination is corrected, then check gas pressure, orifice size, and heat exchanger condition.