Condensing furnace vent sagging under a deck and locking out on pressure switch
A 90,000 BTU/hour, 90% AFUE condensing furnace is installed in a basement. The installer used 2-inch PVC for both intake and exhaust. The exhaust run is about 35 feet with four 90-degree elbows and terminates under a wood deck, 12 inches below the joists and 18 inches from an operable window. The pipe is supported only at the furnace and at the termination; it sags in two places, creating a low spot about 10 feet from the furnace. The termination elbows point down, and the exhaust and intake are 6 inches apart. The condensate trap is dry and has no visible water. The furnace runs, but the homeowner reports occasional exhaust odors near the basement stairs, and the pressure switch has locked out twice.
1. What are the hazards and likely causes of the pressure-switch lockouts?
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The sagging exhaust creates a low point that collects condensate, reducing the effective vent area and causing the pressure switch to sense insufficient draft; that fits the intermittent lockouts. The dry condensate trap can allow flue gases to spill into the basement instead of draining, which is a carbon-monoxide hazard. Termination under a deck, 18 inches from an operable window, and with the exhaust and intake only 6 inches apart increases the chance that exhaust is recirculated into the intake or enters the house through the window. The downward-pointing termination can also collect water and debris. I would treat the exhaust odor and lockouts as a safety issue: recommend the system be evaluated by a qualified HVAC technician before further use, and advise the homeowner to ventilate and call for service if odor persists. Do not reset the furnace repeatedly as a fix.
2. What installation details should you verify against the manufacturer's instructions, and how do you report them without overstating code?
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Verify the vent diameter and equivalent length against the furnace's vent table, counting elbows and termination fittings; check whether 2-inch PVC is permitted for this input and length. Check slope back to the furnace, support spacing, and absence of sags or traps. Check termination clearances from operable windows, doors, intake, grade, and under-deck conditions; check that the intake and exhaust are separated per the manufacturer. Check that the condensate trap is primed and drains. In the report, state what you observed and what the manufacturer requires, and refer to local code for exact clearances; do not invent a number if you are unsure. Recommend correction by a licensed HVAC contractor and mention that pressure-switch lockouts can recur until the vent and condensate issues are corrected.
3. The homeowner proposes drilling a drain hole at the low spot in the PVC to let condensate out. How do you respond?
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That would create an opening in the flue that can release combustion products, including carbon monoxide, into the basement. It would also weaken the pipe and may allow air to enter, affecting the pressure switch and combustion. The correct repair is to re-support and re-slope the vent, eliminate the sag, prime the trap, and correct the termination, not to perforate the flue. I would explain the risk plainly and put the recommendation in writing. If the homeowner has already drilled the pipe, I would advise shutting down the furnace and having it repaired before use. The goal is to remove the water by gravity back to the furnace, not to drain it into the room.