Flue Gas Spillage, a Disconnected Return, and a Tune-Up Invoice
You are called to a 2007 house with a 3-ton R-410A split system: an 80% gas furnace in the attic and a matching outdoor condenser. In the attic you find the furnace's single-wall galvanized flue connector running about 12 feet through the unconditioned space to a Type B chimney, with rust streaks at the draft hood and a corrosion line at the joints. The burner sight glass shows scorching at the top of the burner box. The return duct is a panned joist cavity that has separated at a seam, so the furnace is drawing return air from the 135-degree F attic. The filter is crushed and the blower wheel is visibly dirty. Downstairs, the water heater shares the same chimney flue. The seller's file contains a paid invoice from eight months ago for 'furnace tune-up and safety check, all clear.' The buyer has an infant and asks whether the air is safe to breathe.
1. Explain what the rust, corrosion, and scorching indicate, how the disconnected return fits in, and what you write without diagnosing a cracked heat exchanger.
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Read the clues as a chain, then stop short of a diagnosis. Rust at the draft hood and a corrosion line at the vent joints mean flue gases have been condensing inside the connector - combustion products are lingering long enough to cool and condense, which happens when draft is weak or the vent runs cold. Scorching at the top of the burner box is evidence of flame roll-out: the flame is spilling out of the combustion chamber instead of traveling up the flue. Both point to spillage. Spillage can come from a blocked or undersized flue, negative pressure in the attic, a cracked heat exchanger, or overfiring. I can see the evidence; I cannot see which cause applies.
The disconnected return is the piece most inspectors miss. A panned joist return that has separated pulls return air from the attic - 135-degree F air, plus attic dust and insulation fibers - and depressurizes the house. Depressurization is the classic driver of spillage, and here the water heater is competing for the same chimney. The crushed filter and dirty blower wheel compound it: restricted airflow raises heat-exchanger temperature, which is one of the ways they crack in the first place.
Report it as a safety item with an instruction. 'Evidence of flue gas spillage was observed at the furnace: rust at the draft hood, corrosion at vent joints, and scorching at the burner box. Recommend the appliance not be operated until it is evaluated by a licensed HVAC contractor, and that the venting, combustion air, and heat exchanger be inspected at that time. A cracked heat exchanger cannot be confirmed or ruled out by visual inspection. The single-wall connector in the unconditioned attic is also improper.'
2. Is CO testing required, and what do you actually tell the buyer who asks whether the air is safe?
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It is not required, and it is not a substitute for a combustion analysis. A handheld CO meter read at a register gives one number at one moment under one set of conditions. A furnace reading 0 ppm in the supply during my visit can spill when a bathroom fan, a dryer, or a gust of wind changes the pressure balance. An inspector who writes 'no CO detected, furnace is safe' has made a claim the instrument cannot support, and that sentence will be remembered.
What I actually do. If I carry an analyzer, I use it as a screening tool and describe it that way: 'Spot readings with a handheld CO analyzer at supply registers during the inspection did not indicate elevated CO at that time. This is not a clearance test and does not evaluate the heat exchanger.' If I do not carry one, I say the appliance showed evidence of spillage and the evaluation needs to be done by someone with a combustion analyzer.
The referral, specifically. Ask for combustion analysis, draft measurement, CO in the flue before dilution, CO in the supply plenum, and inspection of the heat exchanger by whatever means the contractor uses, including a camera where accessible. Also ask that the return duct be repaired and the venting corrected.
Answer the parent's question directly. I can say my observations indicate combustion gases may be entering the living space and that the system should not run until evaluated. Recommend CO alarms on each level and outside sleeping areas - they are inexpensive and this household has an infant.
3. The seller has a paid invoice from eight months ago saying the furnace was found 'all clear.' What weight do you give it, and what about the unit's age?
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An invoice documents that someone was there; it does not document condition. 'Tune-up and safety check, all clear' is exactly the paper that makes a buyer stop worrying. It does not tell me what was checked, with what instrument, or whether the heat exchanger was examined at all. Many 'safety checks' are a flame observation and a filter change.
What I write. Acknowledge it without deferring to it: 'The seller provided an invoice dated eight months ago for a furnace tune-up. The scope of that service and the methods used were not documented and could not be verified. The conditions observed at this inspection warrant independent evaluation.' That is honest - it does not accuse anyone of anything, and it does not let the invoice neutralize my observations.
On age. A 2007 furnace is 17 years old against a typical 15-20 year service life. That is context, not a prediction. I report the manufacture date from the data plate, note the age relative to typical life, and do not estimate remaining life or predict failure - the same rule I apply to roofs and water heaters. The buyer's decision should turn on the spillage evidence, not on a life-expectancy guess.
One practical suggestion. If the contractor who wrote 'all clear' is the one the seller recommends for the repair, the buyer should obtain an independent evaluation. Suggesting that is not an accusation; it is ordinary prudence, and a reputable contractor will not object.