The Furnace That Passed Last Year

The rise of 60°F falls inside the manufacturer's stated 35–65°F range, so on its face it is not a defect. But the number deserves scrutiny before you write anything. First, confirm where the readings were taken: a rise measured at the plenum within a few feet of the heat exchanger is not the same as one taken at a distant register, and duct losses can make a marginal rise look acceptable. Second, note that a rise near the top of the range often points to restricted airflow - a dirty blower wheel, a clogged evaporator coil, or undersized return ducting - rather than a combustion problem. That matters because restricted airflow shortens heat exchanger life and can trip the high-limit switch intermittently. Third, the rust staining and chalky residue at the draft hood are separate observations that suggest possible venting or combustion issues. My report would state the measured rise, the manufacturer's range, the conditions of measurement, and a recommendation that a qualified HVAC technician verify airflow and combustion performance. I would not call it a failing condition, but I would not call it clean either.

The sticker changes nothing about what I observed and everything about how I phrase it. My duty is to report the condition of the system at the time of inspection, not to audit the prior technician's work. A certification nine months old does not cover the rust staining, the residue, or any condition that developed since. It also does not tell me what was actually done - 'cleaned and certified' can mean anything from a filter change to a full combustion analysis. In the report I would describe the observed conditions factually, note that the homeowner reported recent servicing, and recommend that a qualified technician evaluate the venting and combustion characteristics. I would avoid language implying the previous contractor was negligent, since I have no basis for that conclusion and it invites a dispute I cannot win. The useful posture is: here is what I saw today, here is why it warrants follow-up, and here is who should look at it.

Rust at a flue connector commonly indicates condensation inside the vent - often from an oversized flue relative to the appliance, a cold chimney, or spillage - and chalky residue near a draft hood can suggest prolonged moisture exposure or spillage of combustion products. Together they raise a legitimate concern about venting performance. But an inspector cannot diagnose the cause from visual evidence alone. Confirming spillage requires a draft test under worst-case conditions, and confirming condensation requires understanding flue sizing, chimney condition, and appliance cycling. My conclusion should be limited to what I can defend: visible corrosion and residue consistent with moisture or spillage, which warrants evaluation by a qualified technician using combustion and draft testing. I would flag it as a safety-relevant observation, recommend prompt evaluation, and avoid asserting a specific root cause I have not verified. Overstating the cause is the more common error than missing the observation.