Furnace Panic Doctor

Overview

The furnace dies at 2 AM in January. Panic. Cold house, kids asleep, unknown repair bill. Homeowners either (a) call emergency service immediately — $300 trip fee for a problem that was a $0 filter change, or (b) freeze until morning with a problem that was actually dangerous (cracked heat exchanger symptoms are not a filter problem).

This skill is the calm voice in the middle of the night: a 10-minute, no-tools diagnostic tree that ends in one of three verdicts:

1. DIY-FIX — exact steps (filter, thermostat batteries, pilot relight, breaker, exhaust vent blockage, drain line)
2. PRO-CALL — what's broken, what it likely costs, what to say on the phone, and whether "emergency" pricing is warranted
3. STOP-NOW — safety lines (soot at registers, sulfur smell, CO alarms, burning electrical smell, fuel odors): shut down per the script and act on the danger, not the comfort

It also produces a technician briefing sheet — symptoms, what you checked, model/serial, timeline. Techs fix documented problems faster and charge less diagnostic time.

When to Use

• "Furnace/heater/boiler not working" — any fuel, any brand
• Weak or uneven heat, blower runs constantly, short cycling
• Thermostat blank or unresponsive
• Pilot light out / won't stay lit (gas)
• Error codes on the control board (LED blink patterns)
• Burning smell when heat starts (dust vs. electrical vs. worse)
• Loud noises: scraping, bang-boom on ignition, screech
• Water around the furnace / AC coil
• "Is this dangerous or just cold?" — the safety triage question

Don't use for: heat pumps in cooling-mode problems, active fire/gas emergency (call the gas utility's emergency line — script covers this), boilers with leaking pressure relief valves spraying water (shut off feed, call plumber). If any CO alarm sounds: everyone out, then 911/gas utility — no diagnostics.

The diagnostic tree (summary)

Full script in references/diagnostic-tree.md; flow:

SAFETY TRIAGE (30 seconds)
  CO alarm / sulfur / fuel smell / smoke → STOP-NOW script
  → otherwise ↓
THERMOSTAT (2 min): blank? batteries? set HEAT+AUTO, fan ON, target 5° above
  room. Did it click?
POWER (2 min): furnace switch ON? breaker? door safety switch engaged?
  (blower test: fan ON at thermostat)
FUEL (2 min): gas valve handle parallel to pipe? pilot lit / hot-surface
  igniter glowing? oil tank level? propane %?
AIRFLOW (2 min): filter (2-inch dust ring = culprit #1), registers open,
  exhaust vent exterior clear (snow/bird nest), condensate drain flowing
OBSERVE ONE CYCLE (3 min): ignition → flame (blue, steady, all burners)
  → blower starts within ~1 min → warm air at nearest register
  → cycle length 5-15 min → quiet shutdown
  Anything else → symptom table match
SYMPTOM TABLE: short-cycling / no-ignition / ignites-then-dies / blows-cold /
  noise-X / smell-Y / water / error-code-N → cause list with DIY flags
VERDICT: DIY-FIX steps | PRO-CALL brief | STOP-NOW script

Files

Usage

# The night it happens: interactive walk-through
python3 scripts/furnace_check.py

# Jump straight to a symptom
python3 scripts/furnace_check.py --symptom "short cycling"

# Non-interactive verdict from flags you already checked
python3 scripts/furnace_check.py --no-heat --filter-clean --pilot-lit

# Print a technician briefing for what you've ruled out
python3 scripts/furnace_check.py --symptom "blows cold" --brief

Workflow

1. Ask the safety-triage questions FIRST (script does this automatically). Any STOP-NOW hit: execute that script (shut fuel/electric per manual, evacuate for CO, utility emergency numbers), nothing else matters.
2. Run the interactive tree — it asks one question at a time, with photos described in words ("handle parallel to the pipe = OPEN").
3. Reach a verdict. DIY-FIX: follow numbered steps exactly; skip "seems similar" shortcuts. PRO-CALL: generate the brief, make normal-hours call if the house is safe and pipes won't freeze (see threshold calc).
4. Freeze protection while you wait: run the pipe-protection math (script's --freeze-risk helper): house temp drop rate, which faucets to drip, when to drain if truly cold.
5. After the fix: log what it was (--log) — next failure starts from history, not zero.

Common Pitfalls

1. Relighting a pilot repeatedly. A pilot that won't STAY lit is a thermocouple (cheap PRO fix) or a drafting problem — not a match problem. Three attempts max, then stop (gas accumulation risk).
2. "Closed" vents save money myth. Closing registers restricts airflow, overheats the exchanger, trips high-limit → short cycling that looks like a control failure. Open them.
3. Filter direction matters. Arrow on the frame = airflow direction, toward the blower. Backwards filters flap, whistle, and shed dust.
4. The furnace switch looks like a light switch. Someone flipped it thinking it was lights. It's the #2 no-heat cause after filters.
5. Testing right after a fix. Furnaces lock out for 1-3 failed ignition attempts; power-cycle at the switch (30 s off) before declaring failure.
6. Space heaters as the plan. Fine for one room overnight; not a whole-house strategy. Do the freeze-risk math instead of hoping.
7. CO "nuisance alarm" assumption. It's never a nuisance until a professional says so. Ageing alarms false-alarm at LOW levels — treat every alarm as real, get out, test doesn't continue until cleared.

Example Session

2 AM, gas furnace, no heat, house 15°C and dropping. Run script: thermostat OK → switch OK → breaker OK → blower runs on FAN-ON (motor alive) → no ignition glow, no flame → filter filthy BUT that's airflow not ignition... symptom table: no-ignition + blower-OK → igniter or flame sensor. DIY: sand flame sensor with dollar bill? — modern call: NO (see pitfalls). Verdict: PRO-CALL, likely flame-sensor clean/replace, $150-350 normal hours. Freeze math: house loses 1.2°C/h → 8 h to pipe-risk threshold at -8°C outside → safe to wait for morning; drip the faucet furthest from the heater. Brief printed with model plate location noted.

Verification Checklist

• Safety triage completed before ANY diagnostic step
• Verdict names the most likely cause(s), not just "call someone"
• DIY steps tried in order, one at a time, furnace power-cycled between
• Freeze-protection math done if waiting until morning
• Technician brief has: symptoms, checks done, model/serial, timeline