No Heat: Furnace Won't Ignite
The no-heat call is the bread and butter of every winter route. Work the ignition sequence in order and the furnace tells you exactly where it stopped — no guessing, no parts cannon.
Stuck at a flash code?
Ventora's AI troubleshooter walks you through the ignition sequence step by step and decodes the board.
In This Guide
Know the Sequence of Operation
Almost every no-heat diagnosis comes down to one question: where in the ignition sequence did the furnace stop? A modern induced-draft gas furnace fires the same way every time, and the ignition control board acts as a referee — if any safety isn't satisfied, the sequence halts and the board throws a flash code. Your job is to walk the same steps the board does and find the first one that fails.
The Standard Ignition Sequence
- Thermostat closes R–W, sending 24VAC to the board (call for heat).
- Inducer (draft) motor energizes and pulls a negative pressure through the flue.
- The pressure switch senses that draft and closes, proving venting.
- Hot surface igniter warms for a 17–30 second pre-purge/warm-up, or the spark ignitor begins arcing.
- The gas valve opens and the burners light.
- The flame sensor rectifies the flame and reports microamps back to the board.
- After a 30–45 second delay, the blower (heat speed) comes on to move conditioned air.
If flame is not proven within the trial-for-ignition period (usually 4–7 seconds), the board closes the valve and retries. After the retry count is exhausted — typically three tries — it goes into a one-hour soft lockout. Recognizing which step never happened is 80% of the diagnosis.
Step 1: Verify the Call and the Board
Before you condemn anything inside the cabinet, confirm the furnace is actually being told to run. Pull the blower door (the door switch must be made or jumpered to power the board) and put your meter on the low-voltage terminals.
- 24VAC across R and C confirms the transformer and secondary power are alive.
- 24VAC across R and W confirms the thermostat is calling. No voltage here means the problem is upstream — thermostat, wiring, or a tripped safety in series with the call.
- Check the 3A or 5A blade fuse on the control board. A blown low-voltage fuse usually means a shorted thermostat wire, often a rubbed-through wire at the condenser.
Read the Flash Code First
Every induced-draft board has a diagnostic LED behind the blower door. Watch it before you probe anything — the legend printed on the inside of the door tells you whether the board sees an open pressure switch, an open limit, failure to ignite, or a flame-sense fault. The code narrows a 20-minute hunt down to a single circuit.
Step 2: Inducer and Pressure Switch
On the call, the inducer motor should spin up immediately. If it doesn't, check for 120VAC at the inducer leads coming off the board. Power present but no rotation means a seized bearing or open motor winding; no power means the board isn't commanding it (bad board relay, or a limit/rollout open in the safety string).
Once the inducer runs, it has to satisfy the pressure switch — a normally-open switch that closes when it senses the negative pressure the inducer creates. This is one of the most common no-heat culprits, especially on 90%+ condensing furnaces.
Prove the Switch, Don't Just Jump It
Put a manometer on the switch port and read the actual draft against the number stamped on the switch (for example, a switch rated -0.60 in. w.c. must see at least that much vacuum to close). Then jumper the switch terminals momentarily to confirm the board advances.
- Adequate draft but switch won't close → failed switch.
- Weak draft → blocked flue, cracked/plugged pressure tubing, water in the trap, or a tired inducer wheel.
- Condensing furnace, intermittent → clogged condensate trap or drain backing up into the collector box.
Never leave a pressure switch jumped. It is a life-safety flue-blockage proof, not a nuisance.
Step 3: The Ignition Source
With draft proven, the board energizes the ignition source. Watch it through the sight port. A hot surface igniter (HSI) should glow bright orange within 17–30 seconds; a spark ignition system should throw a steady, snapping arc at the burner.
If the igniter never glows, kill power and ohm it out. A silicon-carbide HSI (the fragile flat gray type) typically reads 40–90 ohms cold; an open reading means a cracked element. Silicon-nitride igniters read differently and are more robust, but the test is the same — open is dead. If resistance is good but it still won't glow, confirm the board actually sends 120VAC to the igniter during the warm-up window.
Handle Igniters Right
Never touch a silicon-carbide element with bare fingers — skin oil creates a hot spot that shortens its life. Handle by the ceramic base only. And never carry the igniter as a "while I'm here" part swap without ohm-testing first; a good igniter that won't light points you at the board or the gas side, not the igniter.
Step 4: Gas Delivery and Manifold Pressure
Igniter glowing but no flame? The gas isn't getting there. First confirm the obvious: manual gas cock open, LP tank not empty, meter not locked out. Then verify the board commands the valve — you should read 24VAC across the gas valve terminals the instant the trial for ignition begins.
If the valve is energized but burners won't light, put a manometer on the outlet (manifold) tap and read pressure while it's firing — or trying to. Compare against the rating plate.
| Measurement | Natural Gas | LP (Propane) |
|---|---|---|
| Manifold pressure (firing) | ~3.5 in. w.c. | ~10–11 in. w.c. |
| Inlet supply pressure | ~7 in. w.c. (5–10.5 range) | ~11 in. w.c. |
| Typical heat value | ~1,000 BTU/ft³ | ~2,500 BTU/ft³ |
Low or zero manifold pressure with the valve energized points to a failed gas valve, a blocked supply, or a regulator problem. Always verify the furnace is set up for the fuel it's burning — an LP furnace running on a natural-gas orifice/spring (or the reverse) is a dangerous field mistake.
Ventora — Your AI HVAC Companion
Flash-code lookups, sequence-of-operation help, and instant gas-pressure references. Free on iOS.
Step 5: Prove the Flame
Here's the most common no-heat call in the trade: the burners light for a few seconds, then the flame drops out and the furnace tries again — three times — before locking out. That signature is almost always a dirty flame sensor.
The flame sensor is a single metal rod in the flame. Flame conducts a tiny DC current through rectification, and the board reads that current in microamps. Oxidation or carbon on the rod chokes that signal until the board can no longer prove flame and shuts the valve as a safety.
Healthy Flame Signal
A clean rod typically reads 2–6 µA DC. Anything comfortably above the board's minimum will run reliably.
Failing Flame Signal
Drifting below roughly 1 µA — many boards drop out around 0.5–1 µA — causes the short-cycle lockout.
To read it, put your meter on DC microamps in series with the flame-sense wire (many technicians use a jumper harness). Watch the number the moment flame is established. If it starts strong and decays, or reads low from the start, pull the sensor and clean the rod with a Scotch-Brite pad or fine steel wool — never sandpaper, which leaves grit and can etch the surface. Re-seat it and confirm the reading recovers.
If Cleaning Doesn't Fix It
A poor flame signal isn't always the rod. Check the burner ground — rectification needs a solid path back through the burner assembly. A corroded ground screw, a cracked ceramic insulator, or a sensor positioned outside the flame envelope will all read low even with a spotless rod.
Worked Example: Short-Cycle Lockout
The Call
Homeowner says the furnace "runs for a few seconds then quits," and by the time you arrive it's cold with a slow-blinking LED. Rated for a 40–70°F temperature rise. Here's the walk-through.
Step 1: Read the flash code
Door legend: the pattern equals "flame sensed then lost / failed ignition." That points straight at the flame-proving circuit.
Step 2: Watch a cycle
Inducer runs, pressure switch closes, HSI glows, burners light cleanly — then flame drops after ~5 seconds and the board retries. Draft and ignition are fine.
Step 3: Meter the flame signal
DC µA in series with sense wire = 0.4 µA (board minimum ≈ 1.0 µA). Confirmed: signal is too weak to hold.
Step 4: Clean and re-test
Polish rod, check burner ground screw (found loose — retorqued). New reading = 4.2 µA.
Result: Solid Flame, No Lockout
With the signal back to 4.2 µA, the furnace holds flame through the full heat cycle and the blower engages on schedule. Total diagnosis time: under 15 minutes, no parts beyond a pad and a screwdriver.
Quick Reference Values
Keep these targets in your head — or in your pocket — for a no-heat call:
| What You're Checking | Expected / Target | Bad Sign |
|---|---|---|
| Call for heat (R–W) | 24VAC | 0V = upstream fault |
| Flame signal (clean rod) | 2–6 µA DC | < ~1 µA = lockout |
| Silicon-carbide HSI (cold) | 40–90 Ω | Open = cracked element |
| Manifold pressure (NG) | ~3.5 in. w.c. | Low = valve/regulator |
| HSI warm-up time | 17–30 sec | No glow = check 120V/ohms |
| Temperature rise (verify on blower) | Per rating plate (e.g. 40–70°F) | High = low airflow/limit trips |
One More No-Heat Cause: The Limit String
If the furnace lights and heats but shuts down on the high-limit — or won't start at all because a rollout switch is open — you're looking at an airflow or combustion problem, not an ignition problem. A dirty filter, a failed blower, or a cracked heat exchanger can trip these safeties. Rollout switches are manual-reset for a reason: if one is open, find out why before you reset it.
Ventora — the AI HVAC Assistant in Your Pocket
Decode any furnace flash code, walk the sequence of operation, and pull gas-pressure and flame-signal specs on the spot. Built for techs who'd rather diagnose than guess.