WiremanWireman · Furnace no heat (electrical side): troubleshooting for electricians · 2023 NEC ·
Furnace no heat (electrical side)
Follow the ignition sequence: W call, limits, inducer and pressure switch, igniter, flame proving, blower.
Before you start
How much electrical experience do you have?
This sets how much detail each step shows. You can change it any time.
Showing detail
How it works
A modern gas furnace runs a sequence on every call for heat. The thermostat sends 24 V on W, the board checks the limit switches, starts the inducer fan, waits for the pressure switch to prove draft, heats the igniter, opens the gas valve, then proves flame with a flame sensor that passes a tiny DC current through the flame to ground. The blower starts after a delay. The step where it stops tells you which part to test. The gas valve, burners, heat exchanger and venting belong to an HVAC tech.
Every question and answer in this guide. Tap “Step by step” to walk through it.
- What happens when the thermostat calls for heat?
- Nothing at all, inducer never startsTest: Check the W call at the boardExpect: About 24 V AC.
- 0 V at WNo heat call from the thermostat
- 24 V at W, nothing happensTest: Check the limit and rollout switchesExpect: Every limit and rollout switch closed (near 0 Ω).
- A limit or rollout switch is openAn open high limit or rollout switch
- All closedControl board isn’t starting the inducer
- Inducer runs, no glow or sparkTest: Check the pressure switchExpect: Near 0 V across it (switch closed).
- Near 0 V, switch closedTest: Test the hot surface igniterExpect: A steady reading in the range the manufacturer lists. Many silicon carbide igniters read roughly 40 to 90 Ω cold. OL means it’s cracked.
- OL or visibly crackedFailed igniter
- Reads in rangeBoard isn’t powering the igniter
- 24 V across, switch openPressure switch isn’t proving draft
- Near 0 V, switch closedTest: Test the hot surface igniterExpect: A steady reading in the range the manufacturer lists. Many silicon carbide igniters read roughly 40 to 90 Ω cold. OL means it’s cracked.
- Igniter glows or sparks, no flameGas valve or gas supply
- Lights, then shuts off in secondsTest: Check polarity and ground at the furnaceExpect: About 120 V hot to neutral and hot to ground. Near 0 V neutral to ground.
- Reversed, or neutral-to-ground voltageReversed polarity or a poor ground
- CorrectWeak flame signal
- Burners run, blower never startsTest: Check voltage to the blower motorExpect: About 120 V on the heat terminal once the delay is over.
- 120 V at the blower lead, motor doesn’t runFailed blower motor or capacitor
- No voltage from the boardBoard isn’t sending power to the blower
- Nothing at all, inducer never startsTest: Check the W call at the boardExpect: About 24 V AC.