First Fire: A Beginner’s Guide to Starting Up a Two-Stage Gas Furnace

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Tony here. Let me tell you something most manuals won’t:

The first time a furnace fires tells you everything you need to know about how the rest of its life is going to go.

A two-stage gas furnace isn’t complicated—but it is honest. If airflow is wrong, gas is undersized, venting is sloppy, or controls aren’t set right, the furnace will show you immediately. Not always with an error code. Sometimes with noise, short cycling, uneven heat, or lousy efficiency.

This guide is for beginners, homeowners, and new techs who want to understand what actually happens during first fire, what to watch for, and how to set up a two-stage furnace so it runs quiet, efficient, and drama-free.

100,000 BTU 96% AFUE Upflow/Horizontal Two Stage Goodman Gas Furnace - GR9T961004CN

🔍 What “First Fire” Actually Means (And Why It Matters)

“First fire” isn’t just flipping the switch.

It’s the moment when:

Gas pressure meets ignition
Airflow meets heat output
Controls decide staging behavior
Safety systems prove they work

This is when mistakes show up.

Tony rule:

If the furnace struggles on Day One, it won’t magically get better later.

🧠 Understanding Two-Stage Furnaces (Quick and Simple)

Before start-up, you need to understand what you’re working with.

A two-stage gas furnace has:

Low stage (usually ~60–70% capacity)
High stage (100% capacity when needed)

Most of the time, the furnace should run in low stage:

Longer run times
More even heat
Better efficiency
Less noise

High stage is for:

Very cold days
Large recovery setbacks
Heavy demand

👉 Manufacturer overview:
https://www.goodmanmfg.com/products/gas-furnaces

If your furnace immediately jumps to high stage every time, something isn’t right.

⚡ Step 1: Power-Up Checks Before Ignition

Before you allow gas ignition, stop and check these basics.

✅ Electrical checklist:

Furnace is properly grounded
Correct breaker size
No loose low-voltage wiring
Polarity correct (hot/neutral)

✅ Control board readiness:

No diagnostic lights flashing
Door switch engaged
Thermostat calling correctly

Tony tip:

A furnace that won’t power cleanly should never be forced to fire.

🔥 Step 2: Gas On — But Slow and Deliberate

Now we introduce gas.

Before opening the valve:

Verify gas type (natural gas vs propane)
Confirm gas line sizing supports BTU load
Check for a sediment trap
Ensure shutoff valve is accessible

Open the gas valve slowly and listen.

You’re checking for:

Immediate leaks
Pressure instability
Odors that shouldn’t be there

👉 Gas safety reference:
https://www.energy.gov/energysaver/furnaces-and-boilers

If you smell gas, stop. Period.

🔥 Step 3: The Ignition Sequence — What You Should See

When the thermostat calls for heat, a healthy furnace follows a predictable order:

Inducer motor starts
Pressure switch proves draft
Igniter warms
Gas valve opens
Burners ignite smoothly
Flame sensor proves flame
Blower starts after delay

If the furnace stalls, retries, or bangs during ignition, don’t ignore it.

Tony rule:

Smooth ignition now prevents cracked heat exchangers later.

🔥 Step 4: Watching the Flame (This Matters More Than You Think)

Once burners light, watch the flame.

A healthy flame is:

Steady
Mostly blue
Even across all burners
Not lifting or rolling

Warning signs:

Yellow tips
Flame fluttering
Delayed ignition “whoomp”
Flame rollout

These are combustion problems, not “break-in behavior.”

👉 Combustion basics (EPA):
https://www.epa.gov/indoor-air-quality-iaq/carbon-monoxides-impact-indoor-air-quality

🌬️ Step 5: Blower Startup and Airflow Behavior

After ignition, the blower comes on.

In a two-stage furnace:

Low stage should be quiet
Airflow should feel steady, not forceful
No whistling, rattling, or booming

If airflow is loud or uneven, suspect:

Undersized returns
Dirty or restrictive filters
Incorrect blower speed setup

Tony truth:

Airflow problems don’t fix themselves — they get louder.

🧭 Step 6: Confirming Proper Staging Operation

Now the big question:

Is the furnace actually staging correctly?

During mild conditions:

Furnace should stay in low stage
Long, consistent run times
No rapid cycling

High stage should only engage when:

Thermostat calls for it
Timer allows it
Temperature demand requires it

If high stage kicks in immediately, check:

Thermostat configuration
Wiring (W1/W2)
Dip switch or board settings

💧 Step 7: Condensate Drain Check During First Fire

Condensing furnaces produce water — right away.

During first fire:

Verify condensate is flowing
Trap fills properly
No leaks at fittings
Pump (if used) activates correctly

A dry drain during first fire is a red flag.

Tony rule:

No water movement = future shutdown.

📊 Step 8: Measuring What Matters (Not Guessing)

This is where beginners level up.

Measurements to verify:

Manifold gas pressure
Static pressure (return + supply)
Temperature rise across heat exchanger

Compare readings to manufacturer specs.

👉 Goodman technical documentation:
https://www.goodmanmfg.com/resources

If numbers are off, efficiency will be off too.

🧠 Step 9: The First 30 Minutes — Don’t Walk Away

The most important part of first fire?

Staying put.

Watch for:

Short cycling
Error codes
Temperature overshoot
Noise changes
Staging transitions

Most issues appear within the first half hour.

🚫 Common First-Fire Mistakes Beginners Make

Let’s save you some pain.

❌ Assuming noise will “settle in”
❌ Ignoring airflow issues
❌ Skipping pressure checks
❌ Over-cranking thermostat
❌ Leaving before confirming low-stage operation

Tony truth: