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Is a 100,000 BTU R-32 Gas Furnace Right for Your Home? Tony’s Rules for Sizing, Climate & Ductwork

When homeowners call Tony asking if a 100,000 BTU R-32 gas furnace is the right choice, he always gives the same answer:

“It depends on your climate, your home, and your ductwork — not on what your neighbor bought.”

Most people look at BTUs like horsepower numbers on a pickup truck.
More = better, right?

Wrong.

A furnace that’s too big shorts cycles.
A furnace that’s too small runs nonstop.
And a furnace matched to bad ductwork?
That’s a recipe for high gas bills, cracked heat exchangers, noisy airflow, and uneven rooms.

This blog breaks down EXACTLY when a 100,000 BTU R-32 furnace makes sense and when Tony tells homeowners to step down a size.

This is real-world, field-tested sizing — not the watered-down stuff from brochures.

1. The First Rule: Square Footage DOESN’T Determine Furnace Size

Tony never sizes furnaces by square footage alone. That’s rookie-level thinking.

Two 2,600 sq ft houses can need totally different furnace sizes depending on:

insulation levels
window quality
duct system strength
leakage rate
age of the home
ceiling height
regional climate

This is why Tony always references proper load calculation standards rather than “rules of thumb.”
[Residential Furnace Sizing & Heat Load Calculation Standards]

A furnace heats your home’s heat loss — not just its square footage.

2. When a 100,000 BTU R-32 Furnace IS the Right Pick

Tony installs a lot of 100k furnaces — but only in the right situations.

✔ Cold Climate Zones

If winters regularly dip below freezing, a home often needs more heating firepower.
Think:

Michigan
Wisconsin
Minnesota
New York (upstate)
Colorado
North Dakota / South Dakota
Northern New England
[Regional Climate Zone Heating Performance Guidelines]

These climates justify 100k BTUs.

✔ Large Homes (2,800–3,800+ sq ft)

Big volume = big heat requirement.
Especially if ceilings are over 9–12 feet.

✔ Older Homes or Drafty Construction

Homes built before the 1990s often leak heat like crazy unless upgraded.
[Home Insulation & Heat Loss Assessment Manual]

✔ Pairing With a 4–5 Ton AC

A big AC coil often requires a big blower.
Larger furnaces have the blower capacity to match.

✔ Homes With Great Ductwork

A 100k furnace pushes LOTS of airflow.
The ducts need to be able to handle the pressure (more on this later).

**3. When a 100k Furnace Is TOO BIG — And Tony Refuses to Install It**

Oversizing is the #1 mistake Tony sees other installers make.

A furnace that is too big causes:

❌ Short cycling

Starts and stops constantly, never settles into an efficient run.

❌ Loud airflow

Because the blower is trying to shove too much air through undersized ducts.

❌ Uneven room temperatures

Rooms heat quickly, shut off fast, then cool again.

❌ Higher gas bills

Short cycles burn more fuel.

❌ Premature heat exchanger cracks

Rapid expansion/contraction kills metal.

Tony’s rule:
If a furnace heats the home in under 7 minutes, it’s oversized — period.

4. Climate Zones Matter More Than Anything Else

Tony sizes furnaces by climate before anything else.

Cold Climates (100k BTUs Makes Sense)

Midwest
Northern Mountain regions
Great Lakes
Northeast

Homes here need serious output.

Mixed Climates (Sometimes 100k BTUs, Sometimes Not)

Virginia
Tennessee
Missouri
Northern California interior
Utah
Oregon

Load calculations matter. No guessing.

Warm Climates (Almost Never 100k BTUs)

Texas
Florida
Georgia
Arizona
Alabama
Southern California

If you live here and someone recommends a 100k furnace?
Get a second opinion.

5. Ductwork: The Part Nobody Thinks About (But Tony Cares About Most)

If your ductwork can’t breathe, your furnace can’t heat.

A 100k furnace needs a LOT of airflow:

large returns
wide supply trunks
proper static pressure
properly sized branches

Tony sees this mistake constantly:

Big furnace + tiny ducts = loud, uneven, inefficient heat.
[Ductwork Airflow Capacity & Static Pressure Engineering Reference]

Minimum duct requirements for 100k furnaces:

16–20" return trunk
16–22" supply trunk
<0.5 in. w.c. static pressure
properly sized branch lines

If ducts are too small?
Tony steps down to 80k BTUs or fixes the duct system first.

6. Why R-32 Matters in a Gas Furnace Lineup

R-32 isn’t used for heating — it's used for cooling systems paired with modern furnaces.

But R-32 furnace lineups usually bring upgrades:

stronger blowers
higher efficiency heat exchangers
better staging
tighter cabinet construction
improved airflow geometry

Tony likes R-32-compatible furnace lines because manufacturers tend to update the entire design — not just the refrigerant circuit.
[R-32 Heating System Efficiency & Safety Specifications]

That means better long-term performance.

7. Homes That Are Perfect for a 100k R-32 Furnace

Based on Tony’s experience, these homes are a fit:

2,800–3,800 sq ft in cold climates
older homes with high leakage
tall ceilings (especially vaulted)
homes needing strong blower power
houses pairing with 4–5 ton AC systems
houses with strong duct systems

If all the above are true?
100k BTUs is ideal.

**8. Homes That Should NOT Use a 100k Furnace**

Tony turns down these jobs all the time:

1,800–2,400 sq ft newer homes
tight envelope construction (post-2010 builds)
homes in warm climates
houses with restrictive duct systems
homeowners trying to “oversize for safety”
houses with insufficient returns

Oversizing does NOT give comfort.
Oversizing DESTROYS comfort.

9. How Tony Decides: The Real Load Calculation Approach

Forget old “30 BTUs per sq ft” myths.

Tony evaluates:

heat loss
duct capacity
blower capability
climate zone
air leakage
window direction
insulation
homeowner comfort history
furnace staging

He compares the data against modern codes and installation requirements:
[Gas Furnace Installation & Combustion Air Code Requirements]

Only then does he approve 100k BTUs.

10. Signs You Actually Need a Larger Furnace

Tony recommends 100k BTUs when you experience:

✔ furnace running constantly and still cold
✔ home never reaching thermostat setpoint
✔ rising gas bills each winter
✔ cold rooms during windy nights
✔ uneven temperatures throughout the home

These are often signs the furnace is undersized — and 100k BTUs fixes it.