What Size Furnace Do I Need? Avoiding Common Sizing Mistakes in Iowa Homes

If you’ve ever wondered, “What size furnace do I need for my house?” you’re not alone. Picking the right size isn’t about guessing based on square footage or grabbing the biggest unit you can afford. In Ankeny, Des Moines, West Des Moines, Urbandale, Waukee, and across Central Iowa, your furnace has to battle serious cold, wind, and heat loss. Getting the size wrong means higher bills, more repairs, and less comfort. Here’s how to get your furnace choice right, without the headache.

The Myth of “Bigger Is Better”

Bigger furnaces heat faster, right? Not exactly. An oversized furnace short-cycles: it roars to life, overshoots the set temperature, shuts off, and repeats. That yo-yo action wastes fuel, creates hot-and-cold swings, is noisier, and wears components prematurely. On the flip side, an undersized furnace runs constantly and still can’t keep up, especially during sub-zero spells.

Goal: a furnace that runs steady, quiet, and efficient, keeping temps even and bills reasonable.

How Pros Size a Furnace (Manual J, minus the jargon)

Reputable HVAC companies don’t guess; they perform a Manual J load calculation. Think of it as a heat-loss math test for your house. Here’s what we factor in:

• Square footage and layout (two-story vs. ranch)

• Insulation and air sealing (attic, walls, rim joists)

• Window type and count (single-pane vs. double-pane, low-E, etc.)

• Orientation and sun exposure (north-facing rooms lose more heat)

• Infiltration (drafts, leaky doors, chimney effect)

• Ceiling height (volume matters, not just floor area)

• Ductwork (condition, size, leakage, static pressure)

• Basement and garage conditions (finished? room above garage?)

• Occupancy and appliances (internal heat gains)

Once we know how much heat your home loses on a design-cold day, we match a furnace that can replace that heat steadily without short-cycling.

Iowa Factors That Change the Math

Iowa isn’t mild—your furnace selection needs to reflect that reality.

• Wind + drafts: Our winter winds magnify heat loss. Tightening the envelope (weatherstripping, air sealing) sometimes lets you pick a slightly smaller furnace without sacrificing comfort.

• Basements: Finished basements can add significant conditioned space. Unfinished basements still matter—cold concrete slabs and bare walls increase load.

• Rooms above garages: These are notorious cold spots. Duct design, insulation, and air sealing influence the size and airflow strategy.

• Older homes: Beautiful woodwork, original windows, and charming quirks often come with hidden leaks. Right-sizing must account for the envelope as it is—or as it will be if you plan upgrades.

Oversized vs. Undersized: What You’ll Notice

Oversized furnace symptoms

• Short, loud bursts of heat followed by sudden cool-downs

• Uneven temps between rooms or floors

• Higher fuel bills despite “powerful” equipment

• More wear and tear, more repairs

Undersized furnace symptoms

• Runs nearly nonstop on cold days

• Struggles to reach setpoint

• Comfortable on mild days, miserable during cold snaps

• Icing on the cake: higher bills with low comfort

If any of this sounds familiar, you might have a sizing or airflow issue (or both).

Ductwork & Airflow: The Part Everyone Forgets

You can buy the perfect furnace and still feel cold if your ducts aren’t right. We check:

• Static pressure: Like blood pressure for your duct system. Too high = noisy, low airflow, unhappy furnace.

• Return air capacity: Starving the system of return air is the fast lane to short-cycling and noise.

• Leakage: Wasted heat in attics, crawl spaces, or garages = higher bills and cold rooms.

• Filter and grille sizing: Undersized filters choke airflow and can overheat the furnace.

Bottom line: Furnace size and duct design are a matched set. We solve both.

Features That Improve Comfort (Even With the Right Size)

• Two-stage or modulating gas valves: Gentle heat on mild days, full output on frigid nights. Fewer swings, quieter operation.

• Variable-speed ECM blowers: Smooth, quiet airflow that adapts to your home’s needs and can improve filtration.

• Smart thermostats & zoning: Better control, fewer arguments—especially in two-story homes with temperature differences.

• High-efficiency models (95%+ AFUE): Lower fuel use and often stronger rebates.

These features don’t replace proper sizing—but they do make the right-sized system feel luxurious.

What to Expect at an Obsidian Estimate

We keep it straightforward and pressure-free:

1. Load Calculation & Inspection
   We measure, inspect, and listen. We’ll check the envelope, ductwork, and current system, then run a Manual J load calc.

2. Options & Clear Pricing
   You’ll see a few right-sized furnace options (often two-stage or variable-speed) with transparent pricing. No mystery fees.

3. Airflow Plan
   If your ducts need tweaks, we’ll explain what and why—returns, filter cabinets, balancing, or sealing.

4. Installation & Walkthrough
   Our licensed techs install to manufacturer spec, verify safe operation, and show you exactly how to use and maintain your new system.

5. Follow-Up & Support
   Questions later? We’re local, veteran-owned, and here when you need us.

Ready to Size It Right?

If you’re in Ankeny, Des Moines, West Des Moines, Johnston, Urbandale, Altoona, Waukee, or nearby, we’ll design a system that fits your home, not just your square footage.

• Thinking replacement? Book a furnace installation estimate and we’ll bring options.

• Not sure yet? We’ll compare furnace repair vs. replace and show projected energy costs either way.

Obsidian Heating & Cooling: comfort you can count on, winter after winter. Reach out today.