Why Undersized Systems Work Harder (and Cost You More)

When most homeowners pick out a new heating or cooling system, they often think: “Smaller unit, smaller bill.”

But here’s the truth — an undersized HVAC system doesn’t save energy. It wastes it.

It runs longer, works harder, breaks down sooner, and never truly cools or heats your space evenly. In other words, you end up paying more to be less comfortable.

That’s why Jake’s here with a real-world breakdown of what happens when your system is too small — and how to size your next one correctly, whether you’re buying a 12,000 BTU Amana Distinctions PTAC with 3.5 kW Electric Heat or any other type of HVAC unit.

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🧠 What Does “Undersized” Really Mean?

When HVAC pros say a system is undersized, they don’t mean it physically doesn’t fit — they mean it doesn’t have enough BTU capacity to handle your space’s heating or cooling load.

A BTU (British Thermal Unit) measures energy. The higher the BTU rating, the more heat the system can add or remove.

Here’s what that looks like in practice:

• A 12,000 BTU system can comfortably cool about 450–550 sq ft in a typical, moderately insulated home.

• A 8,000 BTU system, by contrast, can handle around 300 sq ft.

Now, imagine putting that smaller unit in a 500 sq ft living room. It’s going to run at full throttle all day and still fall behind.

Jake says:

“Think of it like driving uphill in third gear. You’ll make it to the top, but your engine won’t thank you.”

🔗 Reference: Energy Star – Room Air Conditioner Sizing Guide

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🔥 Why Undersized Systems Burn More Energy, Not Less

It might seem logical that a smaller unit would use less power. But the opposite happens.

Here’s why:

• Undersized units run longer to reach the desired temperature.

• Because they never quite get there, they stay on nearly constantly.

• Longer runtime = higher electricity consumption and more wear and tear on components.

Let’s break that down numerically.

A properly sized 12,000 BTU PTAC might run at 80% load for 30 minutes per hour.
An undersized 8,000 BTU unit might run at 100% load for 55 minutes per hour.

Even though it’s smaller, the constant cycling keeps it drawing power — and since compressors use the most energy during startup, all that extra runtime adds up.

💡 Jake’s Tip:

“An undersized AC is like a treadmill set to sprint speed — it can’t rest, so it burns out early.”

🔗 Reference: U.S. Department of Energy – Energy Saver HVAC Efficiency Tips

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💧 The Hidden Comfort Cost: Humidity and Uneven Cooling

Cooling isn’t just about temperature — it’s about removing humidity.

Air conditioners and PTAC units dehumidify as they cool. But if your system is too small:

• It never runs long enough to pull moisture from the air.

• You end up with a clammy, sticky feeling.

• Mold risk increases in walls and corners.

This is especially noticeable in humid climates like the Southeast.

Jake’s rule of thumb:

“If your thermostat says 72°F but it feels 78°F, you’ve got a humidity problem — usually caused by an undersized unit.”

High humidity also tricks you into setting the thermostat lower, further increasing runtime and energy use.

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⚙️ Mechanical Stress: How Undersizing Kills Equipment Early

Think of your compressor as your HVAC’s heart.

When it runs full speed around the clock, it ages faster. The same goes for fan motors and control boards.

Here’s what happens over time with an undersized system:

1. Compressor fatigue — can’t handle long cycles under high pressure.

2. Blower motor burnout — constant operation without cooling pauses.

3. Coil icing — when airflow can’t keep up with the load.

4. Thermostat drift — inaccurate readings from constant runtime.

These issues aren’t “maybe” problems — they’re guaranteed over time.

🔗 Reference: ACCA – Manual J Residential Load Calculations

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🧾 Real-World Example: The Hidden Cost of Undersizing

Let’s compare two identical spaces — both 500 sq ft — one with a 12,000 BTU PTAC and the other with a 8,000 BTU model.

Over 10 years, that’s roughly $820 more in electricity — not counting replacement or repair costs.

So while you might save $100–150 upfront on a smaller unit, you’ll pay that back (and then some) every year.

Jake’s verdict:

“Undersizing is like skipping oil changes to save gas money. It just doesn’t add up.”

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🌬️ Comfort Zones Matter — Why Room Type Affects Load

Not all rooms need the same BTUs.

A 12,000 BTU Amana Distinctions PTAC may handle a 500 sq ft bedroom just fine, but if that same space is a kitchen or sunroom, the load increases by 15–25%.

Here’s why:

• Kitchens add appliance heat (stoves, ovens).

• Sunrooms face direct solar gain through windows.

• Basements require heating boost because of poor insulation.

That’s why the U.S. DOE recommends adjusting system size based on use type — not just square footage.

🔗 Reference: U.S. Department of Energy – Insulation and Air Sealing Basics

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🧰 Jake’s Quick Fixes If You’re Already Undersized

If replacing your system right now isn’t an option, you can still make it easier for your current unit to keep up.

✅ 1. Improve Insulation

Seal windows, add weatherstripping, and close air leaks.
This reduces heat load so your system doesn’t fight constant infiltration.

✅ 2. Add a Secondary Unit

A small supplemental PTAC or portable AC can even out load between rooms.

✅ 3. Manage Sunlight

Install reflective window films or blackout curtains for west-facing rooms.

✅ 4. Use Fans Wisely

Ceiling fans don’t reduce temperature — but they make air feel cooler by enhancing evaporation.

✅ 5. Schedule Preventive Maintenance

Keep coils clean and filters changed. An overworked system can’t afford dirt buildup.

Jake says:

“You can’t make an 8k BTU act like a 12k, but you can make your room act smaller.”

🔗 Reference: DOE – Energy Saver Cooling Efficiency Improvements

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💵 Why “Buying Small” Costs Big

Many homeowners buy a smaller unit because of price. But if you’re planning to use it every day, undersizing ends up being more expensive long-term.

Let’s consider total cost over 10 years:

That’s a $1,000+ difference — all because the smaller system works too hard.

Jake’s line sums it up perfectly:

“The cheapest system is the one that fits your space the first time.”

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🧊 The Seasonal Strain: Cooling vs. Heating Loads

In cooling mode, undersized systems run constantly — but in heating mode, the problem gets worse.

If your PTAC doesn’t have enough heating capacity, rooms can struggle to stay warm below 40°F. That’s where models with electric heat kits, like the 3.5 kW Amana Distinctions unit, shine.

That kit adds about 11,900 BTU of heating output, giving you both summer cooling and winter backup heat in one compact unit.

Jake explains:

“When temps drop, that heat kit is your insurance policy. It keeps your system from running flat-out just to hit 68°F.”

🔗 Reference: AHRI Directory – PTAC Ratings & Certificates

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⚡ Efficiency Ratings: Why SEER2 and EER Still Matter

Even when your system is correctly sized, efficiency plays a big role in cost.

• EER (Energy Efficiency Ratio) – Measures cooling efficiency at a fixed temperature.

• SEER2 (Seasonal Energy Efficiency Ratio) – Reflects efficiency over varying conditions.

Higher ratings mean more cooling for the same power.

So a high-efficiency 12,000 BTU PTAC may use less electricity than a low-efficiency 10,000 BTU model, even though it’s technically larger.

🔗 Reference: Energy Star – Understanding SEER and EER Ratings

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🏠 Jake’s Real-Life Example

Jake’s neighbor thought his 8,000 BTU unit was “good enough” for a 500 sq ft studio apartment. By mid-July, the AC was running nonstop and the electric bill was up 40%.

Jake helped him upgrade to the Amana Distinctions 12,000 BTU PTAC with 3.5 kW heat. The difference?

• The room hit setpoint faster.

• The compressor cycled normally.

• Monthly bills dropped by 25%.

• Humidity dropped from 70% to 48%.

That’s what “right-sized” comfort looks like.

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⚙️ Signs You Need a Larger System

If any of these sound familiar, your system might be too small:

• Runs almost constantly but can’t reach target temperature.

• Rooms feel cool near the unit but warm elsewhere.

• High humidity or damp walls.

• Airflow feels weak or inconsistent.

• Energy bills keep rising each season.

Upgrading to a properly sized system like the 12,000 BTU Amana Distinctions PTAC fixes those issues in one move.

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🧭 Jake’s Final Word

“You can’t outsmart physics. Undersized systems don’t rest — they overwork, overheat, and overcharge your wallet.”

A correctly sized system doesn’t just cool faster — it cools smarter. It maintains comfort, preserves components, and costs less in the long run.

If your unit feels like it’s always running uphill, it’s time to step up your BTUs.

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In the next topic we will know more about: Oversized Systems Aren’t Always Better: The Comfort Penalty You Don’t See