Key Takeaways
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6k BTU = 150–250 ft²; 10k = 400–450 ft².
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Add 10–20% for poor insulation/extreme weather.
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Add BTUs for windows, people, and appliances.
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Oversizing causes short-cycling and waste.
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Right size + smart controls = 20–30% energy savings.
Why PTAC Sizing Deserves Attention
A packaged terminal air conditioner (PTAC) isn’t like a lamp you can plug in anywhere. Pick the wrong size and you pay twice—first on the utility bill, then on comfort. U.S. hotels and apartments lose millions each year to oversized units that shut off too quickly and undersized units that never quite cool down. In this guide you’ll learn exactly how many BTUs your room needs, when to tweak that number, and how small details such as filter care lock in savings. Along the way we’ll link to hands-on tutorials from The Furnace Outlet blog so you can keep every tip practical and U.S.-code compliant.
Use our sizing guide to get it right the first time.
Reading the BTU Chart: Room Size vs. Cooling Power
ENERGY STAR’s square-footage chart is the industry’s go-to starting point. It lists capacities from 6 k BTU for 150–250 ft² rooms up to 14 k BTU for 550–700 ft² spaces. (energystar.gov) Because PTACs vent directly outdoors, their load calculation is simpler than a whole-house Manual J. Measure the room—including alcoves—then round up to the closest capacity on the chart. Avoid jumping more than one step higher “just in case”; that leads to short cycling. If your room is oddly shaped, break it into rectangles and triangles, total the areas, and use the sum. Bookmark our detailed walkthrough in “BTUs, Sizing, and Square Footage” for real-world examples.
Insulation and Climate: When 10 % More Makes Sense
A 9 k BTU unit might be perfect in mild Seattle but struggle in humid Miami. Poor attic insulation, single-pane windows, or blazing southwest sun loads can spike heat gain by 10–20 %. For hotels along the Gulf Coast, technicians often install a 10 k BTU unit where the chart says 9 k BTU to keep humidity under control during 95 °F afternoons. Conversely, a well-sealed New England apartment with R-30 walls might downsize safely. Not sure how your building stacks up? A quick blower-door test or infrared scan reveals weak spots. The peace of mind beats guessing—and saves electricity for the life of the unit.
In extreme heat zones? Explore high-capacity PTACs built for Gulf Coast conditions.
Counting Windows, Doors, People, and Gadgets
Every south-facing window acts like a mini-greenhouse. Rule of thumb: +1 k BTU per exterior window or door beyond the first two. In busy hotel rooms or dorms, add ≈300 BTU for each extra occupant, plus another 300–500 BTU if a kitchenette, gaming console, or server stays on around the clock. Write these add-ons next to your base BTU figure before you shop. Our blog post “Cost Breakdown of PTAC Heat Pumps” illustrates how these small numbers ripple through lifetime operating costs—worth a skim if you manage a portfolio.
Oversized Units: Cool but Clammy
Bigger PTACs blast cold air so fast the thermostat clicks off within minutes. That leaves moisture behind, creating the muggy “clammy-cold” feeling guests complain about. Short cycles also starve the compressor of lubrication starts and stops, slashing lifespan by up to 30 %. Energy bills spike because motors surge at every restart. Before upsizing, try the tactics in “Cut PTAC Energy Use by 30% with Smarter Sizing, Cleaning, and Controls.” You’ll see why dialing in airflow and set-points usually beats buying a higher tonnage unit.
Avoid muggy rooms—install dual smaller units for even comfort across large spaces.
Undersized Units: The Marathon That Never Wins
Undersized PTACs run non-stop, yet room temperature creeps down only slowly. Continuous operation drives humidity toward the dew point, fogging windows and inviting mildew. It also over-taxes the fan motor, leading to burnt capacitors and nuisance breaker trips. If your unit runs more than 70 % of each hour on a mild day, chances are it’s undersized—or clogged with dust. Our “PTAC Gasping for Air? Save 20% Energy with Simple Maintenance” checklist explains how a five-minute filter rinse can free up enough airflow to match design specs again.
Dehumidification & Multi-Unit Strategies
Properly sized PTACs remove moisture because they stay on long enough for condensate to form on the coil. In wide rooms (over 20 ft deep) or L-shaped suites, two smaller units, spaced apart, beat one large one. They create more even airflow and let you turn one unit off during shoulder seasons. Facility managers tracking kilowatt-hours per occupied room often report 5–8 % lower energy use after switching to dual-unit layouts. For the latest on built-in dry modes, smart thermostats, and auto fan settings, check “PTAC Features to Know in 2025.”
Keep the Benefits: Filters, Coils, and Sleeves
Even the best-sized PTAC wastes energy if airflow is blocked. Dust-clogged filters add up to 15 % efficiency loss over just one cooling season. Wash filters monthly, vacuum the evaporator coil each quarter, and inspect the wall sleeve gasket for daylight gaps. A quick bead of exterior-grade silicone where the unit meets the wall can cut draft-related losses.
Grab replacement parts and install kits here: Shop PTAC accessories
Frequently Asked Questions
Q1. Is there a simple calculator for PTAC sizing?
Yes. Measure the room, find the BTU in the ENERGY STAR chart, then add the adjustments in Sections 3–4. Many U.S. distributors, including The Furnace Outlet’s free sizing tool, automate the math.
Q2. Does ceiling height matter?
Standard charts assume 8-ft ceilings. For each extra foot, multiply base BTU by 1.1 to compensate for the larger air volume.
Q3. Can I replace a 9 k BTU unit with a 12 k BTU to “future-proof” the room?
Usually no. The larger unit may cycle off too soon, hurting humidity control and wasting power.
Q4. How often should I clean filters?
Every 30 days in dusty areas; quarterly in low-dust climates. Consistent cleaning restores up to 15 % lost efficiency.
Q5. What’s the best thermostat set-point for hotels?
Most facilities find 72 °F cooling and 68 °F heating offer a balance between guest comfort and energy savings. Smart controls that relax set-points during vacancy save an extra 10 %.
