Plain‑English summary. Standard BTU rules get you close. But kitchens add internal heat from ovens and ranges, sunrooms flood with solar gain, and upstairs rooms sit under the hottest roof air. Ignore these and a perfectly “right‑sized” unit feels wrong—especially on bright afternoons or during dinner prep.
Bottom line: Apply targeted adders: appliance watts → BTU, glazing → % adders, orientation → % adders, and floor level → % adders. Verify by temperature split and airflow—not just nameplate tonnage.
Key Rules of Thumb
- Base: ~20 BTU per ft² for a single room
- Kitchen adder: +4,000–6,000 BTU
- People: +600–1,000 BTU per occupant
- Sunroom glazing: +15–40% (see table)
- South/West exposure: +10–20%
- Upstairs: +10–25%
- 1 ton = 12,000 BTU/hr
Table of Contents
- Why Kitchens, Sunrooms, and Upstairs Rooms Break the Rules
- Quick Sizing Rules & Modifiers
- Sizing Kitchens: Appliance Loads, Venting, and Examples
- Sizing Sunrooms: Glazing, SHGC, Orientation, and Shading
- Sizing Upstairs Rooms: Stratification, Ducts, and Roof Gain
- Quick Estimator (Rule‑of‑Thumb)
- Worked Examples (Kitchen, Sunroom, Upstairs)
- Common Mistakes & Easy Fixes
- People Also Ask (Short Answers)
- FAQ (Extended)
1) Why Special‑Case Rooms Break the Sizing Rules
Residential cooling load is a blend of sensible (temperature) and latent (humidity) heat. In average rooms, a simple rule like “20 BTU per square foot” roughly tracks the peak sensible load. But three kinds of rooms consistently defy that average:
- Kitchens layer large internal gains (appliance watts → BTU) on top of the envelope load.
- Sunrooms collect solar gains through high window‑to‑wall ratios and higher SHGC glazing.
- Upstairs rooms sit in the warmest air, take more roof and upper‑wall sun, and often get the poorest airflow.
These effects swing daily with use and sun angle: dinner hour peaks kitchen load; late‑afternoon hammers west‑facing sunrooms; cloudless days roast the top floor. The cure is not always “more tonnage”—sometimes it’s better airflow, duct sealing, shading, or a separate zone. But for first‑pass sizing, you’ll want numeric adders.
2) Quick Sizing Rules & Modifiers
| Factor | Rule‑of‑Thumb Adder | Notes |
|---|---|---|
| Base room load | ≈ 20 BTU/ft² | For single rooms/zones with average envelope; refine with Manual J where possible. |
| Kitchen appliance load | +4,000–6,000 BTU | Heavier cooking, multiple burners/ovens at once → toward the high end. |
| People (while occupied) | +600–1,000 BTU/person | Only count regular occupancy during peak hours (e.g., 2–3 people cooking). |
| Sunroom glazing (double‑pane) | +15–25% | Use lower end for low‑SHGC coatings or deep overhangs. |
| Sunroom glazing (single‑pane) | +25–40% | High solar/air leakage; consider films, exterior shades, or upgrading glass. |
| Orientation (south/west) | +10–20% | Late‑afternoon sun and low angles drive peaks. |
| Upstairs/top‑floor | +10–25% | Add if ducting/airflow are unimproved; reduce as attic/duct fixes are applied. |
| Ceiling height > 9 ft | +5–10% | More air volume; stratification control matters (fans, registers at correct height). |
| Shading (deep overhangs, trees) | −5–10% | Applies mostly to sunrooms and west/south windows. |
Reminder: 1 ton of cooling = 12,000 BTU/hr. Use adders to check if a 1‑ton (12k) or 1.5‑ton (18k) unit better fits your peak. Oversizing increases cycling and humidity; undersizing drives long runtimes and hot spots.
3) Kitchens: Appliance Loads, Venting, and Examples
Kitchens behave like mini commercial spaces. Stoves, ovens, dishwashers, refrigerators, and humans all convert watts to heat. A gas cooktop’s energy shows up as convective, radiant, and latent heat; electric ovens dump nearly all input as sensible heat over time. That’s why a kitchen sized like any other 150 ft² room often feels short by dinner.
A) Converting Appliances to BTU
As a quick check, you can convert appliance wattage to BTU/hr with BTU/hr = Watts × 3.412. Not all of that is instantaneous or reaches the room air, but during peak cooking, the effective sensible load rises sharply. Typical quick adders:
- Light cooking: +4,000 BTU
- Regular family cooking: +5,000 BTU
- Heavy cooking / multiple ovens: +6,000 BTU (or more)
B) Occupancy & Hood Effects
Add ~600–1,000 BTU per person present during cooking. A proper vent hood (ducted outdoors) removes heat and moisture from the cooking plume, but if it exhausts 200–600 CFM without make‑up air, it can suck hot outdoor air in through cracks—replacing the plume with infiltration load. Balance is key.
C) Example (12×12 kitchen)
A: Base: 144 ft² × 20 = 2,880 BTU. Add +5,000 BTU for cooking, +1,200 BTU for two people → ≈ 9,000 BTU. If open to a great room, upsize again or separate zones.
If your kitchen opens to a sunny dining/living area, that combined space may warrant a 12k–18k BTU mini‑split head—especially with west windows. Otherwise, a dedicated 9k–12k head aimed at the cook zone can keep sweat off your brow at 6 pm.
4) Sunrooms: Glazing, SHGC, Orientation, and Shading
Sunrooms flip the usual load mix: solar dominates. The more glass, the bigger the gain—and peaks hit when you want to relax. A sunroom that’s fine at 10 am can hit its cooling limit at 4 pm on a clear day.
A) Glazing Type & SHGC
Double‑pane, low‑e, low‑SHGC glass can slash solar gain versus single‑pane clear glass. If you can’t change the glass, add exterior shades, awnings, or reflective films. For first‑pass sizing, apply these multipliers to the base room load:
| Glazing & Shading | Adder to Base Load |
|---|---|
| Double‑pane, low‑e, low‑SHGC | +15–20% |
| Standard double‑pane | +20–25% |
| Single‑pane, clear | +25–40% |
| Exterior shade/awnings | −5–10% from above |
B) Orientation
South and west exposures drive late‑day peaks; east is a morning spike; north is the easiest. Add another +10–20% for strong south/west exposure during peak months.
C) Example (200 ft² sunroom, double‑pane)
A: Base: 200 × 20 = 4,000 BTU. Double‑pane adder +20% → 4,800. West exposure +15% → ≈ 5,500–5,600 BTU. Round to a 6k head, or 9k if it’s routinely packed with people.
Because peaks are narrow but intense, a separate zone (e.g., a small ductless head) keeps the rest of the home comfortable while the sunroom rides out the afternoon sun.
5) Upstairs Rooms: Stratification, Ducts, and Roof Gain
Warm air rises. Attics can exceed outdoor temperature by 20–50°F on sunny days, heating ceilings and upper walls. Meanwhile, second‑floor ducts may be long runs with extra friction and leaks, and a single downstairs thermostat quits early when the downstairs cools first.
- Add +10–25% capacity for upstairs rooms if ducts/airflow aren’t improved.
- Increase return and supply capacity upstairs; consider a second thermostat/zone or a dedicated ductless head for the worst‑case room.
- Boost attic insulation, seal penetrations, and use radiant barriers or cool roofing when possible.
A: Base 8,000 BTU; upstairs +20% → 9,600; west windows +15% → 11,000. A 12,000 BTU unit is reasonable, assuming decent airflow and shading in peak months.
6) Quick Estimator (Rule‑of‑Thumb)
This is a first‑pass calculator. For precise design, use a Manual J load calculation.
7) Worked Examples
A) Compact Kitchen (120 ft², regular cooking)
- Base load: 120 × 20 = 2,400 BTU
- Cooking adder: +5,000 BTU
- Two occupants during dinner: +1,600 BTU
- Total ≈ 9,000 BTU → choose a 9k–12k head depending on openness to adjoining rooms
B) Sunroom (220 ft², double‑pane, west)
- Base load: 220 × 20 = 4,400 BTU
- Glazing adder: +22% → 5,368 BTU
- West orientation: +15% → 6,173 BTU
- Total ≈ 6,200 BTU → 6k–9k head; prefer separate zoning
C) Upstairs Primary (380 ft², south windows, 10 ft ceilings)
- Base load: 380 × 20 = 7,600 BTU
- Upstairs adder (+18%): 8,968 BTU
- South windows (+15%): 10,313 BTU
- High ceiling (+8%): 11,148 BTU
- Total ≈ 11,100 BTU → a 12,000 BTU unit is appropriate if airflow is good
8) Common Mistakes & Easy Fixes
- Oversizing to hide problems. Big units cycle, miss dehumidification, and waste energy. Fix ducting, shading, and infiltration first.
- Ignoring occupancy patterns. If five people gather in the kitchen daily at 6 pm, size for that. If not, prioritize hood capture and ventilation balance.
- Open floor plan myopia. A kitchen open to a west‑glass great room is a combined zone. Either size for both or add a second head.
- Sunroom tied to main stat. Its peaks pull the whole home off target. Separate zoning = comfort and efficiency.
- Top‑floor with a single return. Add a dedicated return upstairs, balance dampers, and seal ducts to deliver design CFM.
- Neglecting envelope upgrades. Films, exterior shades, attic insulation, sealing, and reflective roofs reduce the adder you need.
9) People Also Ask (Concise Answers)
Is 12,000 BTU enough for a kitchen?
Usually for 150–250 ft² kitchens with regular cooking: base 3,000–5,000 + 5,000 cooking + people. If open‑plan, consider 12k–18k or a second zone.
How much extra for a sunroom?
Double‑pane +15–25%; single‑pane +25–40%. Add +10–20% for south/west. Exterior shades or low‑SHGC glass can trim 5–10%.
Why is my upstairs always warmer?
Stratification, higher solar/roof gain, and long/restricted ducts. Add 10–25% capacity or improve ducting and zoning.
Signs your upstairs AC is undersized
Room never reaches setpoint on sunny afternoons, long runtimes with small temperature split, or comfortable downstairs but hot upstairs.
Should I pick a bigger unit or add a head?
For kitchens and sunrooms, a separate small head handles peaks without oversizing the rest of the house.
10) FAQ (Extended)
Educational guidance only. For best results, commission a Manual J load calculation and verify duct design (Manual D) and equipment selection (Manual S).