Our Calculation Methodology & Sources

HVAC Load Calculation Methodology (Whole House)

Step 1: Build the Envelope Geometry

From your conditioned square footage, stories, and ceiling height we reconstruct the surfaces heat flows through:

The ceiling and floor areas both equal the footprint, and net wall = gross wall − windows.

Step 2: Set Design Temperatures by Climate Region

Loads are computed at your region's design conditions against indoor setpoints of 75°F cooling / 70°F heating. The moisture column drives the latent (humidity) load:

Step 3: Assign U-Values by Construction Tier

Each surface gets a heat-transfer coefficient (U-value, BTU/hr·sqft·°F) based on the insulation tier you select:

Windows: single pane U-1.0 (solar factor 32 BTU/sqft), double pane U-0.5 (solar 22), Low-E double U-0.3 (solar 12). The solar factor is orientation-averaged with typical interior shading.

Step 4: Conduction Through Every Surface

Two refinements match Manual J practice: in cooling, the ceiling ΔT gets a +25°F sol-air bump because the attic above it bakes far hotter than the outdoor air; and the floor uses a foundation factor instead of the full ΔT, because the ground moderates it:

• Slab on grade: 0.5 × ΔT heating / 0.2 × ΔT cooling
• Crawl space: 0.7 / 0.3
• Heated basement: 0.25 / 0.1 (below-grade walls see ~50-60°F soil)
• Unheated basement: 0.5 / 0.15 (buffer zone)

Step 5: Air Infiltration (Sensible + Latent)

Outside air leaks in at a natural rate set by your air-tightness selection: 1.0 ACH (drafty), 0.5 ACH (average), or 0.25 ACH (tight new construction). Converted to CFM (ACH × volume ÷ 60), it produces two loads:

Step 6: Internal Gains (Cooling Only)

People and appliances heat the house from inside: 230 BTU sensible + 200 BTU latent per occupant, plus a 1,600 BTU appliance baseline (refrigerator, cooking average, electronics, lighting). Following Manual J convention, the heating calculation ignores these credits - you size the furnace for a cold night with the house empty and the oven off.

Step 7: Duct Gains and Losses

Step 8: Convert Loads to Equipment

The heating BTU figure is the load - a furnace's nameplate input BTU runs higher once AFUE efficiency and sizing allowances are applied (our furnace calculator handles that conversion).

Accuracy and Limitations

For typical homes this block load lands within roughly 10-15% of full ACCA Manual J software - the same accuracy class as other simplified load tools, and far better than square-footage rules of thumb. What it deliberately does not do: room-by-room loads (needed for duct design), window-by-window orientations, or permit-ready reports. For those cases - new construction, additions, code submittals - use ACCA-approved software or a professional service, as explained in our Manual J guide.

Tonnage per Square Foot Methodology

Step 1: Climate Baseline (BTU per Square Foot)

Instead of the outdated flat "1 ton per 400-500 sq ft" rule, we start from a cooling intensity per square foot that varies by US climate:

Step 2: Insulation Adjustment

The baseline is then scaled by construction quality:

• Good (newer build or upgraded): ×0.9
• Average (1990s-2000s, decent condition): ×1.0
• Poor (older, drafty, thin attic insulation): ×1.15

Step 3: Convert to Tonnage

Example: 1,500 sq ft × 22 (mixed climate) × 1.0 (average) = 33,000 BTU → 3 tons, or 500 sq ft per ton. Results above 5 tons trigger a note that homes that size are usually served by two systems rather than one oversized unit.

Step 4: The Advanced Modifiers

The three-input answer above is what the published chart is built from, and it is what you get if you never open the Advanced panel. Opening it adds five more terms, every one of which defaults to no effect so the basic answer is never altered behind your back. With them, the full calculation is:

When any of these move the result off the baseline, the calculator says so and shows you both figures, so you can see what the extra detail actually changed.

Step 5: The Heating Estimate

Ticking the heating box adds a furnace estimate. It uses a separate rate table from the cooling one above, because a furnace is not sized by the cooling load:

• Output BTU = square feet × the rate above × the same insulation factor, rounded to the nearest 500.
• Input BTU = output ÷ 0.80, since furnaces are sold by what they burn rather than what they deliver. An 80% AFUE unit is assumed because it is the common baseline you will be quoted against.
• Required airflow = cooling tons × 400 CFM, which is the figure that has to work for both seasons on shared ductwork.

Where This Tool Sits (and Its Limits)

This is the rule-of-thumb tier of our toolkit - the fastest way to sanity-check a contractor's quote or get in the right range for budgeting. The advanced panel covers ceiling height, storeys, sun, duct location and occupancy, but even with all of it the tool is still working from rates per square foot rather than from your building. It never sees your window area and orientation or your actual air leakage, and those two alone can legitimately move the answer by a full ton in either direction. When the number matters, step up a tier: the whole house HVAC load calculator runs a Manual-J-style block load from your actual construction details, and the room-by-room tonnage calculator handles individual spaces.

Portable AC Calculation Methodology

Steps 1 and 2: The Room Load, Shared With Every Other Calculator

A room's heat gain is a property of the room. It does not change because you are shopping for a portable unit instead of a window unit, so this page does not use its own room-load math. It calls the same engine as the homepage, the window page and the mini split page, which means the identical room returns the identical load on all four.

That shared model is documented in full, with every table, under the Window, Split & Mini Split AC (Shared Method) tab. In outline it works from floor area rather than volume, at 22 BTU per sq ft up to 500 sq ft and easing toward 18 above that, then applies a ceiling-height factor, the climate multiplier for your zone, and the floor, sun and insulation multipliers, with the stacked total clamped at 1.6x. Occupant and appliance heat is added afterwards rather than being multiplied.

Step 3: Conversion to DOE/SACC & Hose Logic

Traditional calculators stop at Step 2 and spit out a number. But portable air conditioners are inherently inefficient because their hot internal components sit inside your room. A window unit rated at 10,000 BTU will drastically outperform a portable unit rated at 10,000 ASHRAE BTU.

To fix this, we map the final room load to the Seasonally Adjusted Cooling Capacity (SACC) - the DOE rating explained in depth in our SACC vs ASHRAE BTU guide:

• SACC Rounding: We take the exact BTU load and round it up to the nearest 500 SACC BTU tier (e.g., a 7,100 BTU load recommends a 7,500 SACC unit).
• ASHRAE Fallback Estimation: Because some retailers still only list the older ASHRAE number, we calculate an estimated ASHRAE equivalent so you know what to actually buy. A single-hose unit delivers about 63% of its ASHRAE rating as SACC, and a dual-hose unit about 77%, so the required ASHRAE size is the SACC figure divided by 0.63 or 0.77 and then snapped to a size retailers stock.
• Dual-Hose Trigger Logic: Three conditions make it "strongly preferred": a Hot/Humid climate, a Kitchen, or Poor insulation. Five more make it "preferred": high sun, a Top floor, a Hot/Dry climate, a Living Room, or any room over 250 sq ft. Single-hose portables create negative air pressure by pushing indoor air outside, which is worst exactly where the load is already highest.

Mobile & Manufactured Home Calculation Methodology

Why Mobile Homes Need Different Math

You cannot use standard residential Manual J estimators for a mobile home. Manufactured homes are built on a chassis, usually sit above the ground, have thinner walls, shallower roof pitches, and distinct ductwork restrictions compared to traditional homes.

Because they are exposed on all six sides (including underneath) and generally have lower R-value insulation, they gain and lose heat much faster. If you size an AC for a mobile home using a standard house calculator, the AC will almost certainly be undersized.

Engine 1: ENERGY STAR Baseline Sizing

Our calculator uses the official ENERGY STAR manufactured-home cooling sizing guidelines as its foundational baseline. This consists of three steps:

• Sizing Group (Climate): We map the U.S. states into 14 distinct Climate Sizing Groups. Group 1 represents the coldest regions (like Alaska) requiring minimal cooling, while Group 14 represents tropical regions (like South Florida or Hawaii) with maximum cooling needs.
• Area Bands: The home's square footage is bucketed into predefined area bands (e.g., 561–840 sq ft, 841–1120 sq ft, etc.).
• Thermal Standards: The baseline tonnage is pulled from a strict lookup table depending on whether the home was built to minimum HUD standards or rigorous ENERGY STAR standards.

Engine 2: Qualification & Load Adjustments

Once the baseline envelope load is established, the calculator applies multiplicative adjustments based on the physical conditions of the home:

• Ceiling Height: Standard mobile homes have 8-foot ceilings. We increase the cooling load by 5% for 9ft ceilings, 10% for 10ft ceilings, and 15% for up to 12ft ceilings to account for the extra air volume.
• Sun Exposure: Heavy shade reduces the required load by 3%, while high direct sun increases the load by 8%.
• Insulation: Poor insulation (common in pre-1995 homes) increases the load by 8%. Good insulation (like modern double-pane windows and thick walls) decreases it by 3%. If the insulation quality is unknown, we assume a slight penalty of +3%.

After the envelope load is scaled, we add flat BTU penalties for internal heat sources:

• Occupants: We add 400 BTU of heat load for every person in the home beyond the first two occupants.
• Add-on Rooms / Porches: If the home has a converted porch or structural add-on sharing the AC, we add a flat 2,000 BTU penalty.

Engine 3: Duct Risk & System Path Routing

Mobile homes often suffer from severe ductwork issues (e.g., crushed underfloor ducts, torn crossover ducts in double-wides). Sizing the AC correctly won't fix comfort issues if the air can't reach the rooms.

Our calculator analyzes symptoms (like hot/cold rooms, coil freezing, noisy airflow) and duct locations to generate a Duct Risk Score. Based on this score, the calculator will recommend one of four system paths:

• Central replacement likely fits (Low duct risk)
• Central can work, but inspect ducts first (Medium duct risk, or older double-wides with unknown crossover duct conditions)
• Duct/airflow diagnosis recommended before buying (High duct risk, replacing the AC alone will likely fail to solve comfort issues)
• Mini-split is strongly worth comparing (If duct risk is high or the user prefers ductless solutions)

Engine 4: Heating Load

Cooling is only half the job, so the calculator also estimates the home's design-day heating load. Here we do not use a BTU-per-square-foot rule of thumb. Instead we run the same block-load physics as our whole-house HVAC load calculator: heat loss through walls, windows, ceiling, floor, and air leakage at your climate's winter design temperature. You do not answer a second climate question. Heating is keyed off your state's own winter severity— not its summer cooling group — because a state can be mild in summer yet brutal in winter (Massachusetts, Colorado, Illinois), so those homes are sized for their winters rather than their summers. Each state maps to a winter design temperature ranging from about −15°F in the coldest states to roughly 35°F in the mild South.

On top of that shared engine we add the three things that make a manufactured home lose heat faster than a site-built house. The base wall, ceiling, and air-leakage values themselves come straight from the whole-house load calculator linked above, so here we only list what is different:

• Exposed underbelly: the floor sits over outdoor air on a chassis instead of a sealed crawlspace, so we scale floor heat loss up by 40% (a 1.4× multiplier).
• Underfloor ducts: when the supply ducts run underfloor through the belly, we add a 15% heat-loss penalty for the heat they shed before it reaches the rooms.
• Leakier, older envelope: homes built before 1995 are modeled with single-pane glass, and if the insulation is unknown we drop them to the poor insulation tier. Poor or older homes also use the higher, leaky air-change rate rather than the average one.

The final heating load is then translated into the sizes people actually shop for:

• Electric furnace (kW): the norm inside a manufactured home's air handler. We divide the load by 3,412 BTU per kW and round up to a stocked size (5, 7.5, 10, 12.5, 15, 17.5, 20, 22.5, or 25 kW).
• Gas furnace (input BTU): we divide the load by an 80% AFUE to get the input rating gas units are sold by.
• Heat pump (tons): the load expressed in tons. In the coldest climate groups the calculator flags that a heat pump loses output on the coldest nights and usually needs backup electric strip heat or a dual-fuel setup.

The heating load is calculated for any floor area, including homes over 3,000 sq ft where the cooling lookup table stops, and the result tells you which season, heating or cooling, actually drives your equipment size. This is a design-day estimate in the same accuracy class as other simplified block-load tools. It is a strong sanity check and a good shortlist, but it is not a substitute for a stamped Manual J where code requires one. If you want to compare fuels or ductless options in more depth, use the furnace sizing calculator or the heat pump sizing calculator.

RV AC Calculation Methodology

Step 1: Reconstruct the Rig's Geometry

Each RV type carries its own default width, interior height, and glass fraction (Class A gets 22% glass for the windshield; a toy hauler only 12%):

Step 2: Design Temperature Difference by Climate

Against a 75°F indoor setpoint: Mild (~85°F days) ΔT 10°F · Moderate (~95°F) ΔT 20°F · Hot (~100°F) ΔT 25°F · Desert (110°F+) ΔT 35°F.

Step 3: RV Envelope U-Values

RV walls are 1-2 inch laminated panels (R-4 to R-9) - nothing like a house wall - so the U-values are much higher:

Step 4: Sol-Air Solar Loading

The single biggest RV-specific factor: the roof sits in direct sun with no attic buffer. We add a sol-air temperature bump to conduction and a direct gain through glass, by parking situation:

• Roof ΔT bump: +35°F full sun / +18°F partial shade / +5°F shaded
• Wall ΔT bump: +8°F / +4°F / +0°F
• Window solar gain: 45 / 22 / 5 BTU per sq ft of glass

Step 5: Infiltration

RVs are leaky boxes with frequent door cycles. Natural air changes per hour by build tier: 1.8 (basic) / 1.2 (standard) / 0.8 (4-season), converted to CFM and priced at 1.08 × CFM × ΔT, then multiplied by a 1.25 latent allowance for the moisture humid outside air carries in.

Step 6: Internal Gains and Pull-Down

Occupants add 450 BTU each (sensible + latent) plus a 600 BTU appliance baseline (12V fridge, converter, electronics). The subtotal is then multiplied by a ×1.10 pull-down factor - capacity headroom to recover a rig that heat-soaked in the sun all afternoon.

Step 7: Match to Real Rooftop Units

The load is matched against the actual RV unit ladder - 11,000 BTU low-profile (vans only), 13,500, 15,000, then multi-unit combos - with three real-world rules most guides skip:

• Hot-ambient derating: rooftop units deliver only ×0.94 of rated capacity in hot climates and ×0.88 in desert heat - exactly when you need them most.
• Forced dual units above 32 ft: one ceiling unit can't distribute air down a long rig regardless of BTUs.
• Short-cycle guard: if a bigger unit would exceed 1.4× the load, the calculator warns against it - oversized RV ACs short-cycle, dehumidify poorly, and die early.

Step 8: Power Requirements

A soft start module cuts the compressor's startup surge to roughly 25-35A, which is what makes small inverter generators and battery banks viable. Two units running together require 50-amp shore service.

SEER Savings Calculation Methodology

Step 1: Standardizing Ratings (SEER vs SEER2)

In 2023, the DOE updated testing standards, replacing the old SEER rating with the more rigorous SEER2 rating. Because SEER2 testing uses higher static pressure that mimics real-world ductwork, a SEER2 rating is mathematically slightly lower than a SEER rating for the exact same unit.

To provide an honest “apples-to-apples” comparison, our calculator automatically converts legacy SEER ratings into modern SEER2 equivalents before doing any math. We use the industry standard conversion factor of roughly 4.5% efficiency loss:

Step 2: Calculating Power Draw (Watts)

SEER (Seasonal Energy Efficiency Ratio) is essentially the ratio of cooling output (in BTUs) to electrical input (in Watt-hours) over a typical cooling season. By definition, if we know the cooling capacity and the SEER2 rating, we can determine the average power draw:

For example, a 3-ton (36,000 BTU) AC with a 14 SEER2 rating has an average power draw of 36,000 ÷ 14 = 2,571 Watts.

Step 3: Estimating Annual Energy Consumption (kWh)

Next, we multiply the power draw by the estimated number of hours the AC runs per year. We provide state-specific averages based on EIA (Energy Information Administration) climate data (e.g., 2,400 hours in Florida vs. 700 hours in Idaho).

Step 4: Financial Savings & Payback Period

Once we have the kWh for both the old unit and the new unit, calculating the financial impact is straightforward math using your local electricity rate:

Note: The payback period assumes electricity rates remain constant. Historically, electricity rates rise, meaning your actual lifetime savings will likely be higher, and the true payback period slightly shorter, than our conservative estimate.

Environmental Impact

To calculate your carbon footprint reduction, we use the EPA's standard conversion metrics for the U.S. electrical grid:

• 1 kWh of electricity saved = 0.85 lbs of CO₂ emissions prevented.
• 1 mature tree absorbs approximately 40 lbs of CO₂ per year.

Heat Pump Size Calculation Methodology

Step 1: Determine the Base Climate Load

Unlike standard AC sizing, heat pumps have to cover both summer cooling and winter heating, so the base tracks a climate's overalldemand rather than a single season. It is highest at both extremes — heavy cooling in hot-humid summers and heavy heating in cold winters — and lowest in mild marine climates where neither season is severe. We multiply your total square footage by that base climate factor:

Note that Temperate/Mild sits at the bottom of the table, below even Hot/Humid. That is deliberate: a mild marine coast has gentle summers andgentle winters, so neither season drives a large load. Climate here is not a simple hot-to-cold ladder — the two ends (heavy cooling, heavy heating) both need capacity, while the mild middle needs the least.

Step 2: Structural Envelope Multipliers

Next, we apply adjustment factors to account for structural thermal loss. These factors either penalize (increase) or reward (decrease) the base BTU requirement:

Step 3: Advanced Condition Adjustments

Our advanced logic applies further granular multipliers to account for unique home layouts and specific regional extremes:

• Home Type: Apartments benefit from shared walls (0.94x multiplier). Two-story homes have complex airflow dynamics (1.03x multiplier), and open-plan homes experience subtle distribution losses (1.02x multiplier).
• Humidity / Latent Load: In areas requiring aggressive dehumidification, we apply a 1.05x penalty. While standard ACs easily handle latent loads, heat pumps often require this slight bump to ensure comfort during sticky summer months.
• Extreme Cold (-15°C / 5°F): If winters drop into extreme negatives, we apply a final 1.03x buffer to ensure the heat pump retains enough capacity before relying on expensive backup heat strips.

The Complete Equation

We present the result as a target BTU and convert it to tons. Because real-world conditions vary, we also output a ±10% range. Finally, we map your specific tonnage to the closest nominal equipment sizes manufactured today (1.5, 2.0, 2.5, 3.0, 3.5, 4.0, or 5.0 tons).

Accuracy & Limitations

This calculator synthesizes guidelines from ENERGY STAR, Trane, and Carrier into a highly accurate estimate model. However, it is fundamentally an estimate.

For cold-climate heat pump sizing especially, it is critical to perform a professional, room-by-room ACCA Manual J load calculation. Factors like specific window U-values, exact wall R-values, and precise air infiltration rates (ACH) can shift the final sizing requirement in ways a generalized calculator cannot fully predict. Our tool also analyzes your inputs and generates a Confidence Score along with targeted warnings if we detect variables that strictly mandate a professional audit (such as very poor insulation in a cold climate).

Dehumidifier Size Calculation Methodology

Step 1: Calculate Base Capacity (Pints/Day)

The foundational metric for a dehumidifier is the number of pints of moisture it can remove per 24 hours. We begin with a baseline of 20 pints per 1,000 square feet (or 0.02 pints per sq ft), assuming slightly damp conditions.

Step 2: Dampness Adjustments

The baseline assumes a "Slightly damp" space. As moisture severity increases, we apply a multiplier to the base pints to handle the increased latent heat load:

• Slightly damp → 1.0x (baseline)
• Damp → 1.25x (+25% capacity)
• Very damp → 1.5x (+50% capacity)
• Wet → 1.75x (+75% capacity)
• Very wet / visible moisture → 2.0x (+100% capacity)

Step 3: Space Type & Layout Modifiers

Different spaces naturally generate or retain more moisture. We adjust the required capacity based on the specific room type and the layout of the area being covered.

Step 4: Environmental Factors (Temp & Height)

Ceiling height affects the total air volume in the room. A standard ceiling is 8 feet. For taller ceilings, we proportionally increase the required capacity (e.g., a 12 ft ceiling means 50% more air, requiring a multiplier of 1.5).

We also adjust for temperature. In cool environments, dehumidifiers are less efficient because coils frost up more easily, requiring a 10% capacity boost (1.1x). In warm environments, the unit operates more efficiently, allowing a slight reduction (0.95x).

Known Humidity Override: If you input a specific current humidity level above 60% relative humidity, we add an extra capacity bonus to pull down that severe moisture level quickly. (e.g. +5% bonus for every 10% RH above 60%).

Step 5: Snap to Standard Sizes

Dehumidifiers are sold in standard pint capacities (e.g., 20, 30, 50, 70 pints). Once the exact required capacity is calculated, we snap the value to the nearest standard size equal to or greater than the required amount to ensure you are never underpowered.

The Complete Formula

Humidifier Size Calculation Methodology

Step 1: Calculate Base Capacity (Gallons/Day)

The primary metric for a humidifier is the number of gallons of moisture it outputs per 24 hours. We begin with a baseline of 0.3 gallons per 100 square feet, assuming slightly dry conditions and a standard 8-foot ceiling.

Step 2: Dryness & Climate Adjustments

The baseline assumes "Slightly dry" air in a moderate winter climate. As the air becomes drier, the moisture deficit increases, requiring a higher multiplier:

• Slightly dry → 1.0x (baseline)
• Dry → 1.3x (+30% capacity)
• Very dry → 1.6x (+60% capacity)
• Extremely dry → 2.0x (+100% capacity)

We also adjust for your climate. In a cold winter climate, indoor heating systems run frequently, stripping moisture from the air at a faster rate (multiplier 1.3x). Very dry climates like deserts require an even higher output (multiplier 1.4x).

Step 3: Space & Coverage Modifiers

Airflow patterns dictate how effectively moisture spreads. We adjust the required capacity based on the specific room type and the layout of the area being covered.

Step 4: Ceiling Height Factor

Ceiling height affects the total air volume. A standard ceiling is 8 feet. For taller ceilings, we proportionally increase the required capacity (e.g., a 10 ft ceiling means 25% more air volume, requiring a multiplier of 1.25).

Step 5: Snap to Standard Sizes

Humidifiers are categorized into standard capacity tiers (e.g., 0.5, 1.0, 1.5, 2.0, 3.0, 4.0 gallons). Once the exact required capacity is calculated, we snap the value up to the nearest standard tier to ensure your unit has enough power to comfortably reach your target humidity without running constantly on its maximum setting.

The Complete Formula

Duct Size Calculation Methodology

Step 1: Determine Required CFM

The core of duct sizing is knowing how much air needs to move through the duct, measured in Cubic Feet per Minute (CFM). In our calculator, you either provide the CFM directly, or we estimate it based on system tonnage (typically 400 CFM per ton of AC capacity).

Step 2: Establish Velocity Limits

We can't just force air through a tiny pipe. If air moves too fast, it creates excessive noise and friction. We establish maximum target velocities (FPM - Feet Per Minute) based on the application:

• Supply Ducts: Target maximum of 900 FPM.
• Return Ducts: Target maximum of 700 FPM (or 600 FPM for main trunks) to ensure quiet operation at the return grilles.

Step 3: Calculate Duct Area (CFM to Size)

Using the formula CFM = Velocity × Area, we calculate the required cross-sectional area (in square feet) to hit the target velocity.

For round ducts, we then convert the square inch area into a diameter and round up to the next standard commercial size (e.g., 6", 7", 8", 9", 10", 12"), never down, because rounding down would push velocity past the limit we just set. We also never recommend smaller than 6", since a 5" branch carrying 100 CFM already runs at 733 FPM.

Step 4: Material Friction Adjustments

Air flows differently depending on the duct material. Rough interiors slow the air down and increase static pressure drop, so we divide the target velocity by the material’s friction multiplier before sizing. In practice this means rougher materials come back a size larger: 400 CFM sizes to a 10" rigid duct but a 12" flex duct.

Step 5: Rectangular Equivalent Conversions

When converting between round and rectangular ducts, we do not simply match the physical area. Because rectangular ducts have corners where air stagnates, they require more physical area to achieve the same airflow as a round duct. We use the ASHRAE Equal-Friction Equivalent Diameter formula, which is the diameter of the round duct that loses the same pressure per foot at the same airflow. This is not the hydraulic diameter (4 × Area ÷ Perimeter), which is a different quantity and would give a smaller answer:

For example, a 12" × 8" duct measures 96 square inches, but it performs like a 10.7" round duct, which is only 90 square inches. When we suggest rectangular alternatives for a round duct, we only offer sizes that match or beat it on this basis, so a listed option is never a downgrade.

We also strictly enforce a maximum Aspect Ratio of 4:1 (the width divided by the height). A duct that is 24" wide and 4" high (6:1) will suffer from severe friction and turbulence, and is rejected by our calculator.

Garage Heating & Cooling Calculation Methodology

Step 1: Reconstruct the Building From What You Know

Most people know their garage by bay count rather than square footage, so either input works. Bays map to typical floor areas:

• Wall area comes from approximating the footprint as a square, so wall area is the square root of the floor area, times four sides, times the ceiling height.
• Door area is 56 sq ft per bay of door, since a standard bay door is 8 ft by 7 ft. Doors on ceilings of 9 ft or more are counted 15% larger. A double-wide door and two singles both come to the same area, which is what actually matters thermally. The total is capped at 45% of the gross wall so an unusual entry cannot produce a building that is mostly door.
• Attached walls barely lose heat, because the room on the other side is already warm. The engine removes a share of the wall accordingly: 0% detached, 25% sharing one wall, 45% sharing two.

Step 2: Build the UA, Surface by Surface

UA is area multiplied by U-value, summed over every surface. It is the number of BTU per hour the building loses for each degree of temperature difference, and it is the honest core of the whole calculation. Lower U is better insulated.

Note that insulating the walls alone does nothing to the ceiling figure. That is not an oversight, it is the single most common mistake in a garage conversion: heat rises, and an open roof deck stays the easiest path out of the building no matter what you do to the walls.

Two more terms complete the UA. Slab edge loss is approximated at 0.8 per foot of perimeter, on the perimeter rather than the whole slab, because the ground under the middle of a slab is close to the slab's own temperature. Infiltration uses the air-change rates above, converted with the standard 1.08 sensible-heat constant.

Step 3: Heating

The design temperature differences below are deliberately smaller than whole-house values, because nobody heats a garage to 70°F. These are measured against a target of roughly 55°F, which is what most people actually want: warm enough to work in and to keep tools and fluids above freezing.

• Heating load = UA × heating ΔT, then × 1.15. That 15% is recovery pickup, because a garage is normally heated on demand from cold rather than held at temperature, and a heater sized only for steady state takes an unpleasantly long time to get there.
• Heater size is rounded up onto sizes actually sold: 17,000 / 25,000 / 30,000 / 45,000 / 50,000 / 60,000 / 80,000 / 100,000 / 125,000 BTU. The low end matters. Electric garage heaters are common at 5 kW (17,065 BTU), 7.5 kW (25,600) and 10 kW (34,120), while gas unit heaters start around 30,000, so a ladder beginning at 30,000 would tell someone with a small insulated garage to buy four times the heater they need.
• Kilowatts for the electric option are the heating BTU divided by 3,412.

Step 4: Cooling

Cooling is not the heating number run backwards, because a garage picks up heat from things a house does not. The envelope gain is UA × cooling ΔT, and then three real gains are added on top:

• Roof solar gain: 6 BTU per sq ft where the ceiling is uninsulated or only the walls are done, falling to 2.5 once the ceiling is insulated. Sun on an uninsulated roof deck is a large load in a building with no attic to buffer it.
• Parked vehicles: 1,500 BTU each. A car driven home radiates heat for a long time, and this is the conservative steady-state figure for it.
• Workshop use: 2,500 BTU if the space is used as a workshop, against 400 otherwise, covering tools, lighting and a person working.

The total is rounded up onto the ductless ladder used elsewhere on this site: 9,000 / 12,000 / 18,000 / 24,000 / 30,000 / 36,000 / 48,000 BTU. Mini splits dominate this category because a garage rarely has ducts and a window unit needs a window the building often does not have.

Why the Result Talks About Your Door So Much

Because the calculator builds UA surface by surface, it can report what share of the total each surface is responsible for. When the overhead door exceeds 18% of total heat loss, the result says so explicitly. On a typical builder garage with an uninsulated steel door, that threshold is passed easily: a bare steel door has a U-value more than three times an uninsulated frame wall and around eight times an insulated one, concentrated in the single largest surface in the building. This is the number that makes an insulated door the cheapest meaningful upgrade for most garages, and it is only visible because the load was built up from surfaces rather than from a square-foot rule.

HVAC Cost Model Methodology

Step 1: Installed Base Price by System Type and Tonnage

Every estimate starts from a national-average installed price - equipment, labor, permit, and old-unit disposal - for a mid-tier brand at standard single-stage efficiency (the 2023 federal minimums: 13.4/14.3 SEER2, 80% AFUE). Central AC uses a lookup by tonnage; the other systems derive from it:

• Tonnage snapping: whatever size you select is snapped to the nearest value on the 1.5 / 2 / 2.5 / 3 / 3.5 / 4 / 5 ton ladder before pricing - equipment is only manufactured in those steps. Mini splits add the 0.75 and 1 ton sizes (9,000 / 12,000 BTU single-zone heads), which central AC and furnaces are not sold in, so those two rungs appear only when a mini split system type is selected. The size dropdown's square-footage hints assume roughly 550 sq ft per ton as a mixed-climate approximation.
• Gas furnace: $4,200 + $450 × (tons − 2, minimum 0) - sized by the cooling tonnage it pairs with (an 80,000 BTU input furnace pairs with a 3-ton coil).
• Electric furnace: $2,600 + $300 × (tons − 2, minimum 0) - noticeably cheaper than gas and with a gentler slope. There is no combustion, so no flue, no gas line, no condensate drain on a high-efficiency model and no combustion-air provision, which removes most of what makes a gas install expensive. It still scales with size, because larger units need heavier elements and a bigger circuit. Note this is the installation price only; electricity generally costs more per delivered BTU than gas, so the cheaper install often carries the higher running cost.
• Furnace + AC combo: (AC base + furnace base) × 0.9 - one crew visit, one permit, shared line-set and plenum work saves about 10%.
• Ducted heat pump: AC base × 1.4 - outdoor unit plus air handler with backup heat strips.
• Mini split (single zone): $3,200 + $800 × tons.
• Mini split (multi-zone, 3 heads): $8,500 + $1,400 × tons.

Step 2: Efficiency Tier Multiplier

• Standard (single-stage, 13.4-15.2 SEER2 / 80% AFUE): ×1.0
• Mid (two-stage, 16-17 SEER2 / 96% AFUE): ×1.2
• High (variable-speed, 18+ SEER2 / 97%+ modulating): ×1.45

Step 3: Regional Labor Multiplier

• Lower-cost area (rural South / Midwest): ×0.85
• Average US market: ×1.0
• High-cost metro (Northeast, West Coast): ×1.2
• Very high (NYC, SF, Seattle, Boston): ×1.35

The regional multiplier applies to the equipment price and to the add-ons below, since both are labor-heavy.

Step 4: Ductwork and Electrical Add-Ons

• Ductwork - reuse as is: $0
• Ductwork - sealing / minor modifications (new plenum, transitions): +$2,000
• Ductwork - full replacement: +$6,500
• Electrical upgrade (new circuit, disconnect, panel work): +$1,000

Step 5: Range and Outputs

Worked example 1 (simple): 3-ton central AC, standard tier, average region, ducts reused. Base $6,000 × 1.0 tier × 1.0 region = $6,000 typical. Low = $6,000 × 0.88 = $5,280 → $5,300; high = $6,000 × 1.12 = $6,720 → $6,700. Per ton = $6,000 ÷ 3 = $2,000. Monthly = $6,000 × 0.00825 ÷ (1 − 1.00825⁻¹²⁰) ≈ $79.

Worked example 2 (everything stacks): 3-ton furnace + AC combo, mid tier, very-high-cost metro, duct sealing, electrical upgrade. Base = ($6,000 + $4,650) × 0.9 = $9,585. Equipment = $9,585 × 1.2 tier × 1.35 region = $15,528. Ductwork = $2,000 × 1.35 = $2,700. Electrical = $1,000 × 1.35 = $1,350. Typical = $15,528 + $2,700 + $1,350 = $19,578 → $19,600, shown as $17,200-$21,900. This is why the same 3-ton job can honestly range from about $8,400 in an average market to over $20,000 in Boston with premium equipment and duct work - every step of that spread is visible above.

How Each of the Three Tools Applies This Model

• HVAC Replacement Cost Calculator: the full model - all seven system types selectable, sizes shown in tons, tiers labeled with both SEER2 and AFUE.
• Furnace Replacement Cost Calculator: the model locked to gas furnace only, with tiers labeled by AFUE alone (80% / 96% / 97%+). Because furnaces are sold by input BTU rather than tons, the size dropdown translates the tonnage ladder like this:

• Central Air Installation Cost Calculator: the model locked to central AC only, with tiers labeled by SEER2 alone. Its two headline scenarios are compositions of the constants above: a like-for-like replacement is the AC base with no add-ons ($5,300-$6,700 at 3 tons, standard tier, average region), while a first-time installation with no existing ducts is AC base + full duct replacement ($6,500) + electrical upgrade ($1,000) = $13,500 typical → the $11,900-$15,100 range quoted on that page.

How the Numbers Were Calibrated

Base prices were calibrated against the consensus of published 2025-2026 cost guides (This Old House, Angi, HomeGuide: typical full replacement $5,000-$12,500, furnace + AC combos $7,000-$15,000) and cross-checked against contractors who publish their pricing on camera. The repair figures in the repair-or-replace tables (capacitor $150-$400 through furnace heat exchanger $2,000-$3,500) come from the same 2026 sources.

What This Model Cannot See (Its Limits)

This is a budgeting anchor, not a quote. It assumes a mid-tier brand (premium lines run 10-20% above, builder-grade about 10% below), it cannot see your home's access difficulties, code-upgrade surprises, or a contractor's overhead - the single biggest reason real quotes for the same job differ by thousands. Use the number to judge quotes, always collect three, and confirm the system size first with the whole house HVAC load calculator - an oversized quote is expensive even when the price per ton is fair.