HVAC & Ventilation: Calculators, Diagrams & Guides
13 calculators · 39 diagrams · 8 guides
Ventilation is the quiet half of a healthy home, and it is where snippet answers go wrong most often. A single number pulled from a search result usually confuses the code-required minimum with the best-practice recommendation, and almost never mentions that a fan rated in a lab moves far less air through a real duct. The calculators here separate those ideas on purpose: each one shows the flat minimum your code demands next to the size that actually fits your room, and then tells you how to buy a unit that still delivers once it is fighting a duct run.
The bathroom exhaust fan calculator is the anchor of this section. It reports the IRC and ASHRAE 62.2 floor of 50 CFM intermittent or 20 CFM continuous, layers on the Home Ventilating Institute methods that scale the fan to your square footage, ceiling height, and fixtures, and applies the delivered-versus-rated derating so the fan you install clears the mildew instead of just spinning. As the section grows it will add range-hood CFM with the makeup-air trigger, whole-house ventilation rate where the ASHRAE and IRC formulas genuinely disagree, ceiling-fan blade span, and dehumidifier capacity on the post-2019 test scale.
Everything is keyed to the current standards — ASHRAE 62.2-2019, the 2021 and 2024 International Residential Code mechanical chapters, and the manufacturers’ own delivered-airflow certifications — and stays materials-and-sizing only, with no pricing. Each calculator is free with no signup, cites the exact code section it rests on, and calls out where a state amendment such as California Title 24 changes the answer, so you can size a fan or a vent once and pass inspection the first time.
HVAC & Ventilation calculators
- Mini Split Breaker CalculatorWhat size breaker and wire for a mini split or central AC? Free NEC Article 440 calculator from the nameplate MCA and MOP — with real spec examples.
- HVAC BTU / Manual J CalculatorCooling + heating BTU, AC tonnage, furnace size, heat-pump balance point — per ACCA Manual J / S, ASHRAE 1%/99%. DOE 2023 SEER2. Free.
- Bathroom Exhaust Fan CalculatorFree bathroom exhaust fan calculator: the CFM you need from room size — code minimum vs HVI best-practice, plus the rated-vs-delivered fix. No signup.
- Range Hood CFM CalculatorFree range hood CFM calculator: size by width or BTU, see it vs the code minimum, and learn if you need makeup air (the 400 CFM rule). No signup.
- Whole-House Ventilation CalculatorFree whole-house ventilation calculator: the continuous CFM your home needs by ASHRAE 62.2 and the IRC, shown side by side. No signup.
- Ceiling Fan Size CalculatorFree ceiling fan size calculator: the right blade span for your room, plus mount type and downrod length by ceiling height. Per ENERGY STAR. No signup.
- Dehumidifier Size CalculatorFree dehumidifier size calculator: the pints/day you need by room size and dampness — on both the current and pre-2019 DOE scales. No signup.
- Manual J CalculatorFree Manual J calculator: estimate your home's heating and cooling load in BTU and tons from size, climate, insulation, and windows. No signup.
- Furnace Size CalculatorFree furnace size calculator: the heating load and furnace BTU (input, output, and AFUE) your home needs by size, climate, and insulation. No signup.
- AC Size CalculatorFree AC size calculator: the cooling load and the right air conditioner tonnage for your home by size, climate, insulation, and windows. No signup.
- Duct Size CalculatorFree duct size calculator: turn a run's CFM and friction rate into a round duct diameter, rectangular equivalents, and velocity. No signup.
- Mini-Split Sizing CalculatorFree mini-split sizing calculator: the right ductless heat pump size for your room, with cold-climate capacity and oversizing checks. No signup.
- Return Air Grille Size CalculatorFree return air grille size calculator: turn CFM into the grille or supply register size by face velocity, with standard sizes that fit. No signup.
Cost guides
Budget-level price ranges per room, computed from a dated cost model — wide brackets by design, updated on an annual review.
Guides & references
- Heat Pump vs. Air Conditioner: Cooling Is a Tie
- Heat Pump vs. Furnace: Cost, Climate & Which to Buy
- Mini Split vs Central Air: Cost, Efficiency & Cold
- HVAC Sizing Guide: Why Bigger Is Not Better
- SEER2, HSPF2 & AFUE: HVAC Efficiency Ratings
- Cold-Climate Heat Pumps: Sizing for Real Winters
- Whole-House Ventilation & Indoor Air Quality
- Basement Humidity & Dehumidifier Sizing Guide
HVAC & Ventilation · 39 diagrams
- HVAC & Ventilation
Three ways to size a bathroom exhaust fan — area, volume, and fixture methods over the code floor
The code floor is a flat 50 CFM that ignores room size; the HVI methods scale to your room. Buy the larger of the two.
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Rated versus delivered bathroom fan airflow — the same fan moves less through a real duct
Fans are rated at 0.1 in. of static, but a real duct sits near 0.25 in., where a typical fan delivers only ~70% of its rating.
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Bathroom fan duct routing — vent outdoors through a roof cap, never into the attic
IRC M1501.1 requires exhaust to terminate outdoors. Dumping humid air into an attic is a top cause of attic mold and sheathing rot.
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Two ways to size a range hood — cooktop width for electric, total BTU for gas
Electric hoods size by cooktop width, gas by total BTU — take the larger, add 25% for an island. The code floor is a flat 100 CFM.
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Range hood makeup air — a hood over 400 CFM can backdraft a water-heater flue
A hood capable of more than 400 CFM (IRC M1503.4) can depressurize a tight house and backdraft combustion appliances — makeup air prevents it.
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Ducted vs recirculating range hood, and duct diameter by airflow
A ducted hood vents outdoors and counts as kitchen ventilation; a recirculating hood only filters and gets no code credit.
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Whole-house ventilation — ASHRAE 62.2 vs the IRC give different rates for the same house
ASHRAE uses a 0.03 area coefficient, the IRC 0.01 (it assumes more natural infiltration) — 90 vs 50 CFM for the same house.
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Whole-house ventilation system types — balanced, exhaust-only, and supply-only
A balanced HRV/ERV supplies and exhausts equally without pressurizing the house; exhaust- and supply-only rely on leakage.
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Whole-house ventilation infiltration credit — total minus infiltration equals mechanical fan flow
The 2019 credit subtracts blower-door infiltration by superposition (Eq 4-2), not the deprecated 2010 "half the excess" rule.
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Ceiling fan blade span by room size — the ENERGY STAR sizing ladder
Blade span scales with room area (ENERGY STAR): 29–36 in. under 75 sq ft up to 56 in. or two fans over 400.
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Ceiling fan mount and downrod by ceiling height — flush at 8 ft, downrod above
An 8-ft ceiling gets a flush mount; taller ceilings use a downrod of (height − 9) × 12 + 6 in. to land blades 8–9 ft up.
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Ceiling fan clearances and when to use two fans
Keep blade tips 18–24 in. from walls and 7 ft off the floor; over ~400 sq ft, two fans beat one oversized fan.
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The 2019 dehumidifier label change — the same machine is rated lower now
The June-2019 DOE test moved from 80°F to 65°F, dropping ratings ~30%: an old 70-pint is now labeled 50. Buy by the current label.
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Dehumidifier sizing by dampness level, with the adjustments that raise it
Capacity scales with dampness (AHAM area × dampness); size up for tall ceilings, laundry, and bare concrete, then round to a standard unit.
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A dehumidifier dries the air — it can’t drain standing water
A dehumidifier pulls water vapor from the air into its tank (30–50% RH). It can’t remove standing water — seepage and puddles are a drainage job.
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Where a heating and cooling load comes from
A Manual J load is the sum of conduction, solar, infiltration, and internal gains — not a square-footage rule. Heating omits solar and internal gains.
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Why square-footage rules of thumb oversize HVAC equipment
Sized by the 500 ft²/ton rule a 2,000 ft² home gets 4 tons; the real load is ~1.75 tons. Rules of thumb oversize 2–3× — and oversized short-cycles.
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Cooling load is sensible plus latent, and the mix flips by climate
Cooling = sensible (temperature) + latent (moisture). Dry climates are almost all sensible; humid climates carry a big latent load, so oversizing leaves a house clammy.
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A furnace’s input rating is not its heat output
Furnaces are sold by INPUT, but only the output heats your home: output = input × AFUE. A 100,000 BTU/h, 95% AFUE furnace delivers 95,000 BTU/h. Size by output.
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Why an oversized furnace short-cycles
An oversized furnace heats fast, overshoots, and shuts off in short bursts — swings and wear. A right-sized furnace runs longer, steadier cycles. Manual S caps it near 140% of the load.
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AFUE — the share of furnace fuel that becomes heat
AFUE is the share of fuel that becomes heat: an 80% furnace loses 20% up the flue, a 98% condensing furnace only 2%. Higher AFUE lowers bills — size by output either way.
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Why an oversized air conditioner leaves a house clammy
An oversized AC cools fast and short-cycles, so it never removes moisture — cold but clammy (~65% RH). A right-sized unit runs longer and dries the air to a comfortable 30–50% RH.
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Window orientation drives the cooling load
Sun through glass is 25–40% of cooling, and orientation decides it: west and east glass catch the low afternoon/morning sun and drive the peak; north is least. Low-SHGC glass or shading on the west pays back most.
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Air conditioner tonnage and the Manual S sizing window
One ton = 12,000 BTU/h. Manual S sizes a single-stage AC at 100–115% of the load, so a 1.75-ton load takes a 2-ton unit. Airflow ≈ 400 CFM per ton.
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The equal-friction duct sizing method
Equal-friction sizing holds one friction rate (0.08 in./100 ft is the residential default) across the whole system and solves for the duct that carries the flow: 1,200 CFM → 16″ round at ~860 fpm.
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Air velocity limits by duct role
A duct can be big enough for the CFM yet still run the air too fast. Branches and returns stay under ~700 fpm, trunks under ~900; past that, ducts get noisy.
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Flexible duct versus smooth metal at the same diameter
Flex has a rougher wall than smooth metal, so at the same diameter it carries less air — size it up one size, and never leave it sagging or compressed.
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Sizing a ductless mini-split head per zone
Ductless systems are sized zone by zone: each head serves one room directly, so there is no duct loss and no shared load — compute a load per zone, then pick the nominal rung at or above it.
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Mini-split heating capacity versus outdoor temperature
Heat-pump capacity is rated at 47°F and fades as it gets colder. Size a cold-climate mini-split on its delivered capacity at your design temperature — from the NEEP list or the manufacturer table — not the nameplate.
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Inverter modulation floor and mini-split oversizing
An inverter throttles down only to its minimum output — about 28% of rated for a single-zone head. Oversize it and the everyday load falls below that floor, so it short-cycles instead of running low and steady.
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Ductless vs. central — ductwork decides it
The honest decision tree starts with ductwork. Ductless puts a head in each room, so every room holds its own setpoint with no ducts to leak. A central system runs one thermostat, so only that room hits setpoint while the others drift hot and cold, and its attic ducts lose ~20–30% of the air (ENERGY STAR). No ducts → ductless; good ducts → central is usually cheaper.
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Sizing a return or filter grille by face velocity
Grilles are sized by face velocity, not the duct friction method: pick the fpm limit for the type (400 for a filter grille), then solve for an opening that keeps the air that slow. 800 CFM → a 20×20 grille.
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Grille net free area versus gross size
Louver bars block about a quarter of a grille face, so only the ~75% net free area passes air. Size the grille up so its open area — not the nominal face — meets the velocity limit.
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The undersized return grille
An undersized return is the most common residential duct fault: the same airflow through a smaller opening speeds up, whistles, and raises static — starving the blower. The fix is more grille area, not a bigger blower.
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Why a bigger AC is not better — oversizing short-cycles and leaves the house cold but clammy
Bigger is not better. An oversized AC short-cycles: it cools fast but never runs long enough to dehumidify, so you get a cold, clammy house plus extra compressor wear. A right-sized unit (ACCA Manual S: 95–115% of the load) runs long, steady cycles that stay cool AND dry.
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Where the cooling load comes from — ceiling, walls, windows, air leaks, and internal gains
Square footage only sets the surfaces — the envelope, climate, and windows set how much heat crosses them. On a design day heat enters through the ceiling (biggest — attic ≈ +15°F), walls, windows (conduction + solar), air leaks, and internal gains. Same 2,000 sq ft, leaky vs. tight envelope = very different equipment.
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A heat pump’s balance point — capacity falls as the heating load rises
A heat pump loses capacity exactly as the house needs more heat. Plot both against outdoor temperature and they cross at the balance point. Above it, the heat pump carries the load; below it, backup heat (electric strip or a dual-fuel furnace) covers the gap. A cold-climate unit crosses lower and needs less backup.
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A heat pump is an air conditioner plus a reversing valve
In cooling mode a heat pump IS an air conditioner — same compressor, coils, and SEER2, so summer running cost is a tie. The only real addition is a reversing valve that lets it run backward to heat, so the whole decision is that modest upcharge.
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Heat pump vs. gas furnace — the operating-cost break-even
A gas furnace’s yearly cost barely moves with the electric rate; a heat pump’s rises with it. They cross near 16¢/kWh (gas ~$1.48/therm, COP 3.0): cheaper electricity favors the heat pump, expensive electricity favors gas. Propane and oil sit far above both.
Frequently Asked Questions
What size exhaust fan do I need for my bathroom?
The code floor is a flat 50 CFM for an on-demand fan or 20 CFM for a continuous one, regardless of room size (IRC M1505.4.4, ASHRAE 62.2-2019). Best practice scales up: about one CFM per square foot at an 8-foot ceiling, or eight air changes per hour for taller rooms. The bathroom exhaust fan calculator shows both numbers and rounds up to a standard fan size.
Why does a fan move less air than its CFM rating?
Fans are rated in the lab at 0.1 inch of static pressure, but an installed duct run — with its length, elbows, and wall cap — sits closer to 0.25 inch, where a typical fan delivers only about 70 percent of its rating. That is why an ENERGY STAR fan, certified to move at least 70 percent at 0.25 inch, matters, and why the calculators size for delivered airflow rather than the box number.
Can a bathroom fan vent into the attic?
No. IRC M1501.1 requires exhaust air to terminate outdoors, never in an attic, soffit, ridge vent, or crawlspace — dumping warm, humid air into an attic is a classic cause of mold, rot, and ruined insulation. Run a smooth duct to an exterior wall or roof cap with a backdraft damper, kept as short and straight as the framing allows to preserve delivered airflow.
What is whole-house ventilation and do I need it?
Whole-house ventilation supplies a continuous trickle of fresh air sized to floor area and bedroom count, required for tight modern homes under ASHRAE 62.2 and referenced by the IRC. The two standards use different coefficients, so the target rate can differ — a full whole-house ventilation calculator is planned for this section; meanwhile a continuous bathroom fan sized on the bathroom exhaust fan calculator can serve as the source in many homes.