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HVAC System Analyzer: 6 Calculators in One Tool

Cost per BTU by fuel, a bills-based heat load estimate, ventilation air changes per hour, heat pump switchover temperature, round and rectangular duct sizing, and room-by-room CFM balancing, all in one tool with shared exports.

A six-tab HVAC workbench that bundles the quick calculations technicians and serious DIYers reach for most. Cost per BTU converts any fuel price (natural gas, propane, oil, electric, heat pump) into dollars per delivered million BTU at your entered efficiency so fuels can be compared on one axis. Heat Load estimates average and design-day load from fuel used over a date range, a lighter version of the dedicated Heat Load from Bills tool. Ventilation computes room volume and the CFM needed to hit a target air changes per hour, or the ACH delivered by a known CFM. Switchover finds the outdoor temperature where a backup fuel becomes cheaper than a heat pump, using capacity ratings at 47, 17, and 0 F. Duct Sizer takes a CFM and recommends round duct diameters (4 to 20 inches) or rectangular equivalents against a 600 to 900 FPM supply velocity target band. CFM Balancer distributes a total system CFM (entered directly or from tons at 400 CFM/ton) across named rooms by floor area, with per-room load-factor adjustments. Every tab is an estimate built from your inputs and simplified rule-of-thumb formulas: the duct and balancing tabs use velocity and area arithmetic only, not a full static-pressure or Manual D design, and the ventilation tab is generic room-volume math, not a code-compliance determination. Share links, CSV/PDF export, and light/dark themes are shared across all six tabs.

Pro Tip: The duct sizer answers "what diameter keeps this CFM in a sane velocity band," which catches gross undersizing fast, but velocity is only half of duct design: a long run with many fittings can be quiet and still move no air because static pressure ran out. Use the velocity check to shortlist a size, then check it against available static pressure and total effective length (a Manual D calculation or friction chart) before cutting metal.

Run the deeper bills-based heat load estimate with house-age benchmarks and diagnostics

Heat Load from Bills Calculator →

Run the full switchover comparison with brand COP curves and multiple backup fuels

Mini-Split Efficiency & Cost Calculator →

Check whether a heat pump upgrade pays for itself from 12 months of bills

Heating Bill ROI Calculator →

Size ductwork with friction-rate inputs in the dedicated duct tool

Duct Sizing Calculator →

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How It Works

  1. Pick a Tab

    The six tabs across the top (Cost per BTU, Heat Load, Ventilation, Switchover, Duct Sizer, CFM Balancer) are independent calculators with shared theme, share-link, and export plumbing. Each tab keeps its own inputs, so you can flip between them mid-job without losing work.

  2. Cost per BTU: Compare Fuels

    Pick a fuel, enter its price in native units and the appliance efficiency, and read dollars per million BTU delivered. Run it once per fuel to rank natural gas, propane, oil, electric resistance, and heat pump heat on one axis.

  3. Heat Load: Estimate from Fuel Use

    Enter fuel used over a date range, the efficiency, indoor setpoint, average outdoor temperature, and design temperature, plus floor area for a per-square-foot figure. The output is an average and design-day BTU/hr estimate.

  4. Ventilation: ACH and CFM

    Enter room length, width, and height plus a target air changes per hour to get required CFM, or enter a known fan CFM to see the ACH it delivers in that volume.

  5. Switchover: Heat Pump vs Backup

    Enter your electric rate, the backup fuel and its price and efficiency, indoor/design temperatures, and the heat pump capacity at 47, 17, and 0 F. The output is the estimated outdoor temperature where the backup becomes cheaper to run.

  6. Duct Sizer and CFM Balancer

    Duct Sizer: enter CFM and duct type to get the round diameters (or rectangular size) that land in the 600 to 900 FPM target band. CFM Balancer: enter total CFM directly or as tons at 400 CFM/ton, list rooms with floor areas and optional load factors, and read the per-room CFM split.

  7. Export

    Share links capture the active tab and its inputs; PDF and CSV export package the results with assumptions and source notes.

Built For

  • A technician ranking propane versus electric resistance versus heat pump heat for a customer in one visit using the cost-per-BTU tab
  • Sizing a branch duct for a 250 CFM bedroom run and checking the trunk is not screaming past 900 FPM
  • Splitting 1,200 CFM across six rooms by floor area for a rough register plan before a Manual D is run
  • Estimating the CFM needed to turn over a workshop at 6 air changes per hour for general ventilation
  • A quick switchover-temperature answer during a dual-fuel thermostat setup, without leaving the analyzer for the full mini-split tool

Features & Capabilities

Six Calculators, One State

Cost per BTU, heat load, ventilation ACH, switchover temperature, duct sizing, and room CFM balancing share one app, one theme, one share-link format, and one export pipeline, so a service call does not need five browser tabs.

Fuel-Agnostic Cost per BTU

Natural gas, propane, heating oil, electric resistance, and heat pump inputs all reduce to dollars per million delivered BTU using standard heat contents and your efficiency, the only fair axis for comparing fuels.

Three-Point Heat Pump Capacity Model

The switchover tab uses capacity at 47, 17, and 0 F so the comparison degrades the heat pump realistically with temperature instead of assuming one fixed COP.

Velocity-Band Duct Sizing Check

Round diameters from 4 to 20 inches and rectangular equivalents are checked against a 600 to 900 FPM supply band, with the velocity for each candidate shown so you can see how close to the edges you are.

Area-Weighted Room Balancing

Total CFM (or tons at 400 CFM/ton) is distributed across named rooms by floor area with per-room load factors for sun-baked or over-glazed rooms.

Honest Boundaries on Every Tab

Each tab states what it is not: the duct and balancing tabs are velocity/area checks rather than static-pressure design, the ventilation tab is generic room-volume arithmetic rather than ASHRAE 62.1/62.2 compliance, and the heat load tab is a fuel-use estimate rather than a Manual J.

Share, PDF, and CSV

Each tab state encodes into a share URL, and exports include the inputs, results, assumptions, and source pointers.

References

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Frequently Asked Questions

At the common supply velocity band of 600 to 900 FPM, 400 CFM wants roughly a 9 to 11 inch round duct (a 10 inch round at 400 CFM runs about 730 FPM). The duct sizer does this lookup for any CFM and also converts to rectangular sizes. Remember the band is a velocity check; long runs and fitting-heavy layouts still need a static-pressure check.
The conventional rule is 400 CFM per ton of nominal cooling capacity, which is the default the CFM balancer uses when you enter tons instead of CFM. Real systems are commonly set up between about 350 CFM/ton (humid climates, more latent removal) and 450 CFM/ton (dry climates); use the direct CFM entry if you know the actual blower setup.
ACH = (CFM x 60) / room volume in cubic feet. The ventilation tab does it both directions: enter a target ACH to get required CFM, or a known CFM to get delivered ACH. Note this is general ventilation arithmetic; ASHRAE 62.1/62.2 compliance, kitchen/bath exhaust codes, and any industrial or confined-space ventilation follow their own procedures.
It changes with local prices, which is why the tab exists. As a feel for the math: natural gas at $1.50/therm and 95 percent AFUE delivers heat near $16 per million BTU; propane at $3.00/gal and 95 percent is around $34; electric resistance at $0.15/kWh is about $44; and a heat pump at COP 3 on the same electricity is about $15. Enter your own rates and the rankings are computed for your zip code, not a national average.
Same idea, lighter version: this tab takes one fuel quantity over a date range and scales it to design conditions. The dedicated tool adds multiple simultaneous heat sources, house-age BTU-per-square-foot benchmarks, input diagnostics, and a quick envelope upgrade estimate. Start here on a service call; use the dedicated tool when the answer matters for an equipment purchase.
No. The duct sizer checks candidate sizes by velocity and the balancer splits CFM by area, which catches gross sizing errors quickly. A Manual D design accounts for available static pressure, total effective length, fittings, filter and coil losses, and blower tables. Use the quick check to get in the neighborhood and a full design to finish.
The outdoor temperature where your backup fuel becomes cheaper to run than the heat pump, given your electric rate, fuel price, and the heat pump efficiency at that temperature. Dual-fuel thermostats use it as the changeover setpoint. This tab computes a quick estimate from three capacity points; the Mini-Split Efficiency Calculator does the full-curve version with brand presets and multiple fuels.
Disclaimer: All six calculators are simplified estimates built from your inputs and simplified published formulas. The duct sizer and CFM balancer use velocity and area arithmetic, not static-pressure or Manual D design; the ventilation tab is generic room-volume math, not an ASHRAE 62 or code compliance determination; the heat load and switchover tabs are bills-and-ratings estimates, not load calculations or equipment selections. Verify any duct construction, equipment purchase, or ventilation requirement with a qualified HVAC contractor and the codes that apply to your project.

Learn More

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How to Analyze Your HVAC System Performance

Seasonal efficiency vs rated efficiency, degree-day analysis, heat load from utility bills, oversizing problems, duct leakage, and when to replace vs repair.

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Ductwork Sizing and Design Guide

How to size supply and return ductwork using the equal friction method. Covers velocity limits, flex duct corrections, fitting equivalent lengths, return air sizing, duct leakage, and static pressure budgets.

Read Guide →

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