Most HVAC software still sizes ducts wrong. Here's what right looks like.
A 5-inch duct for 110 CFM isn't a typo — it's what velocity-only sizing produces. Why rule-of-thumb duct sizing keeps failing, and what proper ACCA Manual J and Manual D actually take.
Ask a typical piece of contractor software to size a duct and you'll usually get one of two things: a lookup against a friction chart, or a number backed out of velocity alone. Both can produce something like a 5-inch duct for 110 CFM — a spec that looks fine on a screen and causes callbacks in the field.
Sizing ducts correctly is one of the few things HVAC software could genuinely get right. Most of it doesn't.
Velocity isn't the whole story
"Size by velocity" is the shortcut most people learn: pick a target velocity, back into a duct size. It's quick, and it's wrong often enough to matter. Velocity describes how fast air moves through a duct. It says nothing about whether the system as a whole can push the required airflow to every room. Two runs at the same velocity behave very differently once you account for length, fittings, and the static pressure the equipment has to overcome.
Airflow is a pressure budget
Getting duct sizing right means treating the whole system as a pressure budget.
It starts with the load. A Manual J calculation establishes how much heating and cooling — and therefore how much airflow — each room needs, from real inputs: window U-factor and SHGC, orientation and solar gain, insulation, infiltration.
From there you size for a friction rate, not a velocity. Manual D's equal-friction method distributes the available static pressure across the whole duct system so every run gets its share, with a velocity cap so air never moves fast enough to become noise. And it accounts for total static pressure: every elbow, boot, and length of run costs pressure. Ignore that, and the fan can't deliver the airflow the load calc called for — the rooms furthest from the air handler starve.
The difference between the two approaches is stark:
| Velocity-only rule of thumb | Manual J + Manual D | |
|---|---|---|
| Sizes from | A target air speed | Each room's heating/cooling load |
| Run length & fittings | Ignored | Accounted for |
| Total static pressure | Ignored | Accounted for |
| Noise control | Incidental | Explicit velocity cap |
| Every room gets its airflow | Not guaranteed | By design |
| Can you check the math? | Trust the number | PDF proofs + test suite |
Why most software avoids it
Because it's hard. A real Manual J/D engine is a genuine piece of engineering, not a chart lookup. Most contractor platforms handle the quotes and invoices and leave sizing to a rule of thumb or a separate tool — software that runs the business but not the work.
What doing it properly looks like
We built this into a contractor platform recently: an equal-friction Manual D engine with a velocity cap and total-system static-pressure loss, fed by a per-room Manual J load calc, and backed by an automated test suite and rendered PDF proofs so the numbers are checkable rather than trusted. An auto-routing step keeps it usable — place the air handler and supply boots, and the system produces a correctly-sized trunk-and-branch layout for supply and return. The engineering is there; the person using it just sees a clean design. There's more on that build in the HVAC platform case study.
What to ask your HVAC software
If you're evaluating a platform, the sizing questions matter more than the feature list:
- Does it run a real per-room Manual J load calc — window U-factor and SHGC, orientation, infiltration — or just a rule of thumb?
- Does it size ducts with Manual D equal-friction using total-system static pressure, not velocity alone?
- Does it apply a velocity cap so runs don't turn into noise?
- Can it show its work — checkable numbers or PDF proofs, not a black box?
- Does it auto-route a usable trunk-and-branch layout, not just return a single number?
Most platforms answer "no" to the first four. That gap is exactly where the callbacks come from.
It shows up as callbacks
Correct sizing isn't academic. It's the difference between a system that's quiet and comfortable and one that produces complaints, warranty visits, and "the back bedroom is always hot." Each of those costs a contractor time and trust.
The methods aren't a secret — ACCA published them decades ago. The rule of thumb survives because building the real thing is harder, not because it works.
Want this kind of thinking on your product?
We design, build, and grow software end-to-end. Tell us what you're working on.