TestTalkHQ • HVAC Hub
Eighth trade vertical on TestTalkHQ — the first dedicated HVAC hub. Duct CFM from velocity, equal-friction duct sizing, external static pressure vs the blower nameplate, piston superheat / TXV subcooling, return-vs-supply delta-T, a sq-ft-per-ton cooling load starting estimate (not Manual J), furnace heating BTU load with AFUE-to-input conversion, ductless mini-split per-zone BTU sizing, and extra refrigerant ounces for line-set length beyond factory charge stack into one diagnostic path. Starting point for field checks, not a Manual J certificate or a refrigerant ticket.
Low airflow and high static look like charge problems until you measure them. Size or verify the duct, clock CFM from an anemometer traverse, compare total external static to the blower nameplate, then decide whether delta-T (air side) or superheat/subcooling (refrigerant side) is the next gauge set. Do not use living-area square footage as duct CFM. Do not charge by delta-T alone.
Airflow · Measure
CFM from anemometer FPM and round or rectangular duct area. Traverse math for the reading you already took — not a Manual D design CFM and not a substitute for a flow hood when registers are the job.
Open Duct CFM Calculator ->Sizing · Equal friction
Equal-friction duct sizing from target CFM and friction rate. Round and rectangular options. Pair with measured CFM and ESP when the installed run underperforms.
Open Duct Sizing Calculator ->Restriction · Nameplate
Total external static (supply + return) vs the blower nameplate. High/low ESP flags, airflow notes, and the next link into CFM when the system is starved or over-pumped.
Open ESP Calculator ->Charge · Refrigerant
Piston superheat or TXV subcooling from line temperature, gauge pressure, and R-410A / R-22 / R-32. Target band with overcharge, undercharge, and airflow flags — not a recovery license.
Open Superheat / Subcooling Calculator ->Air side · Split
Return vs supply air temperature split with a humidity-adjusted expected band (typical AC ~16–22°F). High split often means low airflow; low split often means charge or coil issues — confirm with SH/SC and CFM.
Open Delta-T Calculator ->Air side · Delivered capacity
Total, sensible and latent cooling capacity from measured airflow and the dry-bulb and wet-bulb temperatures entering and leaving the coil, with the barometric pressure corrected for your site elevation. Returns the sensible heat ratio and the moisture removal in pints per hour — the latent half a Delta-T reading cannot see.
Open Sensible Heat Ratio Calculator ->Load · Sizing start
Rough residential cooling tons and BTU/hr from conditioned sq ft, climate band, ceiling height, and insulation quality. Square-footage rule of thumb with a Manual J disclaimer — starting point only, not a load calculation.
Open Cooling Load Calculator ->Load · Heating BTU
Heating BTU output from square footage, DOE-style climate zone, insulation, and ceiling height, then furnace input by AFUE and a round-up to 40k–120k. Rule of thumb with a Manual J heat-loss disclaimer — starting point only, not a load calculation.
Open Furnace Sizing Calculator ->Code · Combustion air
Net free area for gas appliance combustion air to IFGC 304 / IRC G2407, then the gross louver size. Five code methods, side by side.
Open Combustion Air Opening Calculator ->Load · Heat pump
Where heat pump capacity falls to the building load, and how much backup heat has to cover the rest.
Open Balance Point Calculator ->Load · Ductless BTU
Per-zone cooling BTU from climate, insulation, ceiling height, sun, and occupants. Round up to common head sizes, total the system, and flag single-zone vs multi-zone (or two outdoors above ~48k). Starting point only — not Manual J / Manual S.
Open Mini-Split Sizing Calculator ->Charge · Weigh-in
Ounces to add for liquid-line length beyond the factory-charged 15 or 25 ft. R-410A, R-22, R-32, R-454B. Total system charge and a typical max-length flag. Starting point only — verify with superheat / subcooling after the system runs.
Open Charge Calculator ->Quote · Job cost
Labor hours, equipment, refrigerant, ductwork, disposal, permit, overhead, and markup or margin into a customer-facing quote. The pricing counterpart to the sizing and charge tools — not a load calculation.
Open Job Cost Calculator ->From the TestTalkHQ shop
Want a client-ready quote instead of just the numbers? Excel + Google Sheets workbook with a refrigerant compatibility check.
View on Etsy →No. These tools are arithmetic on the readings you enter. Load calculation, duct design sign-off, and charging still need the code path and the credential your jurisdiction requires.
Blowers, condensers, and heat strips live on circuits. Voltage drop, ampacity, and breaker sizing sit on the electrical hub. Airflow and charge stay here. The two hubs link both ways because the equipment does.
Weigh extra line-set ounces on a new or recovered system. After it runs, piston systems verify with superheat and TXV systems with subcooling. Different job from delta-T (air side). See weigh-in vs SH/SC.
Not as a primary method. Delta-T is an air-side symptom. Confirm airflow and static, then use piston superheat or TXV subcooling for charge. The comparison pages spell out which gauge set answers which question.
On the free-calculators index under HVAC, without a dedicated vertical page. This hub is the eighth trade index so airflow, static, and charge tools share one entry point like welding, electrical, and the rest.
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