Delivered capacity

Load calculation

A room-by-room cooling and heating load for Zone 2 construction, with the envelope inputs exposed instead of hidden behind a wizard. In development.

Inputs floor area · orientation · glazing · assembly R-values · measured infiltration
Outputs sensible and latent load · required tonnage · design airflow
Method ACCA Manual J procedure at New Orleans 1% design conditions

Why your contractor's tonnage is a guess

Square-feet-per-ton is a sales tool, not an engineering method. What the rule of thumb assumes about your house, and what happens to the numbers when you check those assumptions against a measured envelope.

Anchor number the sample Uptown shotgun on the PHP homepage: 2.4 tons calculated, 4.0 tons installed

Oversizing, short-cycling, and the humidity you can't get rid of

An oversized coil satisfies the thermostat before it has run long enough to remove water. The result is a cold, clammy house at 68% indoor relative humidity — and a mold complaint that more tonnage makes worse.

Anchor number runtime fraction, latent removal per cycle, and the indoor dew point that follows

Flex duct pressure calculator

What compression actually costs a flex runout. Five percent compression roughly doubles the friction loss; fifteen percent roughly quadruples it. An 8 in. runout carries about 202 CFM pulled taut and about 95 CFM at 15% compression — same duct, same pressure budget.

Inputs diameter · design airflow · run length · compression ratio · pressure budget
Outputs friction rate · carrying capacity vs. round metal · velocity · Reynolds number
Method ASHRAE Duct System Design Guide friction procedure at ε = 0.003 ft, with LBNL 2004 measured compression correction factors

Duct install calculator

Fittings, boots, bends, and takeoffs converted into the effective length they actually add, so a run's total equivalent length comes out of the geometry you can see in the attic rather than out of a memory of the last job.

Inputs run geometry · fitting counts · takeoff and boot types
Outputs total effective length · available static pressure check
Method equivalent-length summation; every fitting figure carries its published source or it doesn't ship

Duct sizing and static pressure: the budget nobody checks

Fan external static pressure minus coil, filter, and accessory losses is what's left to push air through duct. On the one local report carrying full fan data, 0.60 in. w.c. less 0.36 leaves 0.24 — and that 0.24 has to cover the whole critical path.

Anchor number allowable friction rate = 100 × available static pressure ÷ total effective length

The return plenum hole — what a 7 in. cut costs you

Supply leaks get all the attention because they depressurize a house. Return leaks in an attic do something worse: they pull 125 °F air at 150 grains of moisture across the coil, and that load is charged against the capacity your rooms were supposed to get. On a 3-ton system, a 7 in. round hole pulls roughly 20% of the return air out of the attic.

Anchor numbers entering coil temperature rises from 75 °F to 85 °F · supply humidity rises from 57 to 74 grains against a 65-grain room, so the machine adds moisture to the house · effective capacity drops 73%, upper bound with a ±3-point band · the 10×12 case makes effective capacity negative at any indoor temperature

Return leak calculator

Hole size, tonnage, attic temperature, and attic dew point in; leak airflow, the share of return air coming from the attic, capacity lost, supply air condition, and condensate production out. Build spec and constants are written and reconciled against measured data.

Inputs hole size and shape · system tonnage · attic dry bulb · attic dew point · indoor condition
Outputs leak CFM · % of return air from attic · effective capacity in tons and % of nameplate · supply temperature and humidity · gallons of water per hour
Method orifice flow solved as a parallel path against a PSC blower curve, refrigerant-side gross capacity, psychrometric mixing — validated against Texas A&M psychrometric-room measurements to within 2.4 to 6.5 percentage points, reported as an upper bound

Supply leakage and attic duct gain

Two separate losses that get reported as one. Leakage sends conditioned air into the attic; conduction through duct wall warms the air that does arrive. Published field work puts the combined thermal loss at 10% to 45% of delivered capacity.

Anchor number duct leakage to outside, CFM25, from a duct blaster test — not a visual inspection

Filter restriction and the airflow you never had

A deep-pleat filter in an undersized return is a capacity problem wearing an indoor-air-quality costume. What the pressure drop does to airflow, and why the answer is a bigger filter grille rather than a thinner filter.

Anchor number pressure drop across the filter, measured clean and loaded, against the fan's available static pressure

Envelope air-tightness

Return-side leaks pressurize a house instead of depressurizing it, so the leak airflow becomes net mechanical outdoor air. The two pillars share the same physics and the same instrument.

Insulation as a system

Attic temperature is the driver of every duct and plenum loss in Rail 03. The insulation strategy above the ducts changes the capacity arithmetic below them.

Indoor air quality

An attic return leak is an unfiltered air intake drawing from the dirtiest, hottest, wettest space in the house. Capacity loss is the part you can feel; the particulate and moisture load is the part you can't.