INTRO (0:00 to 0:30)
Darrel at the curb, tablet under his arm, nodding at the condenser by the side yard: That is a 5 ton on a 2,000 square foot house, and the service history in my hand says short cycles, two capacitors in four years, and rooms that never even out. The homeowner wants a replacement quote and assumes 5 tons, because that is what fits the pad. We are not going to guess. We are going to measure this house, run a real Manual J, and let the manufacturer's own table at 112 degrees pick the machine. Watch how few of these inputs you could have eyeballed.
ON-SCREEN: M37 Field Demo: one house, one load calc, one defensible selection
MAIN (0:30 to 10:45)
Beat 1: Orientation and glazing, the biggest lever first (0:30 to 2:15)
Darrel pulls up the compass at the front door and calls the facing for the camera: front door faces east, so this back wall of glass faces west, straight into the afternoon sun at the same hour the air peaks. He walks the perimeter with the laser and tape, second camera close on each window as he narrates the capture routine: width, height, count, orientation, every opening. At the big back slider he does the low-E test on camera, holding the card or flame to the glass and reading the reflections: these were retrofitted, dual pane, low-E, that is real SHGC relief, and it goes in the calc as what it is, not as the original single pane and not as some menu default. He points out the patio overhang and the sunscreens on the two west bedroom windows: shading is an input too. Then the teaching line, delivered flat: a west slider entered as north is the single most expensive typo in this trade. Compass in one hand, tape in the other, every entry.
ON-SCREEN: Every window: size, count, orientation, panes, low-E, shading
ON-SCREEN: SHGC = fraction of solar energy the glass passes. West glass peaks with the design hour
Beat 2: Envelope and the attic truth (2:15 to 4:15)
Darrel knocks the exterior wall, then pops an interior outlet cover with the flashlight: frame and stucco, insulated cavity, mid-nineties. Not the eighties block story, and not spray foam either. This house sits in the middle band. Headlamp on, up the attic hatch, second camera following. He sinks a tape into the blown insulation: call it a topped-up modern R, somebody upgraded this, points for them. Then he swings the light onto the duct trunk snaking across the joists: and then there is this. Every foot of that duct lives in this attic, which was a hundred and fifty degrees at five o'clock yesterday. It leaks like every duct system leaks, twenty to thirty percent typical, and it soaks heat through its jacket the whole run. Duct location is a load input, one of the biggest in this market. If we ever seal this attic or move these ducts, the load itself changes and the calc gets rerun. Back down the ladder, he flags infiltration evidence on the way: original weatherstripping with daylight at the bottom corner, six can lights in the hall ceiling, a fireplace with a damper that has not closed since the nineties. No blower door today, so this gets an honest age-and-condition category, not the optimistic button.
ON-SCREEN: Attic ducts: 20 to 30 percent leakage plus conduction gain. A top-three load input here
ON-SCREEN: Infiltration: enter what you saw, not what the menu hopes
Beat 3: Floor area, internal gains, and the witness interview (4:15 to 5:30)
Darrel shoots room dimensions with the laser, narrating: measured conditioned area, 2,000 and change, garage excluded. The listing said 2,200. The listing always says more. Three bedrooms, so occupancy enters at bedrooms plus one, four people, plus the kitchen allowance. Then he sits the homeowner interview for the camera, two questions only: On the hottest afternoons last summer, did it hold 75? Homeowner: yes, always. Did it run steady or stop and start? Homeowner: stop and start, all day, you hear it slam on. Darrel to camera: that is testimony. A 5 ton that cycles on a near-design day is a 5 ton sitting way above the true load. Whatever the software says later has to agree with that witness, or the software is wrong.
ON-SCREEN: Existing equipment is a measuring instrument: holds setpoint plus cycles = oversized
Beat 4: Into the software, then the cross-examination (5:30 to 7:30)
Darrel at the tablet on the tailgate, screen recorded, walking the flow without naming the product: location first, and verify the design conditions it loaded: 112 outdoor, 75 indoor, heating values sane. Never trust the defaults; verify the station. Then assemblies: the walls, the topped-up attic R, the low-E dual panes. Then geometry room by room, every window with its orientation off my notes. Infiltration: the honest category. Gains: four occupants, standard kitchen. Ducts: attic location, the insulation jacket I measured, leakage by condition. He taps run and the output fills the screen, second camera close: total cooling 38,200 BTU per hour. Sensible 35,500, latent 2,700. Now the cross-examination, counted off on fingers: One, 38,200 over 2,000 square feet is about 19 BTU per square foot, dead center in the band for a partially upgraded nineties house. Two, latent is 7 percent, right for a dry climate; if this screen said 25 percent latent I would be hunting a humid default in the inputs. Three, the component breakdown: glazing and attic ducts on top, exactly what the walk predicted. Four, the witness: 38,200 is well under what that cycling 5 ton delivers, so the testimony and the math agree. This number survives. Now it gets to go shopping.
ON-SCREEN: OUTPUT: total 38,200 / sensible 35,500 / latent 2,700
ON-SCREEN: Sanity: 19 BTU per sq ft, 7 percent latent, breakdown matches the walk, witness agrees
Beat 5: Manual S at the table, 112 degrees, no nameplates (7:30 to 9:45)
Darrel spreads the printed expanded performance pages on the tailgate, camera overhead: Manual S says match the machine's real capacity at our real conditions. The window for single-stage: 90 to 115 percent of total load. He writes the math big on the clipboard: 90 percent of 38,200 is about 34,400. 115 percent is about 43,900. That is the target zone. First the incumbent, finger on the 5 ton page: at design this delivers around 52,000. That is 137 percent of the load. The window threw it out before the homeowner finished her sentence. The cycling, the capacitors, the uneven rooms: all of it was this row of the table doing exactly what it said it would. Then the 3.5 ton page, second camera tight on the columns: nameplate says 42,000, but that is the 95 degree humid-lab number. Our row does not exist, so we interpolate: at 105 it posts 38,000 total, at 115 it posts 36,200, both in the dry entering wet bulb column, around 63, because this house is not a humid lab. At 112, call it 36,700 total, 33,900 sensible. He runs the three checks aloud, writing each: total, 36,700 over 38,200 is 96 percent, inside the window. Sensible, 33,900 against 35,500, covered within tolerance. Latent, about 2,800 delivered against 2,700, covered. Then the 4 ton page for honesty: it lands near 42,000 delivered, 110 percent, technically inside the window too. Two machines fit. We take the smaller one, every time we can: longer cycles, fewer starts, and it asks the ducts for 1,400 CFM instead of 1,600 through a duct system we have not redesigned. He taps the suction line of the old unit on the way past: and that is the last point. The capacity on that page is only real at the airflow we can actually deliver. Whether these attic ducts can carry 1,400 to 1,500 CFM inside the static budget is a Manual D question, and that is M38's whole job.
ON-SCREEN: Window: 34,400 to 43,900. Existing 5 ton at design: about 52,000 = 137 percent. OUT
ON-SCREEN: 3.5 ton at 112 F, dry entering wb: about 36,700 total / 33,900 sensible = 96 percent. IN
ON-SCREEN: 4 ton also fits at 110 percent. Smallest that covers wins
Beat 6: The write-up (9:45 to 10:45)
Darrel back at the tablet, building the record on camera: the full Manual J output, exported, every input visible: design conditions, assemblies, the window schedule with orientations, infiltration category, duct location. The expanded data pages with the 112 degree interpolation written in the margin and the three coverage checks. The witness interview, one line: existing 5 ton holds setpoint and cycles at near-design, consistent with calculated load. The selection: nominal 3.5 ton single-stage, 96 percent of total load, sensible and latent covered, pending the M38 duct verification at 1,400 CFM. He closes to camera: a year from now, an inspector, an engineer, or the next tech can open this file and re-walk every step to the same answer. That is the difference between a sized system and a guessed one. The pad fits a smaller machine just fine.
ON-SCREEN: Job record: full calc + table excerpt + interpolation + selection rationale
OUTRO (10:45 to 11:00)
Darrel, tailgate shut: Square footage never met this house. The envelope, the attic ducts, the west glass, and the table at 112 picked this machine, and every one of those is a thing you measured. Next module, M38, we find out if the ducts can deliver what we just promised. Bring the room-by-room sheet.
ON-SCREEN: Next: M38 Duct Design (Manual D). The room-by-room sheet is the entry ticket