Estimated reading time: 2 minutes
HVAC sizing methods got a shake-up when Allison Bailes wrote “You Don’t Need a Load Calculation” over at Energy Vanguard, pitching runtime data as a rule of thumb over Manual J numbers for replacement systems in existing homes.
He’s onto something—why fuss with theoretical calcs when a 3-ton AC chugging 20 minutes for an hour on a 95°F day yells “oversized” faster than any chart data you could conjure? I’m all in for real-world proof trumping book smarts, especially when contractors slap in 4-ton cooling systems like a status symbol!
But here’s the Mississippi catch: runtime’s only gold if your system’s not a junkyard reject. Allison’s method assumes peak performance, and down here—where humidity’s a swamp beast and old homes leak like sieves—the numbers may struggle a bit.
I’ve seen A/C units grind “all the time” on a 94°F scorcher not because HVAC sizing’s spot-on, but because they’re toast: refrigerant leaks, coils packed with crud, compressors gasping. I’ll ask, “Does it keep you cool at full tilt?” I might get a nod from someone stewing at 76°F. Rather than proof of a sizing genius—it’s a breakdown disguised as a working cooling system.
My fix? Before timing runs, let’s play mechanic.
Swap that filthy filter. Scrub those coils.
Check refrigerant and feel the compressor’s pulse.
Allison’s right—Manual J’s a slog, and most A/C techs skip it. The runtime hack is pretty slick, but it flops without street smarts. Pair real data with a real system and HVAC sizing sings. Skip the maintenance grunt work, and you’re guessing with a ghost.
My take on the Energy Vanguard article: Love the option, Mr. Allison—let’s tune the engine before we clock it on the straightaway.
Jump to his excellent article: You Don’t Need a Load Calculation