TruFlo guide
HVAC sizing and Manual J
A practical guide to the residential load calculation that should inform HVAC equipment sizing.
What Manual J does
Manual J is ACCA's residential load calculation procedure. It establishes the heating and cooling loads a home presents so equipment sizing can begin with the building rather than a shortcut.
It uses defined building and design inputs to calculate room and whole-home heating and cooling loads. The linked local planning material shows why those inputs must be evaluated at the address instead of copied into a general rule.
Sizing needs the whole system
A load result is part of a complete design, not the equipment selection by itself. A replacement proposal should identify the calculation used and explain how the proposed system follows from it.
For the address-specific design and distribution questions that come next, use the existing Friendswood and Pasadena planning material linked below. This guide does not duplicate those local passages.
TruFlo repair or replace rules
These are TruFlo rules for discussing an AC system. They are not industry statistics. A technician confirms the condition on site.
| Lean | TruFlo rule |
|---|---|
| Rule of thumb | As a rule of thumb, replacement starts to make sense when a repair would cost more than 50% of a replacement, and a technician confirms that on site. |
| depends | Under about 10 years, a repair is usually the lean, unless the system has a refrigerant leak in a spot that cannot be repaired. |
| depends | Between about 10 and 15 years it depends on the refrigerant, the remaining parts warranty, the repair history and the repair cost against replacement. |
| replace | Over about 15 years the lean is replacement, unless the system is still under parts warranty, has not needed repeat repairs, and the repair is small next to a new system. |
| replace | A system still on R-22 leans replacement, because R-22 is being phased out, is getting more expensive, and matching equipment is getting harder to find. |
| repair | A system still under parts warranty leans repair. |
| replace | Two or three repairs in the last two years leans replacement. |
| replace | A home that will not come down under 78 degrees, after a technician has confirmed the system is working properly and the house is well insulated and sealed, leans replacement. |
| depends | Ductwork generally lasts about 15 to 20 years, and a technician confirms its condition on site. |
Sources
- ACCA, Manual J Residential Load Calculation (fetched 2026-09-18)
- TruFlo A/C & Heating FAQ (fetched 2026-09-18)