If you've pulled a residential HVAC permit in Texas, you've probably seen a reviewer ask for a "Manual J" — and maybe a Manual S or D alongside it. They're a related family of calculations published by ACCA (the Air Conditioning Contractors of America), and each one answers a different question. Here's the whole set in plain English.
The one-line version
Manual J figures out how much heating and cooling the home needs. Manual S confirms the equipment you picked matches that. Manual D sizes the ducts to deliver it. Manual T picks the registers and grilles. They run in that order, and each one depends on the one before it.
| Calc | Answers the question | Output |
|---|---|---|
| Manual J | How much heating and cooling does this house need? | The load, in BTUs (room by room) |
| Manual S | Does the equipment I selected actually match that load? | Verified equipment selection by model |
| Manual D | How do I size the ducts to deliver that capacity to each room? | Duct sizes and layout |
| Manual T | Which registers and grilles move the air correctly? | Register/grille selection |
Manual J — the load
This is the foundation. A Manual J load calculation measures a home's heat loss (winter) and heat gain (summer) from the things that actually drive it — square footage, insulation, windows, air leakage, orientation, and your local design temperatures. The result is the load in BTUs, and it's what every other calc is built on. If you want the full breakdown, see What Is a Manual J Load Calculation?
Manual S — equipment selection
A Manual J tells you the load; Manual S proves the equipment you chose fits it. It takes a specific make and model and checks its rated capacity at your local design conditions against the Manual J numbers — so you don't end up with a 4-ton unit on a 2.5-ton house. Oversized equipment short-cycles, wastes energy, and leaves the air clammy; Manual S is the guardrail against it.
Manual D — duct design
Right-sized equipment still underperforms if the ducts can't carry the air. Manual D sizes the duct system — trunk and branch runs, based on the equipment's airflow and the home's layout — so each room gets the capacity Manual J said it needs. It's most often requested on new construction and full system replacements.
Manual T — registers and grilles
Manual T is the last step: selecting the supply registers and return grilles that move the designed airflow into each room comfortably and quietly. It's requested less often than J, S, and D, but it completes the chain.
Which ones does my permit need?
It depends on your jurisdiction and your scope of work:
- Most Texas cities require Manual J for HVAC permits — codes require equipment to be sized by an approved method.
- Manual S and D are commonly added on new construction and full changeouts.
- Manual T is requested in a minority of cases.
If a permit already got kicked back, the reason is usually a missing or rule-of-thumb Manual J — here's why HVAC permits get rejected and how to fix it.
Why the order matters
Because each calc feeds the next, an error early on cascades. A Manual J that's off by a ton makes the Manual S "pass" the wrong equipment, which makes the Manual D size ducts for the wrong airflow. That's why the load calculation has to be done properly the first time — not estimated per square foot.
Need J, S, and D done together?
Cinch MEP is a TACLA-licensed Texas HVAC contractor (license # 116156C) and provides TACLA-stamped residential Manual J, S, and D as a package — delivered as a permit-ready PDF. Standard turnaround is 3–5 business days, with same-day rush before 10 AM CT.
Frequently asked questions
What is the difference between Manual J and Manual S?
Manual J calculates the load (the BTUs the home needs); Manual S confirms the specific equipment you selected matches that load at your local design conditions. J is the requirement; S is the proof the equipment fits.
Do I need all four — J, S, D, and T?
Not always. Manual J is required in most Texas jurisdictions; S and D are commonly added on new construction and full changeouts; T is requested less often. It depends on your city and scope.
What order are the calculations done in?
J first (the load), then S (equipment that matches it), then D (ducts to deliver it), then T (registers and grilles). Each depends on the one before.
Can one company do all of them?
Yes — Cinch MEP provides TACLA-stamped residential Manual J, S, and D as a package, delivered as a permit-ready PDF.
What software is used?
ACCA-approved software following the published ACCA procedures — what jurisdictions recognize as an approved sizing method. A rule-of-thumb or per-square-foot estimate is not an acceptable substitute.