Manual J has been the industry baseline for decades. Manual S is where designs either hold up—or fall apart under inspection.
As of today, Manual S is fully released in Load Calc Guru as a first-class feature, not a bolt-on and not a checkbox export. That matters, because Manual S is where real-world equipment performance is verified against the load you just calculated.
This post explains what Manual S actually requires, what Load Calc Guru now does automatically, and why this changes how you should be submitting HVAC designs.
Why Manual S Is Where Projects Get Rejected
Most failed HVAC submittals don’t fail because the Manual J math is wrong.
They fail because:
- The selected equipment cannot meet the sensible load
- The unit’s capacity collapses at design temperature
- SHR isn’t verified
- Auxiliary heat is guessed instead of sized
- Oversizing limits are violated
Inspectors increasingly know this. They are no longer satisfied with “3 tons should be fine.”
Manual S exists to eliminate that guesswork.
What Manual S Actually Requires (Not the Shortcut Version)
A compliant Manual S selection must answer all of the following:
- Can this specific model number meet the total and sensible loads?
- Can it do so at the actual design temperatures, not AHRI defaults?
- Does it stay within ACCA oversizing limits?
- For heat pumps, what happens below 47°F?
- Is auxiliary heat sized for the maximum deficit, not nameplate tonnage?
If any of those are unanswered, the selection is incomplete.
What Load Calc Guru’s Manual S Does (Exactly)
Manual S in Load Calc Guru is not a static lookup or a marketing-style “match.”
It performs verification.
1. Extended Performance Interpolation
Equipment capacity changes non-linearly with:
- Outdoor Dry Bulb
- Entering Wet Bulb
- Altitude
Load Calc Guru interpolates OEM extended performance tables to calculate actual capacity at your project’s conditions, not rounded table values.
This avoids the most common failure mode: assuming rated capacity equals delivered capacity.
2. Sensible vs Latent Verification (SHR Matching)
Manual J produces two cooling loads. Manual S must verify both.
Load Calc Guru:
- Calculates building-required SHR
- Calculates equipment SHR at design conditions
- Flags selections that fail to meet sensible demand even if total BTUs look sufficient
This is critical in both dry climates (over-latent equipment) and humid climates (under-dehumidification).
3. Strict ACCA Sizing Limits (Tables N2-1 / N2-2)
Oversizing is not subjective in Manual S.
Load Calc Guru enforces:
- 115% max for single-speed equipment
- 130% max for variable-speed equipment (where allowed)
Selections outside these limits are explicitly marked FAIL, not quietly allowed.
4. Low-Ambient Heating Reality (Heat Pumps)
Brochure ratings at 47°F are meaningless if your design temperature is 5°F.
Load Calc Guru:
- Evaluates heating capacity at design temperature
- Applies conservative penalties when OEM data is sparse
- Calculates the maximum heating deficit
- Sizes auxiliary heat to cover the gap—no guessing
This prevents strip heat undersizing and cold-morning callbacks.
5. Deterministic, Inspector-Traceable Results
Every Manual S result in Load Calc Guru is:
- Deterministic (same inputs → same output)
- Traceable back to published data
- Printed alongside the Manual J load it is verifying
Inspectors can see why a selection passes, not just that it does.
What Changed for You as a Contractor
With Manual S now fully integrated:
- You no longer submit “Manual J + equipment guess”
- You submit Manual J + verified Manual S
- Equipment failures are caught before install
- Inspectors see math, not assumptions
This reduces:
- Rejections
- Redesigns
- Callbacks
- Liability exposure
Manual J Defines the Problem. Manual S Proves the Solution.
Manual J without Manual S leaves a gap between theory and reality.
Load Calc Guru now closes that gap.
If you want to see exactly how these calculations are performed—including interpolation methods, assumptions, and reporting—review our full Calculation Methodology.
Manual S isn’t optional anymore.
Now, it isn’t manual either.