Most residential jobs are still sold and installed as if HVAC design were just picking a tonnage and running some duct. Building codes, energy programs, and homeowner expectations say otherwise.
The core of proper residential HVAC design is the trio:
- Manual J – residential heating and cooling load calculations
- Manual S – HVAC equipment selection
- Manual D – duct design for forced-air systems
Used together, they give you a defensible, repeatable HVAC design workflow that beats rules of thumb on comfort, efficiency, and code compliance.
What Manual J does: the load calculation foundation
Manual J answers one question:
How many BTU/hr of heating and cooling does this specific building need at its design conditions?
A proper Manual J load calculation (done with reliable HVAC load calculation software or an online Manual J calculator) takes into account:
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Location and design temperatures
- Winter outdoor design temperature
- Summer outdoor design temperature
- Indoor heating and cooling setpoints
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Building envelope
- Wall construction and R-values
- Roof/ceiling construction and R-values
- Floor and foundation types (slab, crawlspace, basement)
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Windows and doors
- Area by orientation (north, south, east, west)
- U-factor and SHGC
- Shading, overhangs, and glass type
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Infiltration and ventilation
- Natural air leakage (or blower door–based infiltration)
- Mechanical ventilation (HRV/ERV, exhaust)
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Internal gains
- Occupants
- Lighting and appliances
The result is not just one number. A good Manual J calculator gives you:
- Room-by-room heating loads (BTU/hr)
- Room-by-room cooling loads (sensible and latent BTU/hr)
- System-level heating and cooling loads
This is the starting point. Everything in Manual S and Manual D should trace back to these Manual J loads.
What Manual S does: equipment selection based on Manual J
Once Manual J tells you how much capacity the building needs, Manual S explains how to choose HVAC equipment that actually matches those loads.
Manual S deals with:
- Matching equipment capacity to Manual J design loads
- Using manufacturer performance data, not just nominal tonnage
- Considering sensible vs latent cooling capacity
- Accounting for fan speed, airflow, and staging/modulation
Key Manual S concepts when using your HVAC load calculation software outputs:
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Total cooling capacity vs Manual J cooling load
- Capacity at design outdoor temperature should be close to the Manual J total cooling load, not dramatically higher.
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Sensible capacity vs Manual J sensible load
- Especially important in dry climates or homes with high internal gains.
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Latent capacity vs Manual J latent load
- Critical in hot-humid climates where humidity control is as important as temperature.
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Heating capacity vs Manual J heating load
- For furnaces: output BTU/hr at design conditions
- For heat pumps: capacity from manufacturer tables at winter design temperature
Instead of picking “a 3-ton system” because the house is “about 1,500 ft²,” Manual S has you:
- Start with Manual J design loads (from your online Manual J calculator).
- Look up manufacturer performance tables.
- Select equipment whose capacities track the Manual J loads at design conditions.
Manual S is where you prevent systematic oversizing or undersizing of HVAC equipment.
What Manual D does: duct design that delivers Manual J loads
Manual D answers:
How do we move the right amount of air to each room to satisfy the Manual J loads?
Manual D uses the room-by-room Manual J outputs to:
- Convert BTU/hr per room into required CFM per room
- Size ducts and branches to deliver those CFMs with acceptable pressure drops
- Select registers and grilles that work at realistic velocities
Without Manual D duct design, even a perfect Manual J and perfect Manual S equipment selection can fail in practice. Common duct issues when Manual D is ignored:
- Undersized ducts to distant rooms → hot/cold rooms
- Oversized supplies in small rooms → noise and drafts
- Excessive static pressure → poor airflow and reduced capacity
A realistic workflow:
- Take room-by-room Manual J loads from your HVAC load calculation software.
- Convert each room’s load to CFM (using sensible heat formulas and chosen ∆T).
- Use Manual D principles to design the trunk and branches around those CFMs.
Many modern design tools help with CFM calculations and duct sizing once you’ve entered the Manual J data.
How Manual J, S, and D actually connect on a real job
A complete residential HVAC design process should look like this:
Step 1: Manual J – calculate loads
- Model the home in an online Manual J calculator or dedicated software.
- Use real design temperatures, envelope data, window specs, and infiltration assumptions.
- Generate room-by-room heating and cooling loads and system totals.
- Produce a Manual J report for your files and for AHJs if required.
Step 2: Manual S – select equipment
- Use Manual J design loads as the target, not the building’s square footage.
- Compare those loads to manufacturer performance data (not just nominal ratings).
- Choose equipment whose:
- Heating capacity at winter design temp covers the Manual J heating load
- Cooling capacity at summer design temp matches the Manual J cooling load
- Sensible/latent split lines up with the Manual J sensible and latent loads
Document the selection so you can show exactly how Manual J (load) and Manual S (equipment) align.
Step 3: Manual D – design ducts
- Convert room-by-room Manual J loads to room-by-room CFMs.
- Use Manual D to:
- Size supply and return ducts
- Design trunk and branch layouts
- Select diffusers and registers that work at realistic flows and velocities
- Verify that the total airflow and static pressure profile are compatible with the selected equipment.
By the end, you should be able to trace a line from:
- Room load → room CFM → branch duct size → register → equipment selection
instead of “we ran 8-inch flex to all the bedrooms because that’s what we had on the truck.”
What happens when you skip one of the manuals
Skipping any piece of the Manual J / Manual S / Manual D chain has predictable consequences.
Skipping Manual J
- No real load calculation, just tonnage guesses.
- Equipment is sized based on square footage or habit.
- Manual S and Manual D (if done at all) are built on fake loads.
Result: oversized or undersized systems, poor comfort, more callbacks.
Skipping Manual S
- You run Manual J, but then ignore it when you choose equipment.
- You pick “the next size up” instead of matching capacity to load.
Result: the Manual J report becomes meaningless paperwork; equipment size and performance are still rule-of-thumb.
Skipping Manual D
- You have solid load calculations and reasonable equipment selection, but duct design is guesswork.
- Rooms with higher loads may get the same duct size as rooms with low loads.
- Static pressure and airflow are not verified.
Result: room-by-room comfort is inconsistent even if the Manual J and Manual S work were correct.
Where software fits: turning the three manuals into a usable workflow
Running all three manuals entirely by hand is possible but slow and error-prone. That’s why most professionals rely on:
- Online Manual J calculators for loads and reports
- HVAC load calculation software that integrates loads, equipment data, and duct design helpers