Rescheck, Comcheck, and Manual J Reports
What Is a Manual S?
A Manual S takes the loads from your Manual J and proves that the specific piece of equipment you picked actually delivers those loads at your design conditions. It's ACCA's Residential Equipment Selection standard, and a lot of building departments now want it in the packet alongside the load calculation.
Short version of why it exists: the number on the box is not what the unit produces in your house on the hottest day of the year.
The Thing Nobody Tells You About Tonnage
A 3-ton air conditioner is not a machine that makes 36,000 BTU per hour. It's a machine that made roughly 36,000 BTU per hour on a test stand, under one specific set of conditions, so it could get a label.
Those conditions are 95 degrees outdoor, with 80-degree air at 67 wet bulb coming across the coil. That's the AHRI rating point. It's a benchmark for comparing units against each other, and that's all it is.
Push the outdoor temperature to 105 and capacity drops. Run 75-degree return air across the coil instead of 80 and capacity drops again. Change the airflow and it moves. The same nominal 3-ton unit might give you 34,000 in one house and 30,000 in another, and neither of those is 36,000.
Manual S is the process of going to the manufacturer's expanded performance data — the real tables, not the nameplate — and finding what that unit actually does at your design conditions. Then checking that number against the load.
Run the Numbers on a Real House
Say your Manual J comes back like this:
-
Total cooling load: 33,000 BTU/h
-
Sensible portion: 26,000 BTU/h
-
Latent portion: 7,000 BTU/h
Contractor proposes a 3-ton unit. Nominal 36,000. Looks like plenty of room.
Now pull the expanded data for that specific model — the outdoor unit matched to that indoor coil, at your airflow. At a 95-degree outdoor design temp with 75-degree return air at 62 wet bulb, that combination gives you something like:
-
Total capacity: 34,200 BTU/h
-
Sensible capacity: 26,900 BTU/h
-
Latent capacity: 7,300 BTU/h
Check all three, not two. Total clears the 33,000. Sensible clears the 26,000. Latent clears the 7,000. You're at about 104% of total load, inside the sizing limits. That selection holds up.
Now move that same house to a 105-degree design temperature. Same equipment, same coil, same airflow. Capacity falls off:
-
Total capacity: roughly 30,800 BTU/h
-
Sensible capacity: roughly 25,300 BTU/h
-
Latent capacity: roughly 5,500 BTU/h
You're short on all three. On the hottest afternoon of the year, that "3-ton" unit can't hold the house, and the homeowner's going to be calling somebody in July.
Same box. Same label. Different answer. That's the entire point of Manual S.
One caveat on those numbers: they're illustrative of how expanded data behaves. Real capacity depends on the specific outdoor unit, the indoor coil it's matched to, the metering device, and the airflow you're actually moving. Two systems with the same nominal tonnage and different coils are different machines.
Sensible and Latent: Why the Total Isn't Enough
Every air conditioner does two jobs at once. Sensible cooling drops the air temperature. Latent cooling pulls water out of it. Add them together and you get total capacity.
Sensible heat ratio — SHR — is sensible capacity divided by total. Most residential equipment lands somewhere around 0.70 to 0.80.
Your house has a ratio too. In the example above, 26,000 sensible out of 33,000 total works out to about 0.79.
SHR is a useful shorthand for how the load splits between temperature and moisture. But it isn't the test. Manual S doesn't ask you to match the equipment's SHR to the house's SHR — it asks whether the equipment, at your design conditions and your airflow, actually covers the sensible load and the latent load.
Those are different questions, and the second one is the one that gets skipped. It's entirely possible to pick a unit with enough nominal tonnage, enough total capacity on paper, and still come up short on one side of the split. Go back to the first example — if I'd only checked total and sensible and never looked at latent, I could have signed off on a unit that couldn't keep up with the moisture in that house. That's the failure Manual S is built to catch. That's the same reason an oversized system leaves a house clammy at 72 — more on that on my heat load calculation page.
How Big Is Too Big?
Manual S doesn't just say "make sure it's enough." It caps the top end too, and that ceiling is the number plan reviewers actually enforce.
For conventional air conditioners: 115% of the Manual J total cooling load. Selected total capacity shouldn't exceed it. The equipment also has to satisfy the sensible and latent requirements at design conditions — the percentage is a ceiling, not a pass.
There's a floor in the guidance as well, generally cited in the 90% to 95% range, though it gets enforced far less consistently than the ceiling. Practically speaking, if you're under the load you'll find out in August whether the reviewer flagged it or not.
Heating equipment runs on different rules. Furnaces and boilers are commonly allowed somewhere between 100% and 140% of the design heating load. Gas heat behaves differently than compressor-based cooling, and the penalty for oversizing isn't as steep — though it's still real.
Heat pumps get their own treatment, since you're balancing two seasons with one machine. Allowances shift depending on climate, staging, and whether there's supplemental heat.
Variable-capacity equipment doesn't follow the conventional rule at all. Inverter-driven systems modulate across a range instead of running fixed-capacity, so Manual S has separate provisions for them. If somebody applies a flat 115% to a variable-speed system, they're using the wrong procedure. This comes up more every year as inverter equipment takes over the market.
All of these shift between Manual S editions and between what individual jurisdictions adopt and amend. Treat any percentage you read online — including the ones above — as the shape of the rule. The code edition your county adopted and what your plan reviewer expects are what decide the permit.
Do You Actually Need One?
Depends on where you're building, and the honest answer is that it's required more often than it used to be.
You'll need a Manual S, or equivalent equipment-selection documentation, if:
-
Your building department asks for equipment selection documentation with the permit
-
Your adopted code requires equipment sizing under ACCA Manual S or another approved method
-
You're pursuing ENERGY STAR Single-Family New Homes certification, where equipment selected per Manual S is part of the required HVAC design documentation
-
A plan reviewer kicked back your submittal asking how you arrived at the equipment
If you're not sure, call your permit office and ask whether they want Manual S along with the Manual J. Or send me the address and I'll find out — I deal with these offices every day and I'd rather make a phone call than have you buy something you don't need. The authority having jurisdiction makes the final call, always.
What I Need From You
Three things:
-
The Manual J. If I did it, I've already got it. If somebody else did it, send the report.
-
The equipment you're considering — make and model number, including the indoor coil or air handler. The match matters, not just the condenser. If you don't have one picked yet, tell me the fuel type and system type and I'll identify compatible options for you to review.
-
The project address, so the design conditions are right.
That's it. If you need both reports, order them together and I'll model the building once. The numbers stay consistent between the two, which matters more than people realize — a Manual S built on somebody else's Manual J is only as good as that Manual J was.
Cost and Turnaround
Manual S reports are $89.
Reports go out within hours of your order in most cases. I'm often out in the field running blower door tests and HERS ratings, so I'm not going to promise you a specific clock time. If something's going to take longer than usual, you'll hear from me instead of wondering. One revision is included if the building department comes back with a correction request.
The Right Order: Manual J, S, Then D
These three get mixed up constantly, and they're sequential, not optional variations on each other.
-
Manual J figures out how much heating and cooling the house needs.
-
Manual S picks equipment that actually delivers those numbers at your design conditions.
-
Manual D sizes and lays out the ductwork so the right amount of air reaches each room.
J first, always. S depends on J's output. D depends on both — you can't size a duct without knowing the airflow, and you don't know the airflow until the equipment is selected.
Some jurisdictions want all three. Most want J, and increasingly J and S. Ask before you order, or ask me and I'll ask for you.
Who's Doing the Work?
I'm Brad Buzzell. RESNET-certified HERS rater, 25-plus years in residential energy compliance. Rescheck Review is a division of Certified Energy Raters, LLC, and we handle REScheck, Manual J, Manual S, and COMcheck reports for builders, contractors, architects, and homeowners in all 50 states.
Questions before you order, call (800) 671-1895 x702.
Common Questions
What's the difference between Manual J and Manual S?
Manual J calculates how much heating and cooling the house needs. Manual S proves that the specific equipment you selected actually delivers that capacity at your design conditions. J is the load, S is the equipment. You need J before you can do S.
Why doesn't a 3-ton unit make 36,000 BTU?
Because 36,000 is a nominal rating, not a guaranteed output. Conventional central equipment is rated at 95 degrees outdoor with 80-degree, 67 wet bulb air entering the coil. That's a lab benchmark for comparing units. Actual capacity shifts with outdoor temperature, entering air conditions, the indoor coil it's matched to, and airflow. Manual S uses the manufacturer's expanded data at your design conditions instead.
What is sensible heat ratio?
Sensible capacity divided by total capacity. It describes how an air conditioner splits its work between lowering temperature and removing moisture. Most residential equipment runs around 0.70 to 0.80, and your house has a ratio too. SHR is useful for understanding the load, but Manual S doesn't ask you to match the two ratios — it asks whether the equipment covers the sensible load and the latent load at design conditions.
How much bigger than the load can my equipment be?
For a conventional air conditioner, selected total cooling capacity is commonly capped at 115% of the Manual J total cooling load, and the equipment still has to meet the sensible and latent requirements at design conditions. Furnaces and boilers are commonly allowed 100% to 140% of the heating load. Heat pumps and variable-capacity systems follow different provisions. The applicable Manual S procedure and your local code decide it.
Does variable-speed equipment follow the same sizing rules?
No. Inverter-driven systems modulate across a capacity range rather than running fixed-capacity, and Manual S has separate provisions for them. Applying a flat 115% rule to a variable-capacity system is the wrong procedure.
Can you do a Manual S if someone else did my Manual J?
Yes. Send me the Manual J report and the equipment you're considering. The Manual S is only as good as the load calculation underneath it, so if the J looks off I'll tell you before I build on top of it.
Do I need a Manual S for a system replacement?
Depends on your jurisdiction. Some require equipment selection documentation on any permitted mechanical work, others only on new construction. Worth a call to the permit office before you order.