Running your first load calculation feels intimidating because contractors assume every input must be perfect.
That’s not true.
A good Manual J is built in layers. The mistake most first-time users make is trying to model the house with microscopic precision before they understand which inputs actually move the needle.
This guide walks through how to run your first load in Load Calc Guru without getting stuck, while still producing a defensible result.
Step 1: Start With Geometry, Not Materials
Before worrying about insulation R-values or window SHGC, get the shape of the house right.
Scroll down to the Rooms card and add your rooms.
- Enter the length, width, and ceiling height.
- The software automatically creates 4 walls (N, S, E, W) for each room.
- If a wall is facing another room (interior), open the Room Editor, go to the Walls tab, and click "Convert to Interior Wall".
Why this matters: Area drives load more than almost anything else. If your square footage or volumes are wrong, no amount of insulation tweaking will save the result.
Step 2: Set Your Material Defaults First
New users often skip the Material Defaults section at the top of the project page.
Don’t.
Before you start adding rooms, expand the Material Defaults section.
- Select your typical Ceiling (e.g., "R-38 Blown Attic").
- Select your typical Wall (e.g., "R-13 Frame Wall").
- Select your typical Window (e.g., "Double Pane Low-E").
Any room you create after setting these will automatically inherit them. This saves you from editing every single surface manually.
Step 3: Let the Location Set the Design Conditions
Design conditions matter—but only if you override them intentionally.
When you create a project:
- Grant location permissions if asked. Load Calc Guru will automatically pull the nearest ASHRAE weather data for your exact coordinates.
- Check the Design Conditions card. You’ll see the Outdoor Summer DB (Dry Bulb) and Winter DB automatically filled in.
Unless you have a specific reason (client comfort expectations, aggressive energy goals, or inspector requirements), leave them alone on the first pass.
Step 4: Add Windows to Specific Walls
If there’s one place to slow down, it’s windows. But you don’t add them in a vacuum.
In Load Calc Guru, windows live on walls.
- Open a Room.
- Click the Walls tab.
- Find the exterior wall where the window is located.
- Click "Add Default Window" (uses the assumption from Step 2) or "Add Unique Window" if it’s a sliding glass door or specific feature.
Focus on:
- Correct size
- Correct orientation (which is handled by the wall it's on)
- Reasonable SHGC (don’t assume “low-E” means low SHGC)
Step 5: Ignore Manual S—for Now
This sounds backwards, but it’s deliberate.
Your first goal is to answer:
“Does this load make sense?”
Scroll past the Result Details to the Result Summary.
If the total BTUs feel wildly off compared to experience, stop and fix inputs. Don’t scroll down to the Manual S section yet. Validate the load first.
Step 6: Look at Sensible vs Latent
Many first-time users only look at total cooling in the Result Summary.
That’s a mistake.
Load Calc Guru breaks cooling into:
- Sensible load (temperature)
- Latent load (moisture)
If latent is unusually high:
- Check your Infiltration settings in the Building Data card.
- Check if you accidentally put a window on a West wall that should be North.
This step alone catches a surprising number of bad assumptions.
Step 7: Export the Report and Read It Like an Inspector
Before sending it to anyone, read your own report.
Ask:
- Can I trace where the numbers came from?
- Are design conditions obvious?
- Are assumptions defensible?
If you can follow it, an inspector can too.
The Point of the First Load Isn’t Perfection
It’s orientation.
Your first Manual J should:
- Be complete
- Be reasonable
- Be explainable
Load Calc Guru is designed so you can refine inputs incrementally without breaking the model. That’s how real projects work.
Get the first load done. Then improve it. That’s how confidence—and accuracy—actually build.