Different isn't automatically wrong. The size on the existing unit, or the one a rule of thumb gives you, is not a calculated load. Many houses have equipment bigger than the load needs. The checks below tell you whether the inputs describe this house. The result is an estimate calculated using Manual J methodology, not a certified ACCA Manual J calculation, so verify final equipment selection with a licensed HVAC professional.

Start here: which way is it off?

  • Too high? Look first for extra floor area (a garage or sunroom counted as conditioned space), too much glass, or a location hotter or colder than the job.
  • Too low? Look first for missing floor area, a form total that was replaced by a partial room list, default window area, or several units' worth of house being compared with one whole-house number.

Check 1: the design location

At the top of Calculation Results, the Design location line shows the city, cooling and heating design temperatures, indoor setpoints, climate zone and daily range the calculation used. Confirm they fit the job. The ZIP resolves to design data for a location, so if the city shown is far from the house or the climate is clearly different, tap the line and report it.

In Pro mode you can also edit the conditions in the Design Conditions card on the project screen. Results then label them as custom conditions.

Next, look at Indoor Design Temperatures on the project form. A Custom setpoint is a real input: a cooler summer setting or warmer winter setting raises the load.

Check 2: the floor area and the rooms

Floor area moves the result more visibly than almost anything else, and it's the easiest input to get wrong when rooms are involved.

  • Only conditioned space counts. Leave out garages, unfinished attics, crawlspaces and unheated sunrooms. Hallways, closets and finished basements count.
  • Check the Counted tag. On the Rooms screen, anything marked Counted adds to the floor area. Tap it on a space outside the heated and cooled area to leave it out without deleting it.
  • Watch partial room lists. If your rooms add up to less than the form, Use Room Total (Use Scanned Rooms Only after a scan) replaces the form's area with just those rooms. That's right only if every conditioned room is in the list. Choose Keep Form Area (Apply Ceiling Height after a scan) if you're still measuring.
  • Scanned rooms are a starting point. Tap a room's area or ceiling height to correct it.

Check 3: windows, insulation and the rest of the shell

  • Total Window Area and Window Type. Glass is often a large part of the cooling load. If nobody measured it, it's a guess. In Moderate and Pro, Windows by Orientation becomes the source of the total once filled in.
  • Insulation Grade and Wall Construction. Fill from house age sets typical values for the era, so correct it if the house has been upgraded. Pro uses your Ceiling and Wall R-Values directly.
  • Ceiling Height and Number of Stories. Both change the size of the shell.
  • Ductwork Location. In Optional Details. It can move the total, because the duct adjustment is applied to the load.
  • Foundation, Roof & Attic. In Moderate and Pro, confirm foundation type, roof color and attic type match the house.
  • Attached Unit. For condos and townhouses only. Declaring a ceiling or floor as bordering another conditioned unit lowers the load, and Results says so.

HVAC System Type isn't a big load driver. Its main job is deciding whether Results shows a furnace size or heat pump capacity. If a heat pump job shows a furnace, that's the input to change. Don't expect the load itself to move much, though the heating side can shift a little where duct losses are calculated.

Check 4: read the breakdown

Scroll down Results to Cooling Breakdown and Heating Breakdown. Each lists what makes up the load: Solar Gain, Window Conduction, Insulation, Occupants, Infiltration, Duct Adjustment and so on. Find the largest rows and ask whether they fit this house.

  • Solar Gain or Window Conduction looks large? Recheck window area, type and direction.
  • Infiltration looks large? Recheck Wind Shielding, and enter a Blower Door Test (ACH50) if you have one.
  • Duct Adjustment looks large? Recheck Ductwork Location.

Then change one input at a time and recalculate. Changing several at once hides which one mattered. Each recalculation increases the project's revision number, so you can tell which result you're reading.

Check 5: one unit or several?

AC Size and the furnace or heat pump figure are whole-house numbers, sized as if one system serves the entire home. If the house has two units, don't compare one existing unit with that figure, and don't expect the two units' sizes to add up to it. Version 1.37 can divide the load between systems. See Room & System Loads.

The Equipment Match card lists the standard sizes that fall within Manual S limits for the calculated load. Use it rather than the single size when you're comparing a quote.

Still looks wrong? Send it in

At the bottom of Results, tap Are these results accurate?. Fill in What looks wrong? and, if you can, what you expected and why. Your inputs and results go with the report, so you don't have to retype them, and I can re-run the exact project. Real bugs have been found this way.

Video walkthrough

The checks on a real job, narrated, 1 minute 12 seconds. Every screen is a real capture from the app, and it shows one input moving the result.

Prefer to read? The steps above cover the same ground. Open on YouTube for captions.

FAQ

Why is the result lower than the unit already in the house?

Existing equipment isn't a measurement of the load, and many systems are larger than the house needs. Check the inputs here first. If they describe the house correctly, compare the existing size with the range in the Equipment Match card.

Why did the load barely move when I changed the system type?

HVAC System Type mainly decides whether Results shows a furnace size or heat pump capacity. The house's load comes from its shell, climate and ducts, so the type has little effect on it.

Why don't Simple, Moderate and Pro give the same answer?

Each mode uses more of the house's real details than the one before. Moderate adds foundation, roof, attic and system type. Pro adds your actual R-values. Where Simple relies on typical assumptions, a more detailed mode can land somewhere else.

Can I change the design temperatures?

In the US, the Design Conditions card in Pro mode lets you override them, and Results labels them as custom. In any mode you can set custom indoor setpoints.