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Heating and air · Replacing a system

HVAC load calculation and sizing

A room-by-room Manual J before anyone names a tonnage. Bigger is not better, and it is the mistake that gets made most.

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One flat fee to find out what is wrong. You are told it on the phone before anyone is dispatched, and it comes off the job if you hire us. Read our Plain Terms

What the work is

A load calculation works out how much heat your house actually gains and loses: its orientation, its glass, its insulation, its air leakage, its ceiling heights, room by room. It is the difference between sizing a system for your house and sizing it for its square footage.

Oversizing feels like the safe choice and is not. A system too large for the load cools the air quickly, satisfies the thermostat, and shuts off before it has removed any humidity. You get a house that is cold and clammy, equipment that short cycles, and a compressor that wears out early.

You can have the calculation whether or not you buy the system from us, and you can take it to anyone else.

What goes into it: the orientation of the house, the area and type of every window, the insulation in the walls and the ceiling, the infiltration rate, the ceiling heights, the shading, and the number of people. The output is a heat gain and a heat loss figure for every room and for the house as a whole.

The room-by-room part is what makes it worth having. A whole-house figure sizes the equipment; the room figures tell you which rooms the ducts are failing to serve, and that is usually the actual complaint that started the conversation.

What is included

  • A measured survey of the house, not a rule of thumb
  • The room-by-room result in writing, and yours to keep
  • What it means for the equipment size, in a sentence
  • Window areas and orientation measured, not estimated from a floor plan

What is not

  • Sizing from the label on the old unit, which is how the old one got its size
  • Sizing from square footage alone

Why the number matters more here

St. George recorded 98 days at or above 100°F in 2024, the most on record, and the all-time high is 117°F. A system sized for a mild place is not sized for this one.

Design load is worked out against the temperature the house actually sees, and here that is a long way from the national default. It is also why a rule of thumb carried in from somewhere else lands wrong in both directions.

What to ask anyone quoting this, us included

  • Was a Manual J done, and can I have a copy?
  • What size did it come out at, and what size are you quoting?
  • If those differ, why?
  • Is this room-by-room, or whole-house only?
  • What did you assume about my insulation, and did you check?

What we can show you in writing

Every line traces to a document we read in full. The number after a sentence points to the research below.

  • Manual J is ANSI/ACCA 2 Manual J - 2016, eighth edition — the ANSI-recognized national standard for residential load calculation. It is somebody else's standard, published and named, not a method we invented.1
  • A load calculation takes its outdoor design conditions either from Manual J's Table 1A and 1B or from ASHRAE weather data, and section 18-7 of the standard says the two sets cannot be mixed. Manual J is a paid ANSI standard, so that is one of the few lines on this site you cannot check for free — the section number is here so you can look it up if you buy it.1
  • Utah has adopted the 2021 International Residential Code and the 2021 International Mechanical Code, under Utah Code § 15A-2-103.1
  • The usual sales pitch says an oversized air conditioner wastes energy by short-cycling. NREL found otherwise: the penalty comes from off-cycle parasitic power in components like controls and crankcase heaters, and where that draw is small, oversizing barely affects energy use. Size it for comfort. That was a simulation of a 1960s-vintage home in Houston, Texas, not a field measurement and not this climate.1

Where this comes from

Every line above traces to a document we read in full. The number after a sentence points to the research below; hover it for the section. If you find a line wrong, tell us and we will fix it and say so.

  1. Heating and air research, September 2026 Compiled from the Code of Federal Regulations at 10 CFR 430 and the Federal Register, EPA rules on refrigerant transition, the Utah Code and Utah DOPL's licensing records, ACCA's load calculation standard, ASHRAE and the AHRI directory, Pacific Northwest and Lawrence Berkeley National Laboratory publications, NOAA climate normals, ENERGY STAR, and the published data of Trane, Lennox and Daikin. 3 sections cited. Hover a number above for the section.

The rest of replacing a system

The quote is the number. There is no second number.

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