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Load analysis discrepancies

Andruid_2 Member Posts: 42
I recently had a job go sideways. I installed an air-to-water heat pump with fan coils for heating and cooling. I used LoopCAD to determine the loads. The fan coil rep helped me size the fan coils. During the summer, the system could not cool the home sufficiently (the homeowner wasn't expecting more than what I designed for). I hired an energy consultant to analyze the home and do a third-party load analysis (using Cool Calc). The homeowner hired an engineering firm to do a full design of the system (load analysis using Wrightsoft).

The energy consultant found the unfinished basement to be a source of excessive infiltration, which is one reason the system couldn't keep up. The engineering firm designed a hydronic system very similar to what I already built, with the exception of fan coil sizes. Both the energy consultant and the engineering firm calculated larger loads than I did. The energy consultant was approximately 15% higher and the engineering firm is approximately 50% higher.

It seems to me, numbers are numbers and formulas are formulas. We all used the same basic design criteria (outdoor design and indoor setpoints). I would like to believe we all used good data in the design (window sizing, infiltration rate, insulation r-values). I was a little skeptical when LoopCAD forced me to use the Canada Building Code instead, because I wanted a cooling load. But, realistically that shouldn't matter much.

...which brings me to the question: Are there reasons to avoid certain load analysis programs?


  • GGross
    GGross Member Posts: 412
    I use loopcad constantly without any issue. You have to be thorough and put the correct info into the software. loopcad has never forced me to use a specific code.

    Most likely what happened is that you sized the cooling for the building, but not what is inside that building. Large homes come with lots of lights, TV's, computers, people, refrigerators, various other equipment which all creates heat inside the home, these need to be input individually, I do it room by room. The energy consultant is probably right on the money, and the engineer probably added some load for fudge factor.
  • Jamie Hall
    Jamie Hall Member Posts: 20,524
    I think you will find that any of those programs will give decent results -- but that none of them, except by accident -- will be completely "on the money". You mention that "numbers are numbers and formulas are formulas" and, for some engineering computations, this is quite true -- indeed, in a sense, it is always true. But... there are too many of them, and most of the numbers are actually very poorly known. Therefore, various programs -- never mind various people using the programs -- will come up with different results. Sometimes very different.

    I might add, speaking here as an engineer of many years, an engineer will almost always add what you refer to as a "fudge" factor. There is a reason for this. It is very unusual for an over-designed structure or system to fail. Engineers don't like failures. It's bad for the reputation, and can be very expensive indeed.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • ScottSecor
    ScottSecor Member Posts: 644
    Keep in mind that some of the factors are often educated guesses. For example, do you know for sure what the R value (U value, etc.) of walls, floors and ceilings actually are? Do you know what type of indoor lights will be left on during peak demand (led lights give off little heat, incandescent and old fluorescent much more). Just how good are those windows, for example all replacement windows are not necessarily better than those original single pane windows with storms. Is this a large family that often opens and closes the two car garage door every hour on the hour, or is this a single person that rarely leaves the house?