Review and analysis to verify model quality

Even a simple box model presents many opportunities for unintended inputs. An output review can help verify model quality and confirm that the model represents your intended inputs.

Review input summaries

Many simulation programs provide standard output reports that are useful for checking that model parameters match your intended inputs.

• Envelope - Check floor area, wall area, roof area and window area. Also verify envelope component performance is as expected: U-factor, SHGC, VLT.
• Zone loads - Check lighting power, receptacle load power, occupant density and floor area.
• Air handlers - Verify fan power and cooling and heating efficiency. (Verification of sizing discussed below)
• Chilled water and hot water systems - Check equipment efficiency and loop pressure loss.

Check for unmet cooling and heating load hours

Many simulation programs report the number of hours when the HVAC system is not able to meet space temperature setpoints or HVAC components cannot meet their assigned loads. A small number of hours, say 50 or less per year, is likely not a problem, but higher numbers indicate that the HVAC capacity may not be properly sized or that there is some other issue such as a problem with the inputs for HVAC control.

Compare outputs to typical benchmark values

If model results differ unexpectedly from benchmarks then it’s a good idea to spend some time investigating whether or not the difference is reasonable or is due to some inappropriate inputs. The most common benchmarks are annual whole building energy consumption per square foot, while some sources also provide energy consumption broken down by end use.

Review energy consumption end use breakdown

The fraction of total energy estimated for each end use, such as cooling, lighting or plug loads, will vary between buildings for many reasons. Therefore, there are no firm rules for judging whether or not end use fractions are reasonable. But even a quick review of end-use results can identify problems such as zero heating or cooling energy, which typically indicates a problem with the model.

Review seasonal patterns of end-use energy consumption

Plots of monthly energy consumption by end use are helpful for checking for unusual results, such as cooling in winter or heating in summer. While there may be valid reasons for these results, sometimes it indicates a problem with HVAC control inputs or some other issue.

Review HVAC system and component autosizing

Most of the HVAC system components in a simple box model will be automatically sized by the simulation program. Since sizing affects energy consumption, it is important to check the sizing results to make sure they are reasonable. Two useful approaches are to 1) compare the sizing results to rules of thumb and 2) compare the autosized capacity to the peak simulated load. Common rules of thumb can be applied to air flow (cfm/ft2), cooling capacity (ft2/ton) and heating capacity (Btu/hr-ft2). To compare sizing to simulated load, some simulation programs have standard output reports that show how many hours per year that, for example, a chiller operates in different part-load ranges. If the equipment never operates close to full capacity, that is an indication that it is oversized.

Review hourly or sub-hourly results

Most simulation programs can output hourly or sub-hourly values of energy consumption and many other results. Even a quick review of the hourly data can be useful to identify potential problems. A time series plot of hourly electricity consumption will show whether the systems are operating on the intended schedule and whether there is expected daily, weekly and seasonal variation. Plots of hourly chilled water and hot water load can help identify issues with HVAC system operation. An hourly report of outdoor ventilation air flow for an air handler can be used to verify that economizer cooling is being used and that desired ventilation rates are being used.