Calibration Primer

[Gamaliel Lodge]

When a model fails to calibrate, it’s time to look for errors and revisit assumptions. Although calibration helps to ensure accuracy of savings estimates, you never want to go to extreme lengths to calibrate a model. Don’t modify inputs beyond what could reasonably represent reality. Some models just won’t calibrate for a variety of reasons from abnormal billing records to unusual usage patterns to physical circumstances in the home that can’t be observed or modeled. With that in mind here some things to consider when reviewing a model that isn’t well calibrated

Utility bills – errors can lead to poor estimates of weather normalized usage for all end uses

1. Double check fuel energy units
2. Check for other data entry errors (e.g. dates usages offset by 1 month, missing digits)
3. Remove any bills with abnormal usage by checking the Omit box
4. Make sure there is adequate and accurate representation of base load bills
   • Base load usage is not constant. DHW and lighting loads have pronounced winter peaks. If you are using high/low bill entry it may improve results to pick an average base load bill rather than the absolute lowest.
   • Incorrect base load representation will impact heating and cooling load estimates.

Heating loads – check these inputs for poor heating calibration

1. Check for error/double-counting of envelope areas
   • Floors above unconditioned basements and crawlspaces should not be listed separately as frame floors
   • Attics should only be used to describe boundaries between conditioned space and unconditioned attic buffer space
   • Vaults should only be used to describe roofs over conditioned space
   • Default wall area calculations should be reviewed for split level houses, houses with finished attic space, and other unusual constructions
   • Use/expand thermostat temperature ranges
     • Thermostat set-point ranges can be used to account for variation in how the homeowner uses the thermostat as well as inconsistency in how the home is conditioned. Duct leakage and registers that are closed or blocked by furniture may cause some rooms to be only partially conditioned, reducing loads.
     • Use/expand system efficiency ranges
     • Double check system sizing. Be sure you have entered sizing in the correct units.
     • Review conditioning selections for crawl/basement
       • On the detailed screen there are editable assumptions for the basement average temperature, which is often lower than the upper floors. If you edit these temperatures, be sure to also consider how the temperature will change with any added insulation on the improved side.
       • Be sure that duct location specification accurately represents total distribution of ducts.
         • Note that for multi-story houses a selection of Attic, Basement, Crawlspace, or Slab automatically places 35% of supply ducts in conditioned space.

Cooling – check these inputs for poor cooling calibration

1. Check for error/double-counting of envelope areas
   • Floors above unconditioned basements and crawlspaces should not be listed separately as frame floors
   • Attics should only be used to describe boundaries between conditioned space and unconditioned attic buffer space
   • Vaults should only be used to describe roofs over conditioned space
   • Default wall areas calculations should be reviewed for split level houses, houses with finished attic space, and other unusual constructions
   • Use/expand thermostat temperature ranges
     • Thermostat set-point ranges can be used to account for variation in how the homeowner uses the thermostat as well as inconsistency in how the home is conditioned. Duct leakage and registers that are closed or blocked by furniture may cause some rooms to be only partially conditioned, reducing loads.
     • Use/expand system efficiency ranges
     • Double check system sizing. Be sure you have entered sizing in the correct units.
     • Check window area, treatments (esp. bug screens), and overhangs
       • Bug screens, overhangs, and other window treatments can significantly reduce cooling loads.
       • Be sure to specify roof/wall colors that are very light or reflective.
       • Be sure that duct location specification accurately represents total distribution of ducts.
         • Note that for multi-story houses a selection of Attic, Basement, Crawlspace, or Slab automatically places 35% of supply ducts in conditioned space.

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