Ask a Modeler: What Questions Do You Ask When Doing Sanity Checks on a Model?

What questions do you ask when doing sanity checks on a model?
– Trying to keep my sanity

Dear Sanity,

Where did you go? Sanity seems to be out the window these days. But if you feel like you are losing it while working from home, at least your modeling results don’t have to be a contributor.

Whole Building Results

The trifecta of metrics are the energy use index (EUI), energy cost index (ECI), and unit price of fuel — between these three values it will be possible to see if the building modeling project is landing in an expected range of energy use against peer databases, spending a reasonable amount of money for energy cost, and paying a reasonable price. EUI and unit price for US projects can be easily compared to public data from DOE, EPA, EIA to see if the aggregate results are in the expected ranges. Having all three values allows a cross-check.

Additional questions may relate to results as simply as “did the use go down?” or up as expected by making a change to the model. Look for first principle reasons to support the results such as an increase in reheat — there is a plausible reason for an increase in that end-use if a lighting retrofit takes place resulting in a decrease for lighting and cooling energy, while heating energy may go up during times of year when lighting energy contributes to reheat. 

Annual or daily results may also be analyzed to show reasonable patterns including plots of hourly results to temperature or occupancy.

Energy Charting and Metrics Tool (ECAM+) can be a useful third-party application to visualize the modeling results and verify the expected patterns are evident.

Primary Systems

Boilers and chillers can be checked for their efficiencies throughout the year and annually — again do these make sense? Patterns that evoke your favorite abstract painter may not be responding to the environmental drivers in a way that physics intended. Comparison to energy code or expected rules of thumb based on vintage of machine can be helpful to identify transcription errors entering the COP’s or identify performance curves that aren’t accurate in some way. 

Gains and Loads

When all of the above sanity checks make sense, then you know the model is accurate! Or at least you have offsetting errors instead of compounding errors — that’s progress, right? 

I’ve found the ASHRAE Pocket Guide to be a helpful resource for rules of thumb on heating and cooling gains, occupancy, and as a cross-check on equipment sizing. The figures are a little out of date such that many of them may be used at the lower / more efficient range of values or may be valid at the higher levels for older facilities undergoing a retrofit.

Equivalent Full Load Hours (ELFH) can also be a reasonable sanity check for equipment to verify that it is scheduled or modulating over the course of a year.

Hopefully these ideas provide some checks to maintain sanity.

David Eldridge, P.E., BEMP, BEAP, HBDP, GGA
Energy Efficiency Ninja, Grumman/Butkus Associates

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