Tuning the Model

So from this example, we can see how important it is to make sure that the model matches the actual bills. "Tuning" the model is where the engineer spends most of his time. We have tried to set up procedures to simplify that job.

First let's go back to the example. When we first tried to set up the model (before tuning), here's what it looked like.

Note that our line does not match the points. We have to "tune" the model to match -- in this case we need to increase the amount of space heating in cold weather in order to bring up the left side of the U-shaped line.

In the spreadsheet we have added colored slider bars below the graph. These provide quick adjustments for some specific end uses. In this case, we adjust the red bar to increase space heating.

We see that heating has to be increased by a factor of about 2 in order to match the bills. This should make us think. You know this building is supposed to have a heat pump, and a heat pump is supposed to have a COP of about 2....Hmm. Could it be that the heat pump is not working? How could we find out?

This is an example of "hypothesis testing". The model lends it self to asking "what if" questions. And it directs you to focus on just what information is needed -- so you when you go to the site, you don't waste time collecting information that is not relevant.

(In this case, it turned out that the controls for the heat pump were not working. The heat pump was running on resistance strip heaters and had been doing so for years.)

Adjusting the slider bar is only temporary. Ideally, we would like to specify engineering parameters that give us the same change. This process takes some thought and some judgement. We have to think about what is causing the building to behave that way and how we can incorporate that information as an engineering parameter on the "Detailed Description" page. In this case, it seems clear that, even though the building is supposed to have a heat pump, we want to model it as using resistance heat. Here's what it looks like when we change heating system from heat pump to electric resistance.

Now we have a tuned model!

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