Finding balance temperature for solar benefit


I need your help finding balance temperature in given climate/building type.
Lately I got into topic of solar radiation benefit massing analysis - @chris posted a definition on that to hydrashare: Solar Raditation Benefit

I also read about that in prof. Reinharts “Daylightning Handbook I”. I do not have the book with me to cite it but it proposed modelling a simple office to find the balance temperature in given climate.

I followed these steps and modelled simple core+perimeter office and assigned glazing, materials/schedules, etc.

I found a component calcBalTemperature but have some questions about it:

  1. numDaysToAverage input - changing this input affects the results dramatically. I do not quite understand what it does - pick number of days to avarage their loads?

  2. How much weekend schedules affect the results? I have my model set up for ideal air loads and 20-24 celsius cooling and heating set points. All weekend days + hours after 8 pm are set to 0 - thus energy at these hours would be 0 as there are no setbacks. Wouldn’t this cause to component decide these are the balance hours (so for example winter nights when it -15 outside)?

  3. Is there a better way to do that?

I post my definition. I used *.epw for Kiev (Ukraine) if someone would be willing to take a look. The balance point I got was 15 degree (622.1 KB)


@Wujo ,

I know I am the author of the balance temperature component but I probably should have released it with some warnings that “balance temperature” is a steady state concept, which is a fundamentally different type of calculation than the transient calculation that EnergyPlus runs. It is primarily for this reason that the numDaysToAverage is necessary since, if you average it over too short of a period, the anomalies of individual hours of the energy simulation will give you false results, particularly because the instantaneous heating/cooling demand can be offset from the hour of a given outdoor temperature thanks to thermal lag. This means that I would not recommend going below the default value of 10 days, especially if the schedules are different between weekend/weekdays. Weekend schedules can have a significant affect on the balance temperature, which is why I recommend that the numDaysToAverage be longer than a week, allowing you to “average out” this difference between weekend and weekday.

If you already built an energy model that has hourly heating and cooling, it would probably be better to use that data in the conditional statement that selects out the hours of helpful vs. harmful solar radiation. This should be a bit more accurate than using the outdoor temperature and your estimated balance temperature. Still, at a certain point, you might be splitting hairs and you should ask yourself “would I still draw the same conclusion about a given building massing if I did a radiation benefit study that used an estimated balance temperature vs. hourly heating/cooling from an energy model?”

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