**Overview**

When the Perimeter / Core zoning strategy is selected, our algorithm automatically assigns all perimeter zones to a cardinal direction of either north, east, south or west. **This article explains how we determine which cardinal direction should be assigned to a given zone.**

**Orthogonally Projected Vectors**

The process starts by taking a vector and projecting it perpendicular to the face of all the exterior walls.

Next, we split the vector into orthogonal components which are at a right angle to one another.

The longest orthogonally projected vector is declared the winner, and the associated zone is assigned to the cardinal direction.

**FAQ**

Buildings with walls at a perfect 45 degree angle to a cardinal direction could return unexpected results. In the example below, we only have three perimeter zones for a box. Most people would expect four, one for each exterior wall. So why does this building only have three? Simply put, the orthogonal vectors tied, and further more, zones which share an edge *and* are facing the same cardinal direction are merged. A workaround would be to try rotating your building as little as a half a degree. This should cause the vectors to no longer be equal, and cause the zones not to be merged.

This might beg the question, why are my zones being merged? Let's look at another example. The image below would have eight perimeter zones but since we merged zones facing the same direction with a shared edge, the end result is only four perimeter zones. Merging the zones is very important when using EnergyPlus because there is a direct correlation to number of zones and engine run times.

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