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Impact of bundling spatially separated spaces in a single zone?

asked 2018-05-16 11:06:05 -0500

updated 2018-05-16 13:20:33 -0500

I've just recently discovered something I thought was just not possible in E+: combining spatially separated spaces in a single zone. I was under the dumb impression that a zone needed to be bounded by its surfaces and form an enclosed volume (I didn't think E+ would calculate zone volume correctly in particular when spatially separated).

Not only it apparently works, but it's actually written in the E+ documentation!

If you look a Getting Started - Zone the building, specifically in the subsection Zoning - Concept 2 - Detailed, you find the following picture and notes on it:

Zoning Concept

Take note of Zone 1, Zone 2, Zone 4, and Zone 7. The two important zoning concepts can be demonstrated with the zoning to reinforce the idea of a thermal zone and encourage the use of simplified models.

1.Notice that Zones 4 and 7 include two rooms that are not adjacent to one another but are served by the same system. Because the air temperature in the two spaces is maintained at the same uniform temperature, the two spaces, though separated spatially, may be defined as a single zone. For our purposes, we will define them as separate zones.

So, to take an actual real life example with actual zone designation:

image description

My floor plate has three stairwells, spread across it (spatially separated), and these stairwells are basically unconditioned (perhaps a unit heater on the top floor, maybe not). Similarly, both of my corridors are conditioned by the same HVAC system. I'd be tempted to merge the three stairs in a single zone, and the two corridors in another, since having too many zones is always a pain (especially in OpenStudio App when you get to a large number)

so here are the million dollar questions:

  • Does anyone have any idea how much impact that would have on my simulation results?
  • And on simulation time? (meaning is one zone with more surfaces better/worse than two zones each with half the number of surfaces)
  • Would you actually do it?
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answered 2018-05-16 14:36:51 -0500

Impact on simulation results: My guess would be marginal. As you've mentioned, grouping an exterior space with interior spaces would artificially impact the heat balance of zones neighboring the interior spaces, but even here I think that typically that impact would be small compared to the internal loads in the neighboring zone (depending on the building, of course). This is assuming the volume of the combined spaces is calculated accurately (i.e. exterior loads get 'averaged out' over entire zone air volume.

Impact on simulation time: I think the jury is out on this. I found this old report on EnergyPlus runtime, which characterizes a runtime increase by a factor of 2.6 for a tenfold increase in zone windows, and the same factor for a 2x increase in the number of zones (15 to 30, conditioned via VAV) [pg 31]. The applicability of that study to your question (or generally) is suspect, since it looks like it was done back in the FORTRAN days..but maybe relative performance difference didn't change much in the move to C++.

Would you actually do it?: Yes, and I regularly do. Especially for big models, being able to group unconditioned spaces into one zone makes managing input much easier.

I would love to see more empirical responses to these questions, but that's my take.

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Asked: 2018-05-16 11:06:05 -0500

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Last updated: May 16 '18