Summary

• EnergyPlus works by dividing the building up in separate thermal zones.
• EnergyPlus carries out calculations for each zone and then collates those results to work out the performance of the whole model.
• Each zone is effectively a "homogenous space" with a single sensor inside that assesses the average of all the loads in that space to work out how much heat needs to be added or removed from the space to keep it at a certain condition.
• For energy modeling a thermal zone should be a part of the building where the load conditions are largely consistent across the entire space.
• For HVAC sizing, a thermal zone may also need to represent a space where an actual piece of equipment is going to go, so that the equipment can be sized realistically.
• The more zones in a model, the greater the detail available as to what is going on. However this also means longer the simulation times and more information to absorb and sort through.
• Minimizing the number of zones so that your model includes the minimum needed the task at hand to get the information you need will save you time and effort.
• We provide a table that helps guide you as to which strategy to use.

What is a thermal zone?

Most spaces in most buildings aspire to maintain some kind of comfort. This is usually expressed in terms of temperature and usually as a range (say 21-24C or 70-75F). It could be said that as long as a space is within the range, it's met its comfort aspirations.

To maintain a setpoint range, there will be times heat needs to be added to a space. This is called the heating load. There will also be times heat needs to be removed from a space. This is called the cooling load.

The best way to think of a thermal zone is that, for any given time step, it is a space inside a building where the heat gained and lost can be expected to be constant wherever you are. In other words, wherever you stand in that zone, you should expect the cooling and heating load to be about the same at a given point in time.

Each zone can theoretically be cooled and heated separately from other zones - each zone is its own mini thermal system, with heat coming in and going out as part of a process to provide comfort.

How zones work in real life 

In the real world, a zone is defined by two things:

• The thermostat
• The heating and cooling equipment it controls

In theory, the thermostat reads the temperature and if it's outside the comfort range it tells the equipment to do something to fix it. People in real life are often uncomfortable if the place where the thermostat is is likely to have a different temperature to the space where they are.

In real life, thermostats and controls are expensive. (It's a good idea to walk around buildings and try and count the thermostats on each floor. There probably won't be that many.) It's very common that zones with different internal load profiles will be run off the same thermostat, especially in renovation work.

The point is - knowing how zones work in real life can help a realistic approach to zoning.

How zones work in energy, comfort and sizing models

In a building physics models, it works the same way in the sense that a thermal zone has a thermostat and heating and cooling equipment. Just like in real life, there is only one thermostat per zone.

Zones are much cheaper in energy models than in real life, so the temptation is to have lots. It's a much more common mistake to see models with more zones than they need and indeed more zones that are likely to get installed in real life.

Probably the easiest way to think about zoning a simulation model is as follows:

             More zones = more detail = more data = more time

Good zoning practice for early stage energy models

 If you're making an energy model for early stage, you're probably looking to study things like:

• How envelope strategies affect energy use
• How space layout affects energy use
• How HVAC selection affects energy use 

If these are the tasks at hand, then here are some guidelines for zoning a project:

• Perimeter spaces should be zoned separately to core spaces. A good rule of thumbs is that a space within 4m  / 15 ft of the facade is the perimeter zone. This is because you want to capture the effect of facade impacts on the space it will impact. This impact should not be averaged across the whole building but concentrated on the perimeter area most affected.
• Perimeter zones with substantially different orientations should be zoned separately. Each orientation will be subject to solar gain at different times, so they need separate zones.
• Areas where the facade varies substantially should have their own zone, even if the orientation is the same. For example, if you have a south facade with two glazing systems, one with a lot of glazing and another with a lot less, then you should divide the perimeter zone up so each of the facade types has its own zone
• Space uses which are substantially different should be zoned separately and according to where they are. For example, Meeting rooms should have a separate space use profile to open plan office (but you probably don't need lots of different meeting room types) and should be put roughly where they will be in the model if you know. You don't need lots of different space types for similar spaces (eg living room, TV room, dining room and lounge room can be "living space").

Perimeter core is the default zoning strategy in Sefaira because it's pretty good at quickly getting buildings with a fairly homogenous space use into a decent set of perimeter and core zones. If the building is not very orthogonal or overshadows itself it can be useful to opt for detailed perimeter core. If you have very different space uses you may need to draw your own perimeter core zones broken up for different space uses.

Extra guidance for thermal comfort models

If you're also looking at thermal comfort, the zoning depth and height are of added importance. This is because the "Sensor" that reports the outputs is located in the center of the zone. That means:

• Perimeter zones that are very deep will have the "sensor" a long way from the facade and may give misleading comfort results (especially relating to radiant temperature)
• Zones built to have high ceilings (eg a 6m / 20ft space) will have the sensor 3m / 10ft above the floor. If you're testing shading of the facade this may give you misleading results.

Generally speaking, other than the above the rules that apply to energy model zoning apply to the above.

Extra guidance for HVAC Sizing models

If you're also looking at HVAC sizing then your zoning method will depend on how detailed you want the sizing to be. As a rule

• The guidance for energy models applies to sizing central HVAC equipment such as boilers, chillers, air handling units. 
• If you want to get preliminary zone sizing for early stage pricing (for example) then you will need to make sure the zoning is more representative of what the zoning will be in real life. This still probably doesn't mean zoning every room in the model.

How Sefaira helps with zoning

We provide 4 different zoning strategies. In summary the goal is as follows:

• One zone per floor - desperation zoning strategy for complicated models that fail to apply perimeter core zoning and studies where very little time is available. Only recommended for conceptual analysis.
• Basic Perimeter Core - default strategy - automatically gives your model 4 perimeter zones for each orientation and one core zone. Fast and suitable for lots of typical use cases.
• Detailed Perimeter Core - better perimeter core strategy for models with self-shading properties, non-rectangular floor plans (eg L-shape, H-shape, U-shape buildings) or sites with multiple buildings in the same model
• One zone per room - supports user-defined zoning plan.