Does thermal zoning matter in early energy analysis? The answer is always, yes. But how it is implemented is open for debate depending on who you ask. Is 'one zone per floor' adequate or should it be 'perimeter & core' based? This post will highlight some of the things I have learned about this interesting topic in the context of early energy modeling for architects.
We will start by looking at an industry standard that prescribes thermal zoning, a research paper on the topic, and then my own limited testing. I used Autodesk Revit + Insight which can do both, but some of its competitors only do the later (i.e. per floor) from what I understand.
Read on to learn more...
I won't get into to much detail on what this study says as you can read it for yourself. It suggests that one zone per floor primarily underestimates energy use, which is what I found in my quick test below. Additionally, here is a quote from the conclusion of the paper:
Notice the 'mass floors' appear, even before creating the energy model.
First, to test the ASHRAE standard implementation of thermal zones I just left the default settings as-is; thermal zone division (i.e. per orientation) and a perimeter division 15'-0" deep.
The thermal zones can now be seen in the energy model I created. Notice one is selected.
Now, in a copy of the Revit model I un-check the perimeter zone option.
This still created a perimeter zone as I did not zero-out that value... so, I will not use this one. Just wanted to show this.
Now I zeroed out the perimeter zone value.
And the result is one zone per floor.
And again the results...
Generally, the reason why no including p/c zoning results in less energy... its all about the physics. Imagine a square building, 9am on a cold sunny morning. The east façade could be experiencing a net heat gain which requires cooling while the west façade could be experiencing a net heat loss. If you have one space/zone then the gains from the east are summed with the west which cancel each other out. As you can see this is a true modeling / computational error.
Here is some general advice on when to use this setting in Revit or not. If you have conceptual massing only, or large open plan office spaces without much internal walls (where internal walls naturally provide our model with the internal discretization, that is p/c zoning) then enable p/c zoning. If you are at a detailed stage, with walls, rooms etc., as the building is naturally divided up you can disable it. If in doubt, just leave it on. More zoning is better than less. I say all this because of course early stage modeling might still involve a fairly detailed arch model.
Fun stuff!
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We will start by looking at an industry standard that prescribes thermal zoning, a research paper on the topic, and then my own limited testing. I used Autodesk Revit + Insight which can do both, but some of its competitors only do the later (i.e. per floor) from what I understand.
Read on to learn more...
ASHRAE 90.1 Appendix G Overview
ASHRAE 90.1 Appendix G describes the Performance Rating Method for computer-based energy simulations of proposed and baseline designs. This also includes requirements when HVAC and Zones have not yet been defined in the building; which fits our early energy modeling interest!Note: this standard is protected by copyright and can be purchased here.Here are a few highlights from Appendix G (for our topic, pay attention to the text I highlighted):
G2.1 When comparing proposed and baseline designs, 'Performance Calculations' shall use...G2.2 States the simulation program shall be computer-based; not done by hand or using Excel.
- the same simulation program
- the same weather station data
- the same energy costs
G2.2.4 Requires the simulation program to be ASHRAE 140 validated and the results shall be furnished by the software provider. Click here for Autodesk's published results.
G2.2.1 The simulation software shall have the ability to model...G3.1.1 Exception a only requires multiple building types, within a single building, if they apply to more than 20,000 SF of conditioned floor area. So an office building with 8,000 SF of retail can just be simulated as Office only.
- 8760 hours per year
- hourly variations in occupancy, lighting power, miscellaneous equipment power, thermostat setpoints, and HVAC system operation, defined separately for each day of the week and holidays
- thermal mass effects
- ten or more thermal zones
- part-load performance curves for mechanical equipment
- capacity and efficiency correction curves for mechanical and cooling equipment
- air-side econimizers with integrated control
- baseline building design characteristics specified in Section G3
Table G3.1, Item 1b All conditioned spaces in the proposed design shall be simulated as being both heated and cooled even if no heating or cooling system is to be installed, and temperature and humidification control setpoints and schedules shall be the same for proposed and baseline building designs.
Table G3.1, Item 2 Allows potions of exiting building to be excluded in some cases. Unfortunately Autodesk Insight does not allow this currently.
Table G3.1, Item 5a All uninsulated assemblies (e.g. balconies,... parapets) shall be modeled separately. Some tools extract exact floor and wall area from Revit for analysis; e.g. if you change the parapet height the EUI changes, which is not appropriate.
Table G3.1, Item 5d Manual shading such as blinds shall not be modeled. But fixed design features like sun shades and automated products like Sage glass (electrochromic) can be included. BTW, LHB designed the Sage Glass manufacturing facility; www.lhbcorp.com/project/sage/.
Table G3.1, Item 7 This section described what to do when the HVAC Zones have been designed.
Table G3.1, Item 8 This section described what to do when the HVAC Zones have not been designed... which is what we are interested in for early energy modeling. It says separate thermal blocks shall be assumed for interior and perimeter spaces. Perimeter spaces shall be 15'-0" deep and divided, generally, by orientation.Based on this ASHRAE standard, using thermal zones is required in early energy modeling. On a related note, you may want to review my post from last week for more on the concept of proposed versus baseline analysis: ASHRAE 90.1 Baseline - Revit + Insight
Research Paper
ASHRAE member, Lillian Smith, AIA, wrote a paper on this topic called Beyond the Shoebox: Thermal Zoning Approaches for Complex Building Shapes, click here for a link I found to this PDF.
I won't get into to much detail on what this study says as you can read it for yourself. It suggests that one zone per floor primarily underestimates energy use, which is what I found in my quick test below. Additionally, here is a quote from the conclusion of the paper:
"Overall, these results demonstrate the importance of reasonable zoning in conceptual models. Using software to automatically cut geometric forms with a different floor plan at each level into valid energy models can be a huge benefit for studying form and orientation at the earliest stage of design when changes of this nature can be made and will have a big effect on energy use. Automatic zoning algorithms that can be trusted to return reliable early stage energy simulation results can enable rapid design iteration, which allows for more studies and options to be considered and intelligently analyzed."
My Own Limited Testing
I did a quick test of my own using Revit, a conceptual mass that twists vertically and Insight. One run was performed with a single zone per floor and another per the ASHRAE recommendations (p/c zoning). Both buildings are exactly the same, with a large Window to Wall Ratio (WWR) of 80% to magnify the solar gains. The results corroborate Smith's findings, showing the thermal zone per floor underestimates the energy use.
Here is a quick overview of my tests. A 56-story building modeled as a conceptual mass with 'mass floors' applied. The footprint of the building twists 35 degrees vertically, making each floor plate different.
Notice the 'mass floors' appear, even before creating the energy model.
BTW, it is possible to derive the total gross area of the building from this mass; this is just 10 minutes worth of work so far.
First, to test the ASHRAE standard implementation of thermal zones I just left the default settings as-is; thermal zone division (i.e. per orientation) and a perimeter division 15'-0" deep.
The thermal zones can now be seen in the energy model I created. Notice one is selected.
BTW, the Revit code that creates the energy model is multi-threaded... meaning it uses all of the cores in your CPU.
Now, in a copy of the Revit model I un-check the perimeter zone option.
This still created a perimeter zone as I did not zero-out that value... so, I will not use this one. Just wanted to show this.
Now I zeroed out the perimeter zone value.
And the result is one zone per floor.
And again the results...
Conclusion
Thermal zoning, or p/c zoning, is for when HVAC zoning isn’t designed yet or the internal wall/room layout is not defined yet, which might be more meaningful to the applicability of early stage arch.Generally, the reason why no including p/c zoning results in less energy... its all about the physics. Imagine a square building, 9am on a cold sunny morning. The east façade could be experiencing a net heat gain which requires cooling while the west façade could be experiencing a net heat loss. If you have one space/zone then the gains from the east are summed with the west which cancel each other out. As you can see this is a true modeling / computational error.
Here is some general advice on when to use this setting in Revit or not. If you have conceptual massing only, or large open plan office spaces without much internal walls (where internal walls naturally provide our model with the internal discretization, that is p/c zoning) then enable p/c zoning. If you are at a detailed stage, with walls, rooms etc., as the building is naturally divided up you can disable it. If in doubt, just leave it on. More zoning is better than less. I say all this because of course early stage modeling might still involve a fairly detailed arch model.
For BIM Chapters updates, follow @DanStine_MN on Twitter or connect on LinkedIn
