What are the HVAC and energy analysis detailed hypotheses/informations?

[Anonymous]

Hello,

I am planning to use Revit's energy analysis on some buildings, but would like to know more what's really been used as hypotheses in the calculation of the results. Basically, I would need to show some client that it can compare well to some other energy analysis (with TRNSYS or OpenStudio, e.g.).

But for that I'd like to know if there is some additional documentation that I can find, explaining more the models. For instance, what is the boundary condition used on the ground (some answers here for that)? Is there any radiative exchanges (with view factor matrices, for instance) taken into account in the rooms? What is really behind all the HVAC systems proposed (how are these used, etc.)? Etc, this is a non exhaustive list of questions.

Basically, how can I build a good comparison, on a very simple test model, between Revit's capabilities and other softwares? Even if it's very simple, 4 walls, no schedules etc... even no HVAC at all (is that an option in Revit? I don't think so) --> I need to have a valid comparison. And this implies a good knowledge of all input data and how it's used to compute the outputs in terms of consumption.

Thanks a lot for any help!

Arnaud.

PS: This is based on my researches in the forums, guides available etc.. If some more detailed documentation is available somewhere, it'd be great!

[Anonymous]

As I'm sure you can tell they have been tight-lipped about that info.

Either A) - only a few number of people know, and they're buried deep within the autodesk coffers or B) they are keeping it close chested because it's either very assumptuous (and inaccurate) or they feel it gives them a strategic edge over competitors to not reveal their assumptions used in hvac examples which are now 10+ years old.

I doubt you're ever going to fully get the answers you're after from auto D.

[Anonymous · Accepted solution]

Hi @Anonymous

Insight uses DOE-2 for the energy simulation engine. Insight will also perform energy analysis using EnergyPlus. Email the support staff ([email protected]) and ask for the EnergyPlus key. As you’re probably aware, both of those two simulation engines are highly-regarded tools in the industry.

You can see the inputs used in the analysis by downloading the DOE-2.inp file from Insight. Any specifics not define in the .inp file use DOE-2 defaults. More specific DOE-2 settings can be found in the eQUEST/DOE2 documentation (eQUEST is the DOE-2 interface). http://doe2.com/download/DOE-22/DOE22Vol2-Dictionary_48r.pdf. For EnergyPlus runs, you can look up the documentation for version 8.5 https://energyplus.net/documentation

Insight does not allow the simulation without an HVAC system (a DOE2 limitation).

To download files from Insight log into insight.autodesk.com, navigate to the model in question and expand the model menu to find the download option.

High level descriptions of the HVAC systems available to choose from in the Insight Energy Settings are found here: Insight HVAC Systems

Information for the other settings in Insight are here: References for Energy Optimization

Please click the "Accept as Solution" button if this solves your issue or answers your question so that others may also benefit from the information

[Anonymous]

but the export function from revit doesnt create near uniform and airtight analysis models as required by those 3rd party websites. It just doesnt provide clean exports and is worthless in that regard. its better to just remodel in those programs.

[Anonymous]

Hi @Anonymous

Maybe this section in the Revit help documentation will address some of your concerns about the model's airtightness: About the Energy Model. See the section on Surface edge precision.

Also the section of Surface and Space Resolution might be helpful. Analytical Space Resolution; Analytical Surface Resolution.

[Anonymous]

Hello @Anonymous,

Thanks for the answer, and sorry for my long delay to answer back..

One thing is unclear to me, when you say I can ask for an EnergyPlus key: what is it? Can I then choose to run energy analysis with DOE-2 or EnergyPlus engine?

For the rest, I will take a look at the documentation you mentionned, thanks!

Arnaud.

[Anonymous]

HI @Anonymous

DOE2 is the default Insight analysis engine.  The Energy Plus analysis engine can be used; however you'll have to reach out to the Insight support team and ask for the key. [email protected] 

They will ask you to fill out a survey before determining if you are eligible to use the Energy Plus service (it's expensive for Autodesk so that is why they are being selective).

[Anonymous]

Thanks again @Anonymous!

If I may ask another, more precise question about the ground temperature: In the exported ".inp" file from Insight, I have 1 surface defined as "UNDERGROUND-FLOOR". In the whole ".inp" file, nothing defines what is the boundary condition there.

The keywords "PERIM-CONDUCT" and "PERIM-EXPOSED" aren't present. Furthermore, I read in the DOE-2 manual that

The procedure assumes that the monthly ground temperature is the average outside air temperature damped and delayed by three months, which is how the ground temperatures on the weather file are calculated. To force the program to use the weather file values, do not enter ground temperatures using the GROUND-T keyword in the SITE-PARAMETERS command.

The GROUND-T keyword is absent as well from the ".inp" file. But on the other hand, there is no explicit ground temperature field in the Weather data file I exported from Insight either. So, what is it using then? Does it automatically switch to a computation based on outside temperature from the weather file? If not, what does it do?

Is there a way to visualize ground T° data in GBS/Insight?

Thanks a lot,

Arnaud.

[Anonymous]

Good question about the Ground Temp. There are many DOE2 keywords that Insight lets default to the eQUEST/DOE2 default settings, therefor they are not included in the .inp files. In other cases the information is pretty deeply buried by eQUEST.

It’s a convoluted process, but there is a way to extract the Ground temperature information out of eQUEST using the inp file and the .bin downloaded weather file from GBS. In eQUEST you have to edit the hourly report to include the Ground Temp then run the simulation. Then open the detailed results and search for “Ground” in the hourly reports to find absolute ground temps. It shows the monthly ground temps in Rankine (don’t ask me why eQUEST uses that unit), which you can then convert: Rankine to Fahrenheit conversion or Rankine to Celsius.

If you aren’t familiar with eQUEST you can send me the weather .bin file from GBS and I’ll get the data for you.

[Anonymous]

Hello,

Thank you for this answer. As I am not at all familiar with eQuest, it'd be indeed great if you could extract this for me! I attach it here.

Basically, what "worries" me is that as such, if you both export the energyPlus and weather file, and run energyPlus, you got huge differences in the result. I didn't identify yet all the causes for this, but one of them is that EnergyPlus doesn't find a ground temperature and sets it to default: 18°C.

The HVAC is obviously also differently treated, as for instance EnergyPlus has no fuel consumption, where it is huge in the same run from Revit, in my case.

Anyway, thanks a lot for your help!

Arnaud.

PS: Ok for some reason my post doesn't get sent with the attachement. The file is GBS_04R20_196125.bin (Chicago). Can you get it? Thanks a lot!

[Anonymous]

Thanks Marjorie,

I didn't realize you said that Insight will model an EnergyPlus run for us. I thought you meant that we could download the .inp file in order to run it separately in either energyplus or Equest. That's a pretty neat function you've provided.

My point made before was in reference to using the REVit energy model in another 3rd party software separately from insight - in those cases I still believe the export functions aren't useful as the models aren't "water tight", and that seems to be what you need in most software. Unless you use something like IESVE which converts the model, but even then I  think it's cumbersome.

But I guess I still wonder.... do you know of anyone who is going from Revit > Insight > export > then into a program like equest for compliance modeling??? I think the most helful assett would be not having to remodel the shell/spaces and shading elements, then we could just fine tune R values/hvac/schedules in the 3rd party softwares...

Regards, John

[Anonymous]

Hello @Anonymous,

Did you have a chance to process the weather file?

Thanks a lot!

Arnaud.

[Anonymous · Accepted solution]

Hi @Anonymous,

So sorry about the delayed response. I've attached the ground temperatures for you for GBS_04R20_196125 in Chicago.

[Anonymous]

Thanks a lot @Anonymous !