Wednesday, March 13, 2013

Revit MEP Heating and Cooling Loads Results

I received an email from a Revit MEP user asking about results that don't seem to make sense. I'm writing about this because after some experimentation it seems odd to me too. This doesn't involve a complex model, just a simple test they did using a single room. I attempted to mimic their emailed explanation fully realizing that there are myriad possible things I could do differently. Despite the possible variables I was able to reproduce what they were concerned about.

Still Curious?

When they set up a test and then changed one variable (Outdoor Air) the resulting report shifts peak cooling month by one month (their result).

These are the test conditions: Walls surrounding a Space such that it is 100 SF. I placed a floor, ceiling and roof but I don't know that they did that. I used these settings for Occupancy, lighting and power.

These are the energy settings I used for the first report.

These are the energy settings I altered for the second report, only changed the Outdoor Air setting, nothing else.

These are the two reports side by side. Report on the left is the one without Outdoor Air changes and the one on the right is after changing Outdoor Air. Notice the two month advance? There are also subtle Dry and Wet Bulb values.

Their question to me and mine now as well is, "Is it reasonable to expect a so called simple change like this would cause such a jump in the calculation time frame?" I've recommended they reach out to the Autodesk Revit support team so they can examine their dataset first hand. Seems odd to me though.

I should also mention that I changed the Outdoor Air for both the Zone that the space belongs to and the Energy Settings dialog. I didn't seem to matter when I tried changing only one of those for the Outdoor Air.

Comments?? Did I miss something obvious?


KyleB said...

This is entirely understandable. Depending on the climate, Outside Air can represent a significant - if not the dominant - component of the peak cooling load.

Although you didn't show the previous state of the Outside Air settings, I am assuming that they were undefined in this initial state. In that situation, you only have to care about the outside air dry bulb temperature, and the amount of sun per day. So, since the days are longer in June, I'd expect a peak Cooling load to happen then -> The longer time in the sun outweights the lower dry bulb temperature in June vs. August.

Once you introduce Outside Air into the equation, you have condition that outside air, and the wet bulb temperature starts to matter a lot. This is because it takes a lot of energy to dehumidify humid outside air.

You can see from your image that there is significant difference in wet bulb temperature between June and August. That added component to the overall total cooling load makes August the new peak month -> The increased Outside Air component outweighs the decrease in outside air temperature along with the decrease in total time of Sun hitting the exterior surfaces.

So, this is really a situation where you need to understand the energy flows in the building, and how different components influence the total cooling load that is reported.

Steve said...

Thanks for the input, I thought this might poke you in the eye. :)

I guess the flaw in my thinking is to assume that the so called peak is a rigid or fixed place in time and not affected by multiple factors.

Jordi Xifra said...


A very good post, thank you for having this blog. I just want to ask a question:

The second image qeu samples in this post is about "energy settings", I think this dialogue is used to simulate mass energy, while the result given thermal load calculation offers its own dialog to configure the features. I think it's different energy settings to calculate heat loads, you think I'm right?

Anonymous said...

yes this is completely reasonable

cooldigz said...

@ steve : thanks for taking interest in solving our query.
soon we will post details of it here..
@ Jordi Xifra there are number of ways to assign parameters to model, so it doesn't matter from which dialog box we have given settings.

Hogtown Velo said...

As mentioned above this is completely reasonable. Notice your peak heating load; essentially double with the addition of the outdoor air. This OA value is also very high, standard office is .06 cfm/SF, and does not include an OA per person value. This is why Revit load analysis package is essentially useless for any engineer adhearing to ASHRAE standards.

Unknown said...

After generating the heat and cooling load report from revit 2014, i find Cooling Load Density is very low likewise 40(W/m²) as per the building manual calculation, where can correct it

n8 said...

So why does the help file state that making changes here will only affect an energy analysis and not the H & C load analysis?

Straight from help file: "In the Energy Settings dialog, these settings affect the results of a Energy Analysis for Autodesk® Revit® performed using conceptual masses or building elements.

Note: These settings do not affect room or space based gbXML export or heating and cooling load calculations and cooling in Revit MEP."