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?

Concrete Volume between Slab and Beams

A message via Linked In arrived the other evening asking about the volume of concrete when slabs and beams are involved. The hopeful question hinged on the slab reducing its volume by however much the beam overlapped the slab. The answer is a qualified "yes" because it is aware when Join Geometry is used but in the opposite direction, beam reduces volume not the slab.

Here's a floor slab that is 20'-0" x 20'-0" x 6" thick and a 12" Wide x 24" Deep x 20'-0" long concrete beam reporting a volume of 200 CF and 40 CF respectively, before using Join Geometry (slab properties shown).

Here's the properties of the beam before Join Geometry as well, reporting a volume of 40 CF.

Here's the properties of the beam after Join Geometry, now reporting a volume of 30 CF.

I didn't bother capturing an image of the floor slab, its volume didn't change, still 200 CF. I hope you'll take my word for it. The point of the exercise is to determine how much concrete is really needed, at least in my mind. The floor slab volume is intact (for this particular condition at least) but any beams that are joined with the slab will reduce their volume appropriately.

That means a two schedules, one for slabs and one for beams, should provide a reasonably close total if you look at both schedules together. It might be cool if the slab could inherit the beam volume and report a total that included the beams or the reverse subtract the beams from the slab.

In the field the beam and floor, minus any topping slab, are often poured together so the reinforcing ties it all together. Curiously, the way Revit deals with beam sizes and types is generally in conflict with how engineers think of the beam size as it relates to the slab. It's never simple or easy is it?

Set Calculations to Stun

Attending sessions at a conference is always a good way to come up with ideas for new blog posts. This post was inspired by David Butt's session at Autodesk University 2012 about Pipe and Duct Systems. I was working as one of his lab assistants and he mentioned this so I made a note to myself to mention it again, here.

By the way sorry about the title, I was thinking of the Star Trek "Set phasers to Stun" command that Captain Kirk often uttered. Literary license, I was just rhyming Stun with None.

In Revit MEP you can create systems for pipe and duct, meaning just the act of sketching one of them assigned to a particular system will impose certain design properties and graphical qualities to that element automatically. In the past the only way a duct or pipe could behave this way was to connect it to a component or to another duct or pipe that was already. That made it harder to just sketch pipe or duct (schematic design) and use it to represent different uses, like supply or return air, or hot versus cold water.

One of the properties of these systems is called Calculations and you can choose between None, Flow Only or All.

Since Revit only does fluid based calculations (piping) it doesn't make much sense to "waste" any CPU cycles on trying to do calculations on pipe systems for gases like oxygen or nitrogen. Same is true if you really aren't going to rely on Revit for pipe sizing or related calculations either. Any system that isn't going to need this ought to be assigned to None.

Panel Schedule and Load Calculation Addition

A thread at AUGI that started in December 2011 described a situation that Panel Reports show some connected loads as "0". The thread wandered around a bit but Martin Schmid with Autodesk responded on Tuesday with some insight after the development team did some research. Martin prepared a video to help explain what can cause this.

He wrote in his reply at AUGI:

I was able to reproduce this today with some help with our development team. The issue seems to be related to having your connector's Load Classification Parameter associated with a Parameter that has an empty value specified.

You can see in the video that when I create a Load Classification parameter, it has no value... when I then associate that with the connector, the connector ends up with a load classification with no value. Thus, when you connect this, there is no value that propegates into the panel totals, since the panel totals are dependent on summing values from load classifications. As soon as I set a value on the parameter associatd with the connector's Load Classification, the values tabulate.

A reply at AUGI shortly thereafter reminded members to also check the Part Type setting for their families. Using stock content it isn't likely that this would be the issue but for any custom content that has been downloaded or created internally, checking everything you can won't hurt.

Electrical Panel Load Calculations

Another "inside" gem from Inside the System which offers deeper insight to how the numbers work. If you are a Revit MEP user and electrical is your game, then you'll want to read it. Thanks to Martin for taking the time to put it together.

Occupancy Data Application

I've written about the workaround solution for documenting occupancy information in room tags in the past. I've even shared a sample project file based on the work I did for Scott Davis' past firm WLC Architects in 2005 (before he joined Autodesk). Until the API came along we were faced with a semi-inelegant solution that involved manual data entry and checking before plot day. Even after the API nobody really addressed this issue directly, till now...

Rahul Shah (blog: Revit Sticky Notes) works for Wood Bagot in the UK. He responded to a query at AUGI with a promise to write an application to push a calculated value to make Occupancy information taggable. He posted his solution today on his BLOG.

His written instructions on the blog post are:

NOTE: In order to use this plugin you will have to add "Occupancy Load Factor (as area type)" and "Occupancy Load (as integer type)" shared parameters to your project file and assign them to Room object as Instance. Also, calculated occupany load value is not dynamically linked with other values so if you change room size or occupany load factor then you will have to rerun this tool to update occupancy load value. Please read the Readme.txt file contained in the zip file for more information.

You can DOWNLOAD IT NOW!

Schedule Concepts - Calculated Values and Headers

Prompted by a few questions read at RevitForum.org and AUGI in the past few days I decided to record another short video. I used ducts and the notion of figuring out how much they cost based on a cost/ft scenario. The pricing you'll see in the video and screen captures are quite a bargain so you might want to rush out to buy some duct if you can get a supplier to match them?

The concept discussed in the video for calculated values relies on the stock Cost and Length parameters, Cost x Length. The assumption is that the price entered for Cost is the cost of the duct per linear foot (easily applied to metric values). None of this presumes it's the appropriate way to price them, the concepts can be applied to anything else.

The other concept demonstrated in the video is adding a group header to several columns in the schedule. The trick to getting a group header is how you go about selecting the columns. When you click in a header field Revit thinks you want to edit the value. You need to click and drag to select the other columns so you don't just select the column header you click on first...click and drag...then let go of the mouse button to finish. Watching the video is probably more "obvious". Not hard to do, just subtle. Kind of like learning to drive a manual transmission.