Thursday, January 10, 2013

Getting in the Flow with Revit HVAC

Been spending some time trying to get more into the sizing part of duct for Revit… we’ve been working on a project for a client that required well formed systems.  When I turned the duct systems checking tools on, I kept getting some really nasty little errors that I couldn’t figure out…and a couple of items that should work, that didn’t…at least from my understanding of how duct systems should be defined. So here’s the notes from what I learned.

         Regardless of whether the program can define a system when adding  a duct or not, I found that systems normally aren’t “well connected” when you follow this method. I’ve gotten repeated errors, that I believe came down to how I was adding the duct.  The safest method for defining a system and getting the duct sizing results needed, is to create the system first by selecting the targets, such as air terminals. Selecting the source equipment (such as a VAV) once you’ve defined the duct system usually creates the most stable system, and results in the fewest errors with the data tables.

         This includes upstream systems, such as the primary air supply to the VAV boxes. One big issue we’re having with vendor provided air handling units is the lack of good, clearly named connections. No matter who the vendor is, open the family up and put in a connection name, such as primary supply or return air intake. This way, when you are defining the upstream systems, you can select the correct connection even if there are multiple connections.

         The biggest problem I ran into happened in regards to the flow direction settings at the connectors. In order for a system to be sized correctly, the source (such as the VAV box) should have the flow direction set to OUT, and the Flow Configuration set to Calculated:
         A loss method should also be specified – so if you set this to specific loss, make sure you put a value in here – leave it at 0 and you’ll get an error.

         The reciprocal is on the target, or air terminal. Check the flow connection, and make sure it’s set to In for supply air, and leave the flow configuration set to either preset or system. The default air terminals are set to Preset, but you can change this. If it’s set to Preset, flow factor is disabled, and you can set the loss method to coefficient or specific loss. As with the VAV, if no value is set for specific loss for pressure drop, you’ll get an error.
When using the Calculated/Preset combination, the airflow for the equipment is set to match the total airflow assigned to all components downstream. If you have 10 diffusers at 50 CFM, that means you’ll have 500CFM assigned to the VAV box as the calculated value in this configuration.

System flow configuration is a little different – instead of using an aggregate airflow total from all diffusers, the system will calculate the equipment airflow based on a percentage allocated to each air terminal.  If you set the flow configuration to System, and have the loss method set to specific loss, you can specify a flow factor (using a factor between 0 and 1 – with the total of all air terminals equal to 1):

If the loss method for a system flow configuration is set to Coefficient, then you can also set the loss coefficient value for the connector itself(and pressure drop will be disabled):

As with the specific loss method, if you leave these values set to 0, you’ll get an error.

Be aware that Revit already has a loss coefficient tables defined for fittings, based on ASHRAE  Duct Fitting Database, which assigns a loss to specific fittings and accessories. By assigning a coefficient directly to the air terminal, the Equal Friction/Static Regain methods for duct sizing will return better results.

If you are using specific loss, I’d recommend using a shared parameter for that value, so it can be changed without having to directly edit the connector in the family. Use the Associate Parameter tool to add the parameter. By using a shared parameter, you can also include this as part of a schedule or tag.

Here's the help file's description of what each of these settings is for:

This should help you a few of those nagging duct sizing issues - have a great day!

David B.


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Tim Kramer said...

Thanks Dave! Just to clarify, are you suggesting that one should add the Source equipment AFTER the System has already been created using the Targets? In other words, select all the Targets, create the System, finish sketch mode, then edit the System and add the Source? Just want to make sure I'm reading this correctly since it's an extra step. Thanks again!

David Butts said...

You have to have at least one target to define a system - the source cannot define the system. When you select a source object, you're really using it as a target when you create the system that way. You can add a source at anytime after you define the system, but my method is to pick all the components and define the system at once, since it's easier than one clicking elements using Add to System. IT's actually shorter to select all the items you want as targets in a system, then add the circuit, pipe or duct system.

Adam said...

I know this is an older blog post, but I am having a damned of a time getting the flow of my air terminal's demand flow upstream past the VAV box and to the air handler. This chain seems broken.

Air Terminal, 50 CFM
Air Terminal, 50 CFM

to VAV calculated: 100 CFM
to AHU? 0 CFM?

How do I configure the connectors in the VAV box in such a way to make this possible for a simple VAV system without return air or hydronic connections (simple electric heat with an in and out)?

It's driving me nuts!