topic Re: 100% OA AHU not calculating flow in Revit MEP Forum

100% OA AHU not calculating flow

Charles.Ball — Thu, 28 May 2020 22:07:07 GMT

I have created an AHU that is 100% Outdoor Air. The OA-side is set to IN and the Supply-side is set to OUT. There are no return or exhaust connections. I keep getting the flow direction mismatch error and it seems like it is coming from the OA side. The unit is on the roof so the duct is just open to the world. Do I need some sort of grille at the end of the duct? I know IRL there would be a grille, just trying to see if that is the cause.

Thanks.

Re: 100% OA AHU not calculating flow

AutoCAT95 — Fri, 29 May 2020 06:30:30 GMT

I might be wrong, but on that type of equipment you could use only 1 joint (supply out) which starts the duct system into building. No need to add joint "supply in" for just hanging in space. Let revit thinks that AHU create the flow itself.

I hope you can understand my thinking.

Re: 100% OA AHU not calculating flow

RobDraw — Fri, 29 May 2020 10:36:25 GMT

@Charles.Ball wrote:
There are no return or exhaust connections.

Why not? Is this a make-up air unit?

@Charles.Ball wrote:
The unit is on the roof so the duct is just open to the world. Do I need some sort of grille at the end of the duct? I know IRL there would be a grille,

Not really. These units don't usually have an OA duct, just an intake on the unit. You don't need to include anything for the OA for this unless you need the information for calculations.

@Charles.Ball wrote:
just trying to see if that is the cause.

It would be easier to determine if we could look at the family. Post it.

Re: 100% OA AHU not calculating flow

Charles.Ball — Fri, 29 May 2020 13:46:37 GMT

Yes, it is a make-up air unit. We have to add a duct to the intake side to move the opening away from exiting exhaust fans on the roof (code requirement).

Re: 100% OA AHU not calculating flow

Charles.Ball — Fri, 29 May 2020 13:51:40 GMT

It makes sense.

I do however need a duct connected to the OA intake side as we need the inlet to be a minimum distance away from existing equipment. So I am unsure of how to connect the duct without the connector but also not have it become part of the system.

Re: 100% OA AHU not calculating flow

RobDraw — Fri, 29 May 2020 15:12:36 GMT

@Charles.Ball wrote:
I keep getting the flow direction mismatch error

I tested your family with the open ended duct and a couple air terminals on the supply side. It seems correct and I don't get an error without an air terminal on the OA duct.

What's connected to the SA duct in your model?

@Charles.Ball wrote:
Do I need some sort of grille at the end of the duct?

Not really but Revit doesn't like open systems. The more you have the more likely issues can come up.

Re: 100% OA AHU not calculating flow

Charles.Ball — Fri, 29 May 2020 15:17:05 GMT

This is the same system as the other thread you're helping me with. I have 10 supply air terminals directly connected to the AHU and two terminals each connected to a CAV then to the AHU.

Re: 100% OA AHU not calculating flow

RobDraw — Fri, 29 May 2020 15:21:29 GMT

Can you share the system?

Re: 100% OA AHU not calculating flow

Charles.Ball — Fri, 29 May 2020 15:26:54 GMT

Between this thread and the other, everything seems to be working now. I have no more errors in the system. I think the main culprit was the parameter in the AHU family that I associated with the Flow of the connector was not set to "instance".

Thanks for all your help.