Helpdesk Ticket 2769

What a tangled web we've woven here. CR? Enhancement? At a minimum some additional documentation?
Michael J. Witte on 12 July 2010 08:42 PM

These still are not available - seems like a good idea.
Michael J. Witte on 18 February 2014 05:25 PM

Andy Tindale
User
Posted on: 14 July 2010 10:50 AM
Thanks, also thinking about it, the airflownetwork infiltration/ventilation doesn't need a new report as that is not specified by the user in ac/h terms in any case.

but it's not clear to me that this necessarily reflects what the users are looking for. My guess is that they are expecting the input ach rate to be interpreted either at the indoor density or standard density

I agree that technically minded users who took the time think about it might expect input ac/h rate to be interpreted either at the indoor density or standard density, but most of our users (even technically minded ones) who key in say 1.5 ac/h as the ventilation rate expect to see 1.5 ac/h reported in output.

A switch to user inputs would be ideal as it would make it easy for us to provide an option as to how E+ interprets inputs.

Andy

Michael J. Witte
Posted on: 13 July 2010 07:08 PM
Almost. Using outdoor density only applies to ventilation (ZoneVentilation:) and infiltration.

Zone Mechanical Ventilation uses "Standard Density" to convert from user input to mass flow. So, it needs to be reported in Standard Density to line up with inputs.

These report variables could be added, but it's not clear to me that this necessarily reflects what the users are looking for. My guess is that they are expecting the input ach rate to be interpreted either at the indoor density or standard density, and perhaps we need to add a switch to the ZoneInfiltration and ZoneVentilation objects to allow that.

Mike

Mike,

Thanks for the clarification. I understand now and will update our documentation accordingly.

It would help us enormously if you could add report variables for infiltration, natural ventilation and mechanical ventilation that use external pressure (if this is what will make ac/h outputs tally with inputs):

Something like:

Zone Mechanical Ventilation Air Change Rate Outside Density
AirflowNetwork Zone Infiltration Air Change Rate Outside Density
Zone Infiltration Air Change Rate Outside Density
Zone Ventilation Air Change Rate Outside Density

Many thanks,
Andy

Linda Lawrie
Posted on: 13 July 2010 07:03 PM
Remember that the coefficients you enter will change things. only 1,0,0,0 will show back what the user has input.

Also, the eio file reports the nominal air changes per hour, for whatever the user has entered.

Linda Lawrie

Michael J. Witte
Posted on: 12 July 2010 08:41 PM
From the Input Output variable definitions:

"Zone Infiltration Volume Flow Rate Current Density [m3/s]
These outputs are the total volume and volume flow rate of infiltration air based on the
current density of zone air."

"Zone Infiltration Volume Flow Rate Standard Density [m3/s]
These outputs are the total volume and volume flow rate of infiltration air based on the
standard density of air. Standard density in EnergyPlus corresponds to 20ºC drybulb, dry air,
and nominally adjusted for elevation."

"Zone Infiltration Air Change Rate [ach]
The rate of infiltration in air changes per hour."

I'm not sure it's stated anywhere in the documentation, but I'm 99% certain that all reporting in ach uses the current density of the zone air (since that is the air that is being exchanged).

The kicker is here in the ZoneInfiltration:DesignFlowRate object:

"Field: Design Flow Rate . . . The resulting volume flow rate is converted to mass flow using
the current outdoor air density at each time step."

So, the user inputs a design volume flow rate (whether is be in m3/s or per area or in ach, doesn't matter) and this flow rate is converted to mass using the current outdoor air density. The simulated mass flow rate is then reported back out in the user's choice of current indoor air density or EnergyPlus standard air density, neither of which will ever (or extremely rarely) match the outdoor air density. So, the answer is that they will never be able to compare. Even if the wind and temperature adjustment coefficients are set to zero.

I suppose we could add yet another output variable "Zone Infiltration Volume Flow Rate Outdoor Density [m3/s]"?

Mike

We are continuing to get support requests (at least one a week) in which DesignBuilder users are concerned that the infiltration, natural and mechanical ventilation ac/h values reported by EnergyPlus are different to those entered in the interface. I believe that the reason for this is related to the way that the calculations are now done (or the way that air flow is now reported) using difference in pressure between inside and outside.

Is there any documentation on this so I can produce a definitive answer to our users? Our current help page is too vague to be of any use.

Could you let me know how to report ventilation air flow in a way that would be exactly consistent with user inputs? " Zone Infiltration Air Change Rate [ach]", "Zone Infiltration Volume Flow Rate Standard Density [m3/s]" and "Zone Infiltration Volume Flow Rate Current Density [m3/s]" all give different results to inputs.

Thanks,

Andy