Re: [EnergyPlus_Support] Re: Unmet Hours from Outdoor Air Units in EnergyPlus

Ade,

In model_OAU.idf, you have DesignSpecification:OutdoorAir but they aren't linked to anything...

By the way, you would define ACH under ZoneInfiltration:DesignFlowRate, like you did in m3/s per zone. Using one or the other doesn't matter much.
Also, for DesignSpecificiation:OutsideAair I tend to try to use m3/s per area or per person, makes it simpler (you don't have one different value per zone), but that's just me.

Anyway, your problem is that you have two zone equipments: the VRF and the outdoor unit. If you had an air loop, you could indeed size on "ventilation" rather that "sensible", but here's it's not an option.

I actually tried to set a schedule with a high multiplier for designdays, and E+ produces a fatal error because it's higher than 1... This is a trick I had used in eQuest in the past that worked.
I tried doing the reverse (setting the ZoneInfiltration:DesignFlowRate to be equal to infiltration + OA, having a schedule with a multiplier of 1 for design days and a fractional (<0) multiplier for the rest of all days). That worked, but didn't reduce the unmet hours that much. I though it would, but no: It seems that even though you have specified you outsideair unit to have no heating and cooling coil, the autosizing stills tries to meet the load with this one and therefore undersizes the VRV.

I tried a bunch more things, looked at the controls of each unit etc... I've attached my modified .idf and a spreadsheet to illustrate my "infiltration" trick above

At this point I have no idea how to set this up so that the autosizing occurs correctly... I would ask the Energyplus Help desk...

You could always hardsize your VRV units though.

Let us know if you get an answer from the helpdesk

Best,

Julien

--
Julien Marrec, EBCP, BPI MFBA
Energy&Sustainability Engineer
T: +33 6 95 14 42 13

LinkedIn (en) : www.linkedin.com/in/julienmarrec
LinkedIn (fr) : www.linkedin.com/in/julienmarrec/fr

2015-04-20 23:20 GMT+02:00 ad3_prasetya@xxxxxxxxx [EnergyPlus_Support] <EnergyPlus_Support@xxxxxxxxxxxxxxx>:

Hi Julien

Thank you for the quick reply. Yes I have specified and defined ODA requirements per zone in m3/s (converted from cfm used in LEED's form). I put these in DesignSpecification:OutdoorAir and referenced in Sizing:Zone. Regarding to your suggestion, why should it be in ACH instead of m3/s? Eplus has our volume input already from our model doesn't it? And where should I put these ACHs in eplus item? Would you please be more specific. I had used ZoneInfiltration:DesignFlowRate before and ended up in total energy overcalculation with no significant reduction of unmet hours as I posted in Unmet Hours

Unmet Hours
A question-and-answer resource for the building energy modeling community.

View on unmethours.com

Preview by Yahoo

Another fellow expert advise me to do experiments in simpler model first (I created 5 conditioned zones) to find the solution, then use the same procedure in my project model (109 conditioned zones). Would you mind checking my simple models? I attached them here in case you are interested in examining further my problem.

Since I want to test VRF+OAmixer (ODA via VRF TU's mixer component) and VRF+OAunit (ODA is provided in separated unit, via ZoneHVAC:OutdoorAirUnit works alongside VRF TU) then I created 2 idfs, there are model_mixer.idf and model_OAU.idf respectively. They got identical model and HVAC main system, differentiate only by the method of providing ODA. Therefore, I suppose they both should have produced similar result on total energy as well as unmet hours. But in fact, they doesn't.

I had run both files in v8.3. The result is similar with my problematic project model, VRF with mixers has fewer unmet hours (179.25 hours) and the separated OAU possess way too many (1925 hours). As final goal, I should be able to reduce the number of unmet hours significantly in model_OAU.idf with its total energy consumption does not extend too far from the model_mixer.idf. We couldn't use mixers because our proposed design for LEED implements separated Mechanical Ventilation 100%OA.

Regards,
Ade

---In EnergyPlus_Support@xxxxxxxxxxxxxxx, <julien.marrec@...> wrote :

Hello,

Calculating outdoor airflow is simple. In your model you have already specified this (or you should have), probably something like airflow per occupant, per floor area, or a mix of both. I don't know where your project is located but as far as I can tell, it's the same everywhere, there's a code that mandates it and prescribes those values (ashrae 62.1 for example).

Anyways, in your case see what the peak outdoor air requirement is. Calculate an equivalent ACH (air changes per hour, typically m3/h of required Fresh air divided by space volume), and use that for your infiltration schedule for Design Days only. Or you could use an interior equipment to do the same if cooling is the problem. Same idea...

That'll get the sizing routine to get it right.

The calc isn't complicated, but if you need more guidance let me know,

Good luck,
Julien

EnvoyÃ© de mon iPhone

Le 19 avr. 2015 Ã 05:21, "ad3_prasetya@... [EnergyPlus_Support]" <EnergyPlus_Support@xxxxxxxxxxxxxxx> a Ã©crit :

Hi Julien,

I am still waiting for technical specs from our mechanical team, it seems that it will took a lot of time. Therefore, for now I couldnt use other inputs than autosized things.

In my latter post, I got explanation from Jean that the unmet hours problem arises from uncorrected HVAC sizing performed by eplus as you told me also before.

Here I would like to ask you this, since increasing tolerances or throttling range doesn't produce realistic results, how about I modify the sizing:parameters to compensate the undersizing phenomenon? Because when I increased the cooling sizing factor it seems everything is lighted up. Yes, as the result, total energy consumption is increased, but at logical rate, I think. Moreover, unmet hours problem is reduced significantly due to the increased HVAC capacity from factoring the autosizes. Then, if I use the identical number in sizing:parameters on LEED's baseline case, wouldn't the results be logical enough in context of comparing between proposed and standardized scenarios mandated by LEED?

Thank you for your attention.

Regards,
A