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RE: [EnergyPlus_Support] Re: Why Autosize does not consider System Outdoor Air Loads



Hi,

I model these systems a lot and so I took a quick look at your file. Two things struck me straight away

1. Plant sizing object has a delta T of 2.2°K on the hot water loop. This seems to me like a very small delta T for a hot water loop, mine is usually closer to 20°K

2. You have a VAV no reheat object
AirTerminal:SingleDuct:VAV:NoReheat,
    AT9,                     !- Name
    Always On Discrete,      !- Availability Schedule Name
    Node 180,                !- Air Outlet Node Name
    Node 186,                !- Air Inlet Node Name
    autosize,                !- Maximum Air Flow Rate {m3/s}
    Constant,                !- Zone Minimum Air Flow Input Method
    0,                       !- Constant Minimum Air Flow Fraction
    ,                        !- Fixed Minimum Air Flow Rate {m3/s}
    ,                        !- Minimum Air Flow Fraction Schedule Name
    ASHRAE_189.1-2009 ClimateZone 4-8 SecondarySchool Library Ventilation;  !- Design Specification Outdoor Air Object Name

Which is asking that the minimum air flow input is constantly zero. Changing this value could help.

It might also help to change the rated inlet air temperature on your heating coils from 16.6°C to something much lower, that way the coils will size larger. It is also likely that the temperature coming into these coils will be cooler than 16.6°C.

Annie

To: EnergyPlus_Support@xxxxxxxxxxxxxxx
From: EnergyPlus_Support@xxxxxxxxxxxxxxx
Date: Mon, 7 Jul 2014 18:32:30 -0400
Subject: Re: [EnergyPlus_Support] Re: Why Autosize does not consider System Outdoor Air Loads [1 Attachment]

 
Thanks. I attached a file that has a radiant system and a DOAS and four pipe fan coil to serve mechanical and restrooms. The DOAS will supply 12.8 C air temperature (by setpoint manager) all time (no reheat VAV box). The radiant system will provide cooling and heating to meet the rest of the load. Now my problem is during heating time, there is no air flow at all going through the DOAS system. I tried to increase the minimum airflow rate passing the VAV box to 0.3, now I see airflow rate but the supply air temperature is the same as outdoor air temperature (no heating at all). I would really appreciate if someone could look into this file and help me with this. This seems really strange to me because basically E+ thinks there is no zone load or ventilation load at all during heating time. (Cooling works fine though)

HJ



On 7/7/2014 11:58 AM, Jean Marais jeannieboef@xxxxxxxxx [EnergyPlus_Support] wrote:
 
Well then if your system supply temperature is not 30 C but 13 C, then leave the sizing:zone object with the 30 C for heating (and I think you said 17 C for cooling) and introduce some extra load by some other means...you could for example consider the enthalpy difference from 13 C 100%RH to 20 C at the given flowrate of your AHU (min ODA) and put in a negative process load equalling that (you may need to adjust your idd file to accommidate negative process loads.


2014-07-07 17:26 GMT+02:00 HJ wanghaojie630@xxxxxxxxx [EnergyPlus_Support] <EnergyPlus_Support@xxxxxxxxxxxxxxx>:
 
The problem I have during sizing period is that the room temperature is only 10C when I have radiant plus  radiant system serving the zone. During annual simulation, this problem disappear and I think it is due to the higher internal load such as people and receptacle (they are zero during winter design day). I think the problem that causing the room temperature so low is the radiant system sizing is only for zone load but not ventilation load (ventilation SA Temp is 13 C, so it need to be heated up to 20C as the heating setpoint by radiant system).

HJ



On 7/7/2014 9:57 AM, Jean Marais jeannieboef@xxxxxxxxx [EnergyPlus_Support] wrote:
 
It will work as long as the supply air temperature in hte sizing:zone object can combat the zone load (so for heating it must be more than the zone heating setpoint, and for cooling less than the zone cooling setpoint). In your sizing:system object you set the design supply air temperature as what you expect the air side system to bring (that excludes fancoils which are not system equipment, but zone equipment).The ADU sizing routine has a flag that will properly size the ADU on ventilation rate if the sizing:system object uses ventilation rate as the sizing method, otherwise it is sized from the information from the sizing:zone object.


2014-07-07 15:41 GMT+02:00 HJ wanghaojie630@xxxxxxxxx [EnergyPlus_Support] <EnergyPlus_Support@xxxxxxxxxxxxxxx>:
 
Thanks Jean. So for example, if I have heating setpoint of 20C. My DOAS is supplying 13 C and the sizing:system is also set as 13 C for central heating, but the sizing:zone heating supply air temperature is set as 30C. Will it properly size the system? The problem is that I have radiant system but the only place I can specify the water temperature is in sizing:plant. From the  EnergyPlus engineering reference, it says the sizing:zone will perform first to calculate how much air flow needed based on supply air temperature (specified in sizing:zone) and zone load. Then sizing:system will run to calculate the air system side. Then sizing:plant will run to calculate the plant water flow rate. However, based on this sequence, I think it will not work properly for radiant system. Or maybe they use different procedure for radiant system?

Thanks,

HJ



On 7/4/2014 1:06 AM, 'jeannieboef@xxxxxxxxx' jeannieboef@xxxxxxxxx [EnergyPlus_Support] wrote:
 
Hi Karen, as soon as you posted this, I went to test it, because it messes up my current understanding of the autosizing in e+. For Germàn's case, it does not work and fits my current mental model. For HJ's case, it would work as long as the zone setpoint is able to be corrected by the system's air supply. So if he has a zone heating setpoint of 20 and is trying to heat with 13 C system supply air, it will not size the equipment properly and the issue is the same as Germàn's.  

Mit freundlichen Grüßen- Sent from my iPhone (excuse the brevity)

i. A.
Jean Marais
b.i.g. bechtold

On 03.07.2014, at 23:53, "Karen Walkerman kwalkerman@xxxxxxxxx [EnergyPlus_Support]" <EnergyPlus_Support@xxxxxxxxxxxxxxx> wrote:

 

I think that you might not be setting the design heating and cooling supply air temperatures correctly in the Sizing:System object. If you have an air handler providing unconditional ventilation air to zones, set the design heating temperature equal to the outdoor temperature of your heating design day at the system level. Same with cooling. Then your ZoneHVAC equipment should size correctly.

Karen

On Jul 3, 2014 12:30 PM, "HJ wanghaojie630@xxxxxxxxx [EnergyPlus_Support]" <EnergyPlus_Support@xxxxxxxxxxxxxxx> wrote:
 
Thanks. I misspoke, actually s1 provide 13 air temperature all time. So in summer it provide some cooling, and winter it preheat air to 13 C. In this case, will s2 be oversized for summer and undersized for winter?


HJ



On 7/3/2014 12:16 PM, ecoeficiente@xxxxxxxxx [EnergyPlus_Support] wrote:
 
In your case, s2 will only be sized for zone loads without accounting for s1 ventilation load. But, as you say, your s1 system provides neutral air temperature, that is, no load. In that case you do not have the problem I mentioned.

Regards.









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Posted by: annie <anniemarston@xxxxxxxxxxx>


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