Trouble Meeting heating set-point in the morning?

[NebK22]

Hi all,

I'm simulating an office building with a VAV-reheat system but I seem to be having trouble meeting heating set point (70 deg F) for morning start up.

The office space is broken up into perimeter and interior zones, with only the perimeter zones having heating. The HVAC system is purely out of the box, I haven't done much with it. The system is autosized, and the office opens at 6 AM and the HVAC system has a 1 hr warm up period per the systems schedules and set points dialogue box.

I can't figure out why it isn't meeting set point or why it seems to be undersized...more so the interior zones seem to be having trouble meeting heating set point in the morning, which makes sense since they don't have heating coils, but is there a way to fix this?

Thanks

[farahghanem]

check this guide to help you troubleshoot unmet hours

http://www.iesve.com/support/faq/pdf/un ... -hours.pdf

Can you show screenshot of your system?

[NebK22]

Hi Farahghanem,

Thanks for the response!

As it turns out, it's having trouble in both heating and cooling. See below for my ApHVAC System:



It looks like the hot and chilled water loops are fine, they are meeting their target supply temperatures, however, I'm having some trouble when it comes to the target air temperature off the coils:

Heating:


Cooling:


I thought that it may be a control issue, but looking at the airflow controllers, they make sense:

Heating airflow controller:


Cooling Airflow Controller



Any thoughts on how to meet the supply air temperature target?

[farahghanem]

this looks like a coil capacity/control issue. Your coils (and probably chillers/boilers in turn) were not:

1. sized properly

OR

2. The coils are not switching on.

Check for coil control first to make sure it is operating on similar profile to your flow controllers. If not then adjust that. Check also if all rooms are demanding air flow during those times when the coils are not meeting the set-point.

Next check the coil sizing parameters. Make sure the coil has the same total flow specified as required by your zones, and that the entering air conditions are sized properly. Also make sure that when you run sizing to do so for entire year instead of the default months. If all else fails, manually increase the coil sizes and see if that helps.

How large is the project in terms of ft2? If it is not especially large I would be interested to take a look at it if the above did not help solve the issue.

[Lemcarr]

just my opinion,

when I'm making my own system, I start it simple with less controller (only the parameters) , to see if its working.
if it passed, then I'll add controller (sensor) bit by bit and check if it's still working. it may take time, but I know what or which is wrong if something goes off.

cheers,

[NebK22]

Farahghanem,

Thank you for offering your help! I hope to resolve this issue soon, but if I continue to struggle, what would be the best way to share my project with you? The project is about 160,000 sq.ft. (8 stories) but I'm currently playing around with 3 stories to minimize run times.

My current sizing process:

1) Take out all diversity in loads by turning variation profiles ON continuously, ALSO using Peak design values (example - peak design occupancy for office would be 100 sq.ft. per person)
2) Change ventilation rates (ASHRAE to Title 24 in the ISM)
2) Run room load sizing in ApHVAC
4) Run system load sizing in ApHVAC
5) Re-apply diversity to variation profiles and use expected design values (example expected design value may be 200 sq.ft. per person)
6) Run dynamic simulation in Apache

Is the above sizing process correct? I'm assuming that when a design change is made in ApHVAC, say a change in SAT of a coil, then the process above has to be repeated. But if a change happens to an economizer control, the above process should not need to be repeated because the loads are sized assuming the economizer is not functioning.

So if the above sizing process is correct and the room heating/cooling set points and HVAC schedule are correct, is it typically safe to assume that there is a control issue as you have mentioned?


Lemcarr,

Thanks for the response! I'm currently using the out-of-the-box VAV-reheat system, but will need to customize the IDDE VAV system in a minute. I'll take to your advice and follow that process.


Thanks,

[farahghanem]

hmmm... I am not sure you should be changing the diversity factors, this could be the problem. Try just using the default diversities and loads from the ASHRAE database already included in VE.

How many unmet hours are we talking about here btw?

I was hoping your model is not that large :p nevertheless if all hope is gone I'll try to take a sneak peak. You can send me the cab file via e-mail (probably need to upload it to wetransfer first and send the link).