IES-VE User Forum

Dear Community,

I am working on a project with a very odd problem. I have setup the model and the baseline HVAC network using the appropriate prototype systems. After I run the room and system loads calculation and assign the sizing values, I went ahead and ran the PRM sim to be shocked with 4000+ unmet hours in the baseline.

After some investigation, I found out that I would need to "oversize" the loads by up to x6 in order to get rid of the unmet hours. I have checked the loads in my zones and they mostly report higher load than what was originally sized by VE. This is also observed when checking the air temperature leaving the coils, it seldom meets the SAT setpoints.

I am wondering if anyone has experienced this, or has some idea on what might be going on.

Thanks for your support.

*bump*

Hi,

There can of course be numerous reasons why a model has unmet load hours even after the autosizing, it seems your model must have encountered some scenario where the loads were not calculated showing up as they do in the full simulation or else the sizing wasn't being applied correctly. If using VE 2016 then the ISM System Parameters dilaog should help you to query the rooms in your networks and run the engineering checks etc

we have collected a while back a few of the more typical reasons autosizing may not be adequate or may require more input from the user

http://www.iesve.com/support/knowledgebase/faq/3615

http://www.iesve.com/support/knowledgebase/faq/2915

Phil

Hi Phill,

Thanks for your response, yes I typically use these guides to solve unmet hours. I have identified the problem as too low cooling load is being sized for majority of the rooms (some rooms even with 0 cooling load), despite the fact I have went over and double checked my internal gains assignment and profiles.

It would be helpful if IES can provide us with more info/methods to troubleshoot incorrect autosizing by VE, as sometimes it is quite embarrassing to answer clients "why have you oversized by x6"...

While we are on the subject of autosizing, I believe there is another major problem when using the PTHP system. The baseline prototype introduces fresh air to the spaces directly, by passing the cooling coil. This of course results in the sizing runs not seeing the extra load caused by the fresh air. I often need to manually calculate the load and then add that load manually to each zone.

Thanks and Best Regards,

Hi,

I refer to the notes i posted as examples because I'm not sure we could make a robust guide that would fit all the different modelling scenarios out there, just the most typical. Probably just my take on this but if we could preempt and handle every situation then we'd build them all into the autosizing. What you have to keep in mind is the thermal model that calculates the loads initially and then the full dynamic model that is simulating with the HVAC system linked in and a DSM climate file, if fresh air being supplied in the HVAC network is causing heating and cooling demands to be wrongly estimated then add that into your thermal model via the thermal template for the purposes of calculating those loads, remembering as well that the sizing is supposed to be based on worst case conditions.

There likely will always be a requirement for the skilled user to look through simulation results and detect and then investigate any areas where sizing wasn't adequate so the system configurations and controls can b adjusted but at least a simple check like unmet load hours gives you a head start in that QA process.

Phil

Thanks for the feedback, I guess every simulation has its limitation. These kind of problems only seem to appear in some projects only.

If I'm experiencing the same issue, the AHU coils (simple CHW) are continually autosized to the same value of 0.034 kbtu/h.

MatthewSteen wrote:If I'm experiencing the same issue, the AHU coils (simple CHW) are continually autosized to the same value of 0.034 kbtu/h.

I have seen this happen before, no matter what resizing you do you always get the same value, it seems to be some sort of bug but i don't really know what causes it. I have found that changing the coil from the Advanced model to the Simple model and then running the sizing you will get a better calculation. Usually you can usually change back to the advanced coil immediately and resize, and get the correct sizing.

Thanks Simon, I haven't thought of that, it might actually work. Will try it next time this issue presents itself.

I've found the same workaround that @Simon describes to work. Definitely seems like a bug.