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

Note: e+ v8.3 was released just a few days ago.

It sounds like you may be having several problems contributing.

If you have long run times, stop what you are doing and create a new 1 Zone model for testing. Give it the same HVAC systems and the same loads and air per squaremeter.

Strictly, sizing should not be done for the designcase model. You need the actual sizing information from the design. If you are using e+ as a design tool, it does make sense to autosize.

Sizing works from the sizing:zone object (which specifies how the hvacidealloads will run during the sizing simulation). However, it works also in conjuction with the designoutdoorair object. Your job is to specify these objects as if they must bring both lots of ODA and cool it down from the entry temperature of (for example) 38 deg C design to thermostat setpoint.

So terminal units will size their flowrates based on the airflow rate deturmined from the zone:sizing object at the supply temperature that you specified. Some terminal units have heating/cooling coils like your VRF terminals. These coils should be sized for the correct airflow rates and in your case it means knowing the design supply air temperature of your VRF. How much will be deturmined during the sizing summerdesignday simulation. The amount of extra outdoor air load should be taken into account by the program via the designoutdoorair object. Your ODA units are zone hvac. Many objects can be used to bring ODA into a zone. The workaround of the infiltration object should not be required, but ask the helpdesk to be sure.

Make sure that all your schedules observe the designdays day types in a way that makes sense. For example, on the winterdesignday you would have all the lights off all day, whilst on the summerdesignday they would be either on the whole time or follow the normal schedule. Check this for all schedules. Try not to step temperature setpoints, but ramp them.

Check the .err file error report. It usually has some clues.

Check your massflows and temperatures at fluid nodes and try and spot when the loads are not being met.

Check the eio file to see if the sizes of the equipment make sense.

Don't simulate more than a day until you've found and sorted out all the issues.

SetpointManagers have been an issue for me in the past. Report the setpoints on each node on which you are having a setpointmanager place setpoints...are they what they should be?

There are many things I've not mentioned...there are loads of discussions on this topic.

As for e+ not accounting for ODA loads in the sizing...I think most of these issues have been worked out since 2014 (I searched around on gitHub a bit). That means that if you understand how to use the sizing objects, you should be okay and the workarounds of the past may not be nessecary anymore.

If you have bits of idf that you are unsure of, post those. Post errors that you don't understand.

Mit freundlichen GrÃ¼Ã?en- Sent from my iPhone (excuse the brevity)

Jean Marais

b.i.g. bechtold

On 04.04.2015, at 20:43, "ad3_prasetya@xxxxxxxxx [EnergyPlus_Support]" <EnergyPlus_Support@xxxxxxxxxxxxxxx> wrote:

We are undergoing energy model calculation in e+ to meet LEED's mandatory requirement. Our new construction will be a medical office and lab conditioned with VRF system complemented by independent Outdoor Air Units (OAU) to meet IAQ requirement. This OAU consists of supply and exhaust fans only (no coils, mixers, or exchangers). The result of this combination is a huge amount of unmet hours (up to 3700 hours a year). Therefore in e+, it is also possible to introduce OA without adding independent OAU by using mixer embedded inside VRF's Terminal Units. When I implemented this (both cooling and OA provision only through VRF Terminal Units), with the exact technical specifications and the same amount of outdoor air provisions, there was no unmet hours problem occurred (only around 30 hours a year). How this could be happened? Logically, it would behave similar with the OAU equipped. Or maybe I missed something?

For additional information, our project location is in tropical humid climate, Surabaya-Indonesia. Most of the equipment were autosized. The two and only defined input were just the rated COP of each condensor and the volume flow rate (debit) of outdoor air required in each zone. This is because we are still collecting technical information from our MEP team, this is not an easy task since this is new to us and we are not used to in terms of documenting details needed by LEED.

I have asked about this topic in another forum, and experts there suggested that I utilize ZoneInfiltration:DesignFlowRate to take into account ventilation loads from OA units since autosizing in EPlus does not include this type of load as thermal and envelope loads does, leads to undersizing and lots of unmet hours. Nevertheless, when I examined the infiltration + VRF result, it showed far higher Cooling Electricity End Use compared to when I used OA mixer inside VRF TU (811 to 622 GJ). Even though they both have similar amount of OA volume flow rate (m3/s) introduced into the zones (109 conditioned and 26 mech vented zones). My conclusion, I don't think that this method would solve my case since unmet hours are still appeared (700 hours). Any suggestion?

Additionaly, I was told that EPlus team is trying to solve this in their future version of EPlus. Anyone knows how long will it be?

Regards,
Ade