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Our team has just got a laboratory project that some of its rooms need to be pressurized (positively induced) whereas others are in contrary need to be reduced (negatively induced) by using mechanical ventilation (intake and exhaust fans). Is there any way to simulate this in EPlus? We had tried by utilizing Outdoor Air Unit and Exhaust Fan but it resulted on high number of unmet hours.. Yet our aim is to achieve LEED rating by produce less unmet hours (<300 or so).

We've observed EPlus report also and didn't find anything which depicts this positive/negative zone case, any information on this would be appreciated.

Yes, that is exactly what I did. Except that I combine VRF Supply Inlet and DOAS Supply Inlet within the same Nodelist. Then DOAS Return is connected into Zone Return Air Node and VRF Return is in Zone Exhaust Node. Is it possible to connect VRF Return in Zone Return Air and therefore not using Zone Exhaust Node in attempt to connect DOAS Supply into VRF's OA Mixer? Is there any way to share idf file in this website? I would send you mine if possible. Regards,

Could you mention which example files I should look for? In EP v8.4 there are bunch of them. Yet mainly they need sensors, actuators, and programs to give orders. Moreover, I guess there are some keywords in giving orders which should be learned first. Since I'm not proficient in computer language, could you provide an example file which is more specific in adjusting VRF TU's inlet condition by using EMS?

Which one do you mean, OA node of VRF TU or ADU TU? For your information, if zone fresh air is served by DOAS, I exclude OA mixer in VRF's TU and directly connects the return air to the TU's cooling coil hence VRF's TU acts like conventional split wall AC which recirculates air inside the zone without any means in providing fresh air from outside. Is it logical? Regards

For my second question, with the logic that the used air inside zone is exactly the same as the air which is sucked into the return and later be conditioned inside VRF TU, is it okay for us to assume that the mixing is occurring inside the zone? In fact, location points of DOAS supply in our designed zone are always installed near the VRF's return (either with and without duct). With this logic, could we just model DOAS and VRF conventionally in EnergyPlus?

diagram.jpg Here I attached the complete diagram of my HVAC system, is there any suggestion how should I model these in EPlus?

Thank you for the very quick reply. Indeed, when I checked the sizing report it shows each TU has the same output. At first it worries me because I thought the energy consumed would be doubled too. But in the end its not, since total cooling energy used is actually lower. Please check my answer in this post to get clearer diagram of my HVAC system in my second question.

Hi, Unmethours,

Is it possible to model more than one VRF indoor units in the same zone? My case is a big laboratory space served by three VRF indoor units with the same outdoor condensing unit. Can I use ZoneHVAC:EquipmentList to add similar TU system? Or should I divide the zone into smaller zones representing an imaginary zone served by each indoor unit and connect all three zones with voids? Which one is best? Since I recently managed to model multiple indoor units and the result showed slight reduction in cooling usage.

Second question, is it possible to link the supply air from secondary outdoor air system (DOAS) into each primary VRF indoor unit's mixer rather than directly blown into the zone? Presumably by defining ZoneHVAC:EquipmentConnections and tweaking OutdoorAir:Mixer? This attempts always produce severe error messages because DOAS' supply node which was initially serving the zone wont connect into VRF's mixer node.

Any help would be greatly appreciated,

Regards,

Thank you MJWitte, I have tried a simple simulation and when I close the return air with 0 fraction I got a great number of unmethours (2xxx hours).. more problems to solved yet again. I guess I need to hardsize the DOAS fan then. More question; in EPlus, is it possible to model more than one similar indoor units (ex. 3 VRF indoor units) inside a big zone? Or I need to break down the zone into smaller zones, each in regard to the placement of installed indoor units? Regards,

Is it possible to model DOAS system without Return Path? Frankly, we are trying to introduce outdoor air from outside via ducting network with a box-in-line fan which delivers fresh air into several diffusers/grilles in different rooms. Fresh air is then blown into destined rooms without any return network, the air is assumed gone via exfiltration. Should I go with DOAS system or Unitary Outdoor Air Unit? Any help would be appreciated.

Nice, and what exactly is Setpoint for Design? Is it as DesignDay for Sizing the HVAC? When I tried different number, the results are changed too. Then for unconditioned outdoor air (NeutralSupplyAir), is it okay to set the low setpoint same as winter designdays and high setpoint same as summer designdays?

Is there any update about EP 8.4? Can it model DOAS effect on zone sizing inside conditioned VRF rooms?

@MJWitte thank you for the solution, I would try that later. I've read in an eplus source (bigladdersoftware.com) that we can model a lone exhaust fan in a single zone. Can we do the opposite ie. model a lone supply fan without any return or exhaust fan? Our planner has installed supply fan only without any return grill or exhaust fan to provide fresh air from outside. Is it possible to model that in eplus? The fresh air circulation relies on exfiltration through doors or windows.

@MJWitte, thank you. I do hope so. Is there any other method to compensate this deficiency rather than waiting for the next version of EPlus? Aside from increasing the infiltration rate either.

Currently there is a new published version of eplus (v8.3). Does it include ventilation loads from OAU already? How long we should wait for the mentioned version?

Best regards,

.. it only produces around 1800 GJ without any issue on unmet hours (30 hours a year). It is set with the exact same autosized settings. When I allowed VRFs TU to provide autosized oda, instead of just relying to OAU, unmet hours is decreased further to just 700 hours. Yes this shows us that OAU is neglected from HVAC sizing. Our model has 109 conditioned zones and 26 mech vented zones.

Thank you for the advice. I had tried to include infiltration rate equal to oda's cfm. The result shows a slight reduction in unmet hours (3700 become 2200 hrs). In the other hand it increases total energy consumption from 1700 to 2000 GJ. I'm afraid this wouldnt be realistic since in a case where I include oda inside VRFs TU

Yes, most of them 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. Are they still far from enough? We are still collecting other technical information from our MEP team and it seems that this is not an easy task since this is new to us hence we are not used to in terms of documenting details needed by LEED.

Best regards,

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

Regards,