Central AHU outlet air condition problem

Hi at all! I am working at my master thesis on EnergyPlus, I have to modifying the DOE Large Hotel Post 1980 model in order to test different HVAC system. I am finding some problem during the definition of a central AHU. My idea is to create a central AHU that pre-conditioning the outdoor air (I defined a range of RH and a range of T that have to be respected in some nodes) and then satisfy the remaining loads in the different zones with fan-coils.

I had two problem:

1 - I defined the outdoor air flow rate according to the minimum outdoor air requirement (sum of all zone minimum requirements). I checked the outdoor air flow rate in each zones, it changes according occupation but work only at 0 or 100%. There is a way to regulate the outdoor air flow rate according to the real occupation of a zone?

2 - I checked the RH and the Temperature in the nodes in which I defined the SetpointManager and they are not respected sometimes. What could be the cause?

For defining the AHU I used the following elements: - AirLoopHVAC:UnitarySystem - SizingSystem - Coil:Heating:Water - Coil:Cooling:DX:SingleSpeed - Humidifier:Steam:Gas - SetpointManager:MultiZone:Heating:Average - SetpointManager:MultiZone:Cooling:Average - SetpointManager:MultiZone:MinimumHumidity:Average - SetpointManager:MultiZone:MaximumHumidity:Average

Thank you!

Andrea

AndreTarta

asked 2016-10-14 03:20:19 -0500

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__AmirRoth__

updated 2016-10-18 14:37:15 -0500

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From your description I suspect cycling fan operation. An AirloopHVAC:UnitarySystem using Setpoint control should maintain the node set point temperatures and also provide outdoor air with non-zero values. The OA control per occupancy also requires use of the mechanical ventilation object.

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rraustad

answered 2016-10-18 19:37:19 -0500

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Thank you for your reply! In the AirloopHVAC:UnitarySystem I used Fan:VariableVolume element. I checked the air mass flow rate and it's always different from zero, it vary from 14 to 26 kg/s. In Design:Specification:OutdoorAir I created a schedule for each zones, but the schedule can be only 1 or 0, for example I cannot insert 0,2. There is a way to solve this problem?

AndreTarta (2016-10-19 10:58:45 -0500) edit

The Design:Specification:OutdoorAir object tells E+ the "maximum" OA per zone, although a schedule can be used to change that maximum based on time of day. The Controller:MechanicalVentilation object is used to tell E+ to modulate OA based on other inputs (occupancy or IAQ).

rraustad (2016-10-19 11:51:49 -0500) edit

Thank you rraustad! I solved this problem and now the OA is in function of the real zones occupation. Now I have some problem with "SimHVAC: Maximum iterations (20) exceeded for all HVAC loops" I read a lot of previous discussion on the possible causes and I tried to fix the most common causes (only one fan in the air loop, SetpointManager after the coils) but the problem persist and I have more than 900000 warnings. There is something else that I can try?

AndreTarta (2016-10-28 07:11:16 -0500) edit

SimHVAC warnings usually occur when the SA temperature and/or flow rate are changing each iteration of a single time step while the simulation tries to converge. Try using constant fan mode to see if that helps. Also try using a constant SA temperature set point for the main air loop cooling and heating coils to see if that makes a difference.

rraustad (2016-10-28 07:52:48 -0500) edit

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Central AHU outlet air condition problem