Why is this Energy Plus VAV system operating at maximum airflow when all connected zones require heating?
I've been trying to troubleshoot this issue and have run out of ideas. Any suggestions welcome. The system is a VAV AHU with chilled water and heating hot water coils. There are only two zones served by the system. The hot water coil is located upstream of the chilled water coil, but I don't believe that is relevant. There is no OSA flow on the system (this is due to low occupancy and the use of IndoorAirQualityProcedure to determine the ventilation required.) Currently the minimum outside air value is input as zero (I'm aware there should be area-based ventilation provided such that the minimum is not zero, and I plan to fix this later.) The AHU heating coil is set up to heat only up to 10C/50F. The AHU cooling coil is controlled using the SetpointManager:Warmest. At times when there is a heating load in the connected zones, the AHU is supplying at the maximum design flow rate. The system supply temperature is at the maximum reset temp of 15.3C/60F, (as expected since no connected zone required cooling). This results in considerable cooling at the AHU cooling coil, as needed to cool the mixed air (which is 100% return air since zero system OSA flow), followed by considerable reheat at the zone terminal reheat coils as needed to maintain the heating setpoint temperature in the zones.
No heating occurs at the AHU heating coil.
In the case of heating load at all connected zones, and a system OSA flow rate of zero (economizer is not active), the AHU should supply at minimum flow, but here it is supplying at maximum flow.
One idea I had was that the DesignSpecification:OutdoorAir Inputs at the terminal units could be forcing the terminal flow rates up in an effort to deliver needed outside air (since the system is operating at zero outside air for the time of interest, but the DSOA object would indicate some is needed in the zone.) I have already tried removing that input, but the results did not change.
Another idea I had about what might be going on is that the heating load at the connected zones, combined with a very low heating temp (10C/50F) from the AHU could erroneously result in maximum flow at the system? (In this case we have a negative supply air to room air temp difference, and the airstream delivered will actually result in cooling, not heating of the zone. This should not be the result but I was thinking about the fact that very low delta T could result in max system flow) Any thoughts on how I might test to see if this is the problem?
Any ideas about what might be the problem? I've been looking at this for a long time - any new ideas will be much appreciated.
UPDATE 2/15/18: I ran the model for a short period with 10 timesteps per hour and reported at the timestep ...
Looks like a time when outdoor air temp is below the mixer return temp and economizer kicks in. Do you have an economizer?
Thanks for your comment @rraustad. Yes, I do have an economizer on the system with Fixed DB high limit at 75F. But, for the time period of interest, the system OSA flow is zero. I took that to indicate the economizer was not influencing the system flow rate. The graph shows there is zero flow rate at the system outside air inlet node.
Sorry the graph is not very legible. I will attempt to improve it. I also have added the MA SP to the graph.
Are the VAV terminals Damper Heating Action set to Reverse? Are the Zone Heating Sizing Parameters, Heating Maximum Air Flow Fraction set to 1 or less?
Thanks for your comment, @TomB. Yes, for the VAV reheat terminals, Damper Heating Action field is set to "Reverse". In the Sizing:Zone object, the "Heating Maximum Air Flow Fraction" field is left blank. The I/O reference indicates the default value is 0.3.
This graph is a little misleading. I see what I think is 13.29 m3/s of mixed air outlet flow rate. I also see another mixed air outlet at 2.7 m3/s. To me the mixed air outlet is at the outlet of the OA mixing box so I'm not sure what I am looking at. Do these 2 data points represent air loop flow rate or zone terminal unit flow rate? Is this just 1 air handler? I assume just 1. What are the 2 terminal unit flow rates? Is the other zone at 10.6 m3/s? One zone load at 0 W (2.7 m3/s?) and the other at 895W (10.6 m3/s?). If you 0 the people, do things begin to control as you expect?
@rraustad: Whoops, sorry about that. When I updated the graph, I inadvertently added a variable from a different model. I have updated them again. The 13.29 is the air loop flow rate. I need to rerun to get the flow rates at the two terminal units, so I'll do that later and update again. I'll also try your suggestion to remove all people.
I believe @rraustad was on track with the comment regarding economizer. The system ought to be using using the economizer, rather than the cooling coil to get to the needed AHU SAT. For some reason, the economizer is not operating. I will next try changing the
Controller:MechanicalVentilationsystem outdoor air method. I ran the model with 10 timesteps per hour for a test period and will add info at the bottom of my original question post. You will see the behavior still occurs but not as severely as in the hourly simulation, which had only 4 timesteps per hour (below the recommended min)The mixed air T = 21.26C and mixed air set point = 14.52C. So yes, the economizer should open if capable. What is the OA Controller min/max OA flow rate in eio file? What is the economizer control type? What are the economizer limits?
Also, it's when the OA mixer OA temp goes below the OA mixer return temp that the economizer kicks in.
Thanks for the help, @rraustad. I found the reason Economizer is not working: "MinimumFlowWithBypass" is input for the field "Economizer Control Action Type" in the
Controller:OutdoorAir, at these low occupancy times minimum OSA flow is zero, so no economizer operation. I still don't understand why the airloop flow is going so high. How is airloop flow determined when connected zones need heating? I will try adding a value for the "Maximum Flow Fraction During Reheat" at the connected air terminal objects, as @TomB mentioned, to see what that does.The damper modulates to meet the zone load given the current SAT. OA requirement can increase amount of flow. You can limit max flow during reheat using
Maximum Flow Fraction During Reheat. CompareZone Predicted Sensible Load to Setpoint Heat Transfer Ratewith Q = Mdot * (hair,out - hair,zone) to see if the load is met (i.e., use Output:Variable reports for mass flow and enthalpy).You might also answer your own question when you figure out exactly what caused this so others could benefit.
If the damper action is reverse, then perhaps the system is actually doing what it 'should' be. what happens if the damper action is set to normal.?
@TomB: Thanks for the suggestion. Good idea to check that. Oddly, the pattern of high mixed air flows during morning warmup still occurs when I set the field "Damper Heating Action" to "Normal" on the
AirTerminal:SingleDuct:VAVReheatobject.