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MAU Airflow Night Setback

asked 2015-08-11 17:20:05 -0500

updated 2017-05-17 12:33:07 -0500

I am trying to control a MAU that serves corridors in a apartment building to do a night setback of 50% airflow in OpenStudio. The unit is 100% outdoor air. The fan is variable. I have tried using the Maximum Fraction of Outdoor Air Schedule because it is the only input that allows a 0-1 fractional schedule, but this does not seem to be working. Has anyone been successful in implementing a measure like this? It seems like this should be a common control strategy and would be used in other applications like labs.

On another note, I am running into a weird outdoor air issue. On a VAV system, the outdoor air flow rate I put into the Design Outdoor Air Flow Rate in the OS:Sizing:System object and the Minimum Outdoor Air Flow Rate in the OS:Controller:OutdoorAir object overrides the Design Specification Outdoor Air from the zones. With the MAU system (it is controlled like a single zone system) the Design Specification Outdoor Air from the zones overrides the Design Outdoor Air Flow Rate and Minimum Outdoor Air Flow Rate. Does anyone know why this is?

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I've ran into these kinds of issues before. Have you tried specifying the Outdoor Air Flow Rate Fraction Schedule in the Design Specification Outdoor Air obj?

Lincoln's avatar Lincoln  ( 2015-08-11 21:41:00 -0500 )edit

Maybe it would be better to post two separate questions here.

Julien Marrec's avatar Julien Marrec  ( 2015-08-12 07:35:12 -0500 )edit

So using the Outdoor Air Flow Rate Faction Schedule and the Maximum Fraction of Outdoor Air Schedule I can finally get the OA damper to modulate to 50% at night. I am not able to get the fan to turn down to 50% though, the unit just does 50% OA and 50% RA. Any ideas on forcing the fan to turn down? Also, the 50% modulation of the OA damper doesn't work if I put in a lot of OA (say 2 cfm/sf). It seems like there is a point at about 1 cfm/sf where this works, and then if the OA goes above that it stops working. Any ideas on that, it is very weird.

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TaylorRoberts's avatar TaylorRoberts  ( 2015-08-18 09:09:26 -0500 )edit

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answered 2015-08-12 08:11:31 -0500

updated 2015-08-12 08:29:37 -0500

Question 1: There are 5 fields that apply to the amount of outdoor air.

1) Minimum Outdoor Air Flow Rate 2) Maximum Outdoor Air Flow Rate 3) Minimum Outdoor Air Schedule Name 4) Minimum Fraction of Outdoor Air Schedule Name 5) Maximum Fraction of Outdoor Air Schedule Name

Since this is a 100% outdoor air unit, I assume the minimum and maximum outdoor air flow rate fields are set to the same value. Try using the same schedule for 4) and 5) above and see if you get the variation needed. 3) above is used to modify the minimum and maximum outdoor air flow rate input field fraction to dynamically adjust the minimum outdoor air quantity. The reason there is a check for an air loop is because the actual system flow rate may be different than the maximum outdoor air flow rate specified in 2).

The Minimum Outdoor Air Schedule, 3) above, is used to modify the fraction of outdoor air based on the inputs in 1) and 2) as follows:

// set OutAirMinFrac
if ( AirLoopNum > 0 ) {
if ( AirLoopFlow( AirLoopNum ).DesSupply >= SmallAirVolFlow ) {
    OutAirMinFrac = MinOAMassFlowRate / AirLoopFlow( AirLoopNum ).DesSupply; (kg/s)
} else {
    OutAirMinFrac = 0.0;
} else {
if ( OAController( OAControllerNum ).MaxOA >= SmallAirVolFlow ) {
    OutAirMinFrac = MinOA / MaxOA; (inputs in m3/s)
} else {
    OutAirMinFrac = 0.0;
if ( OAController( OAControllerNum ).MinOASchPtr > 0 ) {
MinOASchedVal = GetCurrentScheduleValue( OAController( OAControllerNum ).MinOASchPtr );
MinOASchedVal = min( max( MinOASchedVal, 0.0 ), 1.0 );
OutAirMinFrac *= MinOASchedVal;

The flow fraction schedules in 4) and 5) are used to limit outdoor air flow rate:

if ( OAController( OAControllerNum ).MinOAflowSchPtr > 0 ) {
MinOAflowfracVal = GetCurrentScheduleValue( OAController( OAControllerNum ).MinOAflowSchPtr );
MinOAflowfracVal = min( max( MinOAflowfracVal, 0.0 ), 1.0 );
if ( MinOAflowfracVal > OutAirMinFrac ) {
    OutAirMinFrac = MinOAflowfracVal;
OASignal = max( MinOAflowfracVal, OASignal );

if ( OAController( OAControllerNum ).MaxOAflowSchPtr > 0 ) {
MaxOAflowfracVal = GetCurrentScheduleValue( OAController( OAControllerNum ).MaxOAflowSchPtr );
MaxOAflowfracVal = min( max( MaxOAflowfracVal, 0.0 ), 1.0 );
if ( MaxOAflowfracVal < OutAirMinFrac ) {
    OutAirMinFrac = MaxOAflowfracVal;
OASignal = min( MaxOAflowfracVal, OASignal );

Question 2: Regarding MAU, I assume this refers to the ZoneHVAC:OutdoorAirUnit. I would expect this system type to control using the zone outdoor air flow rate specifications.

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Asked: 2015-08-11 17:20:05 -0500

Seen: 298 times

Last updated: Aug 12 '15