Question-and-Answer Resource for the Building Energy Modeling Community
Get started with the Help page

# Outside air flow rate is not constant for WSHP OS

I am modeling a building served by water source heat pumps, the first option was to serve the outside air by DOAS, an alternative design is introducing the OA directly to the WSHP. When I used the DOAS the outside air was delivered at a constant rate and fluctuates between the minimum OA and 0 based on the operation schedule of the DOAS as shown in the first image. However when the outside air is delivered directly to the WSHP it seems to fluctuate randomly and never reaches the minimum OA rate as shown in the second image, I was expecting the OA in this case to be constant 24/7 since there is no schedule governs the operation of the WSHP. Any ideas what is causing this?

Thank you,

Air Outlet Node

OA inlet Node

Air Outlet Node and Air Outlet Node

edit retag close merge delete

OA can only flow through the WSHP when the WSHP fan is on. You need to check OA flow at the inlet to each WSHP to see more detail. Plot WSHP supply node and OA inlet node flow to the WSHP.

( 2019-03-05 08:24:32 -0500 )edit

I plotted the supply node and OA inlet node flow and added them to the question. The OA is still looks fluctuating. Also I am not sure why the supply air flow is not constant since the fan is “Fan:OnOff”

( 2019-03-06 08:47:36 -0500 )edit

It appears that the WSHP is cycling to meet the load which will vary the air flow rate during each time step. What is the object name? Is it ZoneHVAC:WaterToAirHeatPump? Is it AirLoopHVAC:UnitaryHeatPump:WaterToAir? Answer this question and a more specific answer can be provided.

( 2019-03-06 09:45:15 -0500 )edit

it is ZoneHVAC:WaterToAirHeatPump. If the fan is constant flow shouldn't the flow stay constant all the time? I am thinking of this unit like the AC in my apartment either it is off and the flow is zero or it is on and the flow is 500 cfm, not sure if this hold true in EnergyPlus simulation

( 2019-03-06 10:14:19 -0500 )edit

In EnergyPlus a cycling fan cycles based on load each time step. This is analogous to the AC system in your home operating for only a portion of the hour. If the load is high, the AC would operate the entire hour, if load is low the system will cycle.

( 2019-03-06 11:17:22 -0500 )edit

Sort by » oldest newest most voted

The ZoneHVAC:WaterToAirHeatPump has an input field that allows a schedule to be used to control the fan operation. If this schedule is not used the fan defaults to cycling fan. The fact that the fan is Fan:OnOff does not determine whether or not the fan cycles, it's the schedule value (1 means constant fan).

From the description of the field in the IDD (the Input Output Reference has similar text).

ZoneHVAC:WaterToAirHeatPump,

A17, \field Supply Air Fan Operating Mode Schedule Name
\type object-list
\object-list ScheduleNames
\note Enter the name of a schedule that controls fan operation. Schedule values of 0
\note denote cycling fan operation (fan cycles with cooling or heating coil). Schedule
\note values greater than 0 denote constant fan operation (fan runs continually
\note regardless of coil operation). The fan operating mode defaults to cycling fan
\note operation if this field is left blank.

more

The issue is solved when schedule value is grater than zero, but when the fan is cycling the flow rate is not constant and the fan power follow it linearly as you can see here. The Fan:OnOff documentation says This object models a constant air volume fan that is intended to cycle on and off in tandem with a cooling or heating system (i.e., AUTO fan control mode). its clearly states that the air volume is constant, so I am not sure if this is an issue in EnergyPlus?

( 2019-03-06 15:20:31 -0500 )edit
1

The way E+ models the on off fan is to calculate a part load ratio (PLR is a fraction from 0 to 1) for each time step. The fan is on for PLR of the time step and off for (1 - PLR) of the time step. The average air flow over the time step is PLR * max fan flow rate. The fan in your home does the same thing except over a longer period of time. If your fan at home is on for 15-min and off for 15-min, then PLR = 0.5. The air volume flow is constant when the fan is on (also when it's off but then it's 0). Nothing is wrong with the E+ fan model.

( 2019-03-06 15:26:57 -0500 )edit