Add thermostatic fan control to series fan-powered induction unit

[mjwitte]

Issue overview

#5918 and #5879 added thermostatic fan control to parallel fan-powered induction units (AirTerminal:SingleDuct:ParallelPIU:Reheat).

Addressed in #6285 - This suggestion is to add the same for series fan-powered inductions units (AirTerminal:SingleDuct:SeriesPIU:Reheat.

Addressed in #6285 - For parallel PIUs, the thermostatic fan control is selected by entering zero for "Fan On Flow Fraction". Series PIUs have no such field, so a new field will be needed, unless there is consensus that series PIU fans should always be thermostatically controlled and this is a bug-fix. If a new field is added here, then the same field should probably be added to the parallel PIU.

User description of test file results

In the attached 5 zone VAV system example, during unoccupied period, when there is only one PIU zone call for heating in unoccupied period, all PIU fans activated, all zone heating baseboards turned off if PIU reheat coil can meet zone setpoint.

In the attached example file, I have baseboard heater for each zone to provide heating during unoccupied period, they are set to be always available. But for space 2-1, the baseboard heater is intentionally undersized (capacity 345 W) so that at times it will need the PIU to turn on to provide heating during unoccupied period.

Following are two examples.

Example 1:
At HVAC timestep 441 (8:10), all PIUs are off. Space 2-1 temperature drops below heating setpoint (18.33 C) with baseboard heater at full capacity. All other zones are either at heating setpoint with baseboard heating or in deadband with baseboard at 0 W.

At the next HVAC timestep 442 (8:11:40), PIUs for space 1-1, 2-1 and 3-1 turned on with reheat coil on. The parallel PIU fan for Space 4-1 is off since it’s in deadband. For space 2-1, looks like the PIU heating is not enough to bring zone to heating setpoint, so the baseboard heater is still on. For Space 1-1 and 3-1, PIU reheat coil is enough to maintain zone at heating setpoint, so baseboard heaters are off. This is determined by the fact that we specify baseboard heater as 2nd heating equipment for the zone in the “ZoneHVAC:EquipmentList” object. However, in this situation, it is more logical to keep the baseboard heater running and use PIU as the 2nd heating equipment, since officially it is still in “unoccupied period”. But I understand this might be a separate issue.

Example 2
At timestep 468 (10:40 :00). Again space 2-1 is below heating setpoint by more than half of the thermostat tolerance. Space 1-1 temperature is in deadband.

Then at timestep 469 (10:43:19). PIU fans for Space 1-1, 2-1, 3-1 are turned on. For Space 1-1, with the series PIU fan drawing cold air from plenum, the space temperature starts to drop. If its PIU were kept off, the zone should be in deadband.

Details

Some additional details for this issue (if relevant):

• Version of EnergyPlus v8.7.0
• Helpdesk ticket number 12019

Checklist

Add to this list or remove from it as applicable. This is a simple templated set of guidelines.

• Defect file added EnergyPlusDevSupport\DefectFiles
• Ticket added to Pivotal for defect (development team task)
• Pull request created (the pull request will have additional tasks related to reviewing changes that fix this defect)

[mjwitte]

Actually, as I re-read the issue description, it's only a problem during unoccupied times - we just need the PIU fans to be smarter about who turns on during night-cycle operation. I base this on these two documents

p. 44 of
http://www.trane.com/download/equipmentpdfs/series_vavprc008en.pdf

and

p. 7 of
https://www.titus-hvac.com/docs/2014%20catalog/fanpowered_2013.pdf

Now that the night-cycle manager has the concept of "run just enough", maybe the zonefans night cycle flag should be recast to be per zone and then this could all be automatic without needing to change the FPIU inputs at all.

[mjwitte]

Closed via #6285