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[EnergyPlus_Support] Simultaneous heating and cooling in RTU


Hi everyone,

I've inherited an E+ energy model, and I'm experiencing a problem of simultaneous heating and cooling in a rooftop unit.

The RTU has: mixing box > cooling coil > heating coil > supply fan.

It then serves multiple VAV boxes with reheat (no cooling coil).

I'm trying to set a discharge temperature of 55°F when in cooling mode and 75°F when in heating mode. The spaces served have a Tstat of 77°F in cooling mode and 65°F in heating mode.

Currently, I'm seeing that, at all times (winter or summer), the mixed air is trying to meet 55°F (due to SetpointManager:MixedAir), if above it is cooled to 55°F and then reheated to 75°F.

I clearly have a controls problem.

Right now, there are two SetpointManager:Scheduled for the RTU: one is controlling the temperature after the cooling coil (setpoint of 55°F), and the other is controlling the temperature after the heating coil (setpoint of 75°F).
I also have two setpointmanager:Mixed air: one on cooling for the mixed air and one of heating for the supply temperature.

SetpointManager:Scheduled,
    RTU 1 Cooling Supply Air Temp Manager,  !- Name
    Temperature,             !- Control Variable
    Always 13.89C or 57F,    !- Schedule Name
    RTU 1 Supply Fan Outlet; !- Setpoint Node or NodeList Name

SetpointManager:Scheduled,
    RTU 1 Heating Supply Air Temp Manager,  !- Name
    Temperature,             !- Control Variable
    Always 24.72C or 76.5F,  !- Schedule Name
    RTU 1 Supply Path Inlet; !- Setpoint Node or NodeList Name
   
   
SetpointManager:MixedAir,
    RTU 1 Cooling Coil Air Temp Manager,  !- Name
    Temperature,             !- Control Variable
    RTU 1 Supply Fan Outlet, !- Reference Setpoint Node Name
    RTU 1 Heating Coil Outlet,  !- Fan Inlet Node Name
    RTU 1 Supply Fan Outlet, !- Fan Outlet Node Name
    RTU 1 Mixed Air Nodes;   !- Setpoint Node or NodeList Name

(RTU 1 Mixed Air Nodes = RTU 1 Cooling Coil Outlet + RTU 1 Mixed Air Outlet)

SetpointManager:MixedAir,
    RTU 1 Heating Coil Air Temp Manager,  !- Name
    Temperature,             !- Control Variable
    RTU 1 Supply Path Inlet, !- Reference Setpoint Node Name
    RTU 1 Heating Coil Outlet,  !- Fan Inlet Node Name
    RTU 1 Supply Fan Outlet, !- Fan Outlet Node Name
    RTU 1 Heating Coil Outlet;  !- Setpoint Node or NodeList Name


I'm wondering how I can properly set my controls to make sure I don't have simultaneous heating and cooling. I've thought about simply switching the coils, so that the heating coil is put before the cooling one, but there must be another way (and it would still try to meet 55°F in winter mode for the mixed air).

I must admit that I'm lost between all the setpointmanagers, availabilitymanagers, etc, so any help will be greatly appreciated.

Thanks,
Julien
--
Julien Marrec, EBCP, BPI MFBA
Energy And Sustainability Engineer
T: +33 6 95 14 42 13

LinkedIn (en) : <http://www.linkedin.com/in/julienmarrec>www.linkedin.com/in/julienmarrec

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 Posted by: Julien Marrec <julien.marrec@xxxxxxxxx>

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