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PTAC Zone equipment, no cooling load, no electrical load

asked 2016-01-28 00:42:52 -0600

olivier's avatar

updated 2017-05-04 06:09:25 -0600

__AmirRoth__'s avatar

Hi,

Being still a novice in Openstudio I do not know how to properly design HVAC systems. therefore I need help to correctly design one.

For the moment I am using PTAC DX Clg Elec Htg, in reality my AC has no heating. I have hard sized most of the parameters, namely:

OS:ZOneHVAC:PackagedTerminalAirConditioner

  • Supply Air Flow Rate During Cooling Operation: 0.431 m^3/s
  • Supply Air Flow Rate During Heating Operation: ~ 0 (because no heating)
  • Supply Air Flow Rate When No Cooling or heating is Needed: 0 (because in reality the coils are always on when the fan is on, it's in Thailand so it is very hot and humid --> AC is working hard)
  • Outdoor Air Flow Rate During Cooling Operation: 0.431 m^3/s
  • Outdoor Air Flow Rate During Heating Operation: 0 (because no heating)
  • Outdoor Air Flow Rate When No Cooling or heating is Needed: 0 (because in reality the coils are always on when the fan is on, it's in Thailand so it is very hot and humid --> AC is working hard)

OS:Coil:Heating:Electric

  • Nominal Capacity: 0 (because no heating)

OS:Fan:ConstantVolume

  • Maximum Flow Rate: 0.431 m^3/s (I had to hard size this because every time I ran the simulation it couldn't autosize because there was No "Zone:Sizing" objects even though I had set the Do Zone Sizing Calculation and Do System Sizing Calculation to on in Simulation Control, it works when hard sized)

OS:Coil:Cooling:DX:Single:Speed

  • Rated Total Cooling Capacity: 10550 W
  • Rated Sensible Heat Ratio: 0.7 (had to hard size, same reason explaine in OS:Fan:ConstantVolume)
  • Rated COP 2.8

The rest was left alone.

When I run the simulation with the above parameters it is able to complete, however when I look at HVAC load I have this (0 cooling load).

image description

I've left the Cooling Sizing Parameters and Heating Sizing Parameters to their default values and have not done any design days, I have only used the weather file from the NREL website. I have however made my own Availability Schedule For OS:ZOneHVAC:PackagedTerminalAirConditioner, it is an OnOff Schedule type and looks like this:

image description

However the other Schedules have been left as there default schedule namely:

  • Supply Air Fan Operating Mode Schedule for OS:ZOneHVAC:PackagedTerminalAirConditioner: Always On Discrete 17
  • Availability Schdule for OS:Coil:Heating:Electric: Always On Discrete 19
  • Availability Schdule for OS:Fan:ConstantVolume: Always On Discrete 18

And therefore there isn't any electrical consumption from the PTAC

Any help on how to correctly design this type of HVAC system to get cooling loads and electrical consumption would be greatly appreciated.

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answered 2016-01-28 03:07:41 -0600

ngkhanh's avatar

try too add design day first. in Openstudio E+ folder you can see many data input for Thailand or you can download from Energyplus.net.Autosize for such simple calculation is pretty accuracy in OS. If you want to hard sizing, you need to hard sized all value in PTAC

Make sure that you have enabled all sizing option in simulation control Zone sizing, system sizing and simulation sizing days.

check your heating and cooling schedule value. If they are out of normal range, the hvac load is removed. in your case, it's often cooling setpoint.

Are you created space type and set default internal load ? (check in input summary in E+). If you open a new file in OS (untitled) and forget set load and their schedule , it may take that result.

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Hi, thanks for your answer. I have added design days to my model, the .ddy file from the openstudio website. I have made a Cooling Setpoint Schedule for my thermal zone (20 C all year round). I have created a space type with internal loads (only lighting, elec eq), is there an internal load for the AC? I have also re-run the simulation with new PTACs that are all autosized. I have heating loads... Which is not very logical because I only need cooling loads...

olivier's avatar olivier  ( 2016-01-28 18:50:55 -0600 )edit

Hi I added a heating thermostat schedule set it to 0 C and a cooling thermostat schedule and set it to 25 and got a cooling load.

olivier's avatar olivier  ( 2016-01-28 23:05:47 -0600 )edit

1/ Try to use default schedule cooling and heating setpoint. Becareful with all schedule type unit : fraction, Activity level, temperature 2/ Check is your design day data are reasonable for hot and humid climate by see Temperature and Humidity tab at DDY 3/ you need both heat/cool setpoint schedule. You can check current applied heating setpoint schedule. heating setpoint value is always less than cooling. 4/ It seems you applied a very big heat loss or too small heat gain (small internal load). Try to correct load definition as usual practice in your place

ngkhanh's avatar ngkhanh  ( 2016-01-29 01:55:44 -0600 )edit
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Asked: 2016-01-28 00:42:52 -0600

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Last updated: Jan 28 '16

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