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PTHP Template: how to fix this warning: Full load outlet air dry-bulb temperature < 2C"

asked 2020-02-06 11:35:59 -0600

Cristiano's avatar

updated 2020-02-08 12:00:49 -0600

I'm using the HVACTemplate:Zone:PTHP template to estimate the cooling and heating loads of a AC split unit with COP of 3.29 and thermal capacity of 12000 BTU during one year and I'm getting the following warnings:

** Warning ** Calculated design heating load for zone=DORM2 is zero.
   **   ~~~   ** Check Sizing:Zone and ZoneControl:Thermostat inputs.
   ************* SizeDXCoil: Potential issue with equipment sizing for Coil:Cooling:DX:SingleSpeed DORM2 PTHP COOLING COIL
   **   ~~~   ** User-Specified Gross Rated Total Cooling Capacity [W] = 3516.00000
   **   ~~~   ** differs from Design Size Gross Rated Total Cooling Capacity [W] = 2960.30983
   **   ~~~   ** This may, or may not, indicate mismatched component sizes.
   **   ~~~   ** Verify that the value entered is intended and is consistent with other components.
   ** Warning ** SizeHeatingCoil: : Potential issue with equipment sizing for Coil:Heating:Electric DORM2 PTHP SUPP HEATING COIL
   **   ~~~   ** ...Rated Total Heating Capacity = 0.00 [W]
   **   ~~~   ** ...Capacity used to size child component set to 0 [W]
   ************* Testing Individual Branch Integrity
   ************* All Branches passed integrity testing
   ************* Testing Individual Supply Air Path Integrity
   ************* All Supply Air Paths passed integrity testing
   ************* Testing Individual Return Air Path Integrity
   ************* All Return Air Paths passed integrity testing
   ************* No node connection errors were found.
   ************* Beginning Simulation
   ** Warning ** CalcDoe2DXCoil: Coil:Cooling:DX:SingleSpeed "DORM2 PTHP COOLING COIL" - Full load outlet air dry-bulb temperature < 2C. This indicates the possibility of coil frost/freeze. Outlet temperature = 1.43 C.
   **   ~~~   **  ...Occurrence info = ANO, 01/01 00:00 - 00:05
   **   ~~~   ** ... Possible reasons for low outlet air dry-bulb temperatures are: This DX coil
   **   ~~~   **    1) may have a low inlet air dry-bulb temperature. Inlet air temperature = 16.999 C.
   **   ~~~   **    2) may have a low air flow rate per watt of cooling capacity. Check inputs.
   **   ~~~   **    3) is used as part of a HX assisted cooling coil which uses a high sensible effectiveness. Check inputs.
   ************* Simulation Error Summary *************
   ************* ===== Recurring Error Summary =====
   ************* The following recurring error messages occurred.
   *************  ** Warning ** CalcDoe2DXCoil: Coil:Cooling:DX:SingleSpeed="DORM2 PTHP COOLING COIL" - Full load outlet temperature indicates a possibility of frost/freeze error continues. Outlet air temperature statistics follow:
   *************  **   ~~~   **   This error occurred 30222 total times;
   *************  **   ~~~   **   during Warmup 0 times;
   *************  **   ~~~   **   during Sizing 0 times.
   *************  **   ~~~   **   Max=1.999957  Min=0.6961

I believe that the first warning is not a problem once the weather file is from a very hot climate. What worries me is the third which indicates the possibility of coil frost/freeze. I believe the problem is because the constant cooling setpoint is set to 17 °C, but this is the value that I have to use. Does anyone know how I can fix this problem? are these warnings something to worry about? If someone wants to check the IDF file here it is:

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answered 2020-02-07 07:52:38 -0600

updated 2020-02-07 22:32:13 -0600

The warning shows a cooling coil air outlet temperature of 1.43 C. The recurring warning at the end shows a minimum cooling coil air outlet temperature of 0.7 C. This is not all that far below 2 C so you should be able to adjust the coil performance. If you have room to adjust the performance, meaning you are not trying to model a specific cooling coil where you know rated air flow rate, rated SHR and total cooling capacity, then you should be able to make some minor adjustments.

The first step is to know the air flow to capacity ratio of your cooling coil. Divide the rated air volume flow rate by total cooling capacity (12,000 BTU/hr / 3.4129 in your example). This should give you a number near 5E-5 m3/s/W. The allowed range for a DX cooling coil is 2.684E-5 (200 cfm/ton) to 6.713E-5 (500 cfm/tom). Adjusting this ratio can alter the cooling coil outlet air temperature.

if you are autosizing then you can adjust the Sizing:Zone input for Zone Cooling Design Supply Air Temperature by decreasing that value slightly. This should increase the zone air flow rate needed to meet the zone load. Since the zone load doesn't change then the end result will be to increase the autosized air flow rate while, hopefully, maintaining nearly the same total cooling capacity.

If you are not autosizing then slightly increase the cooling coil air flow rate (without changing the cooling capacity) and the outlet temperature should rise. Or lowering the cooling coil total capacity (without changing air flow) should have the same affect.

This may be an iterative process so each time you make an adjustment look at the temperatures reported in the warnings to watch how they change. Make very small adjustments at first so you can still see the warning and therefore see how your change affected coil performance.

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I see. I raised the air flow rate and it worked. In this example I used a cooling capacity of 5274 W (12000 BTU/hr) and an air flow of 0,25 which gave me an air flow to capacity ratio of 4.74E-5 m³/s/W.

Cristiano's avatar Cristiano  ( 2020-02-07 10:55:47 -0600 )edit

answered 2020-02-08 09:53:53 -0600

Jim Dirkes's avatar

EnergyPlus is finicky regarding hard-sizing the PTHP airflow, but not hard-sizing the FAN airflow at the same time. (This seems true for all DX coils in E+, not just heat pumps.)

Another consideration: many newer heat pumps operate with good capacity down to 4F (-15C). If yours is such a heat pump, adjust the input for minimum outdoor air temp for compressor operation. Also review the part load performance curves to confirm whether they are a good representation of the actual equipment's performance.

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Asked: 2020-02-06 11:35:59 -0600

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Last updated: Feb 08 '20