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Coil, Fan and VRF Terminal not Operating Continuously on VRF System

asked 2019-03-13 10:17:29 -0500

lrebello gravatar image

updated 2019-03-13 15:41:14 -0500

Hello Team,

I am facing an strange problem with my energetic simulation. I am trying to subimit a VRF system to LEED and as I understand some VRF terminals can not read the schedule that is associated to them.

So I have a VRF System with multiple VRF Terminals and most of them are thermal zones associated with people thermal comfort and some others are critical mission thermal zones.

As an exemple of a critial mission thermal zone, I am evaluating thermal zone named 003_SL_TEC_LESTE and it is associate with VRF ZONE TERMINAL COOLING COIL 347. This zone is an IT ROOM and the HVAC must be ON 24/7. The explained bellow is happening for all critial mission thermal zones.

First thing I analyzed was COOLING COIL RUNTIME FRACTION and FAN AIR MASS FLOW RATE, and dispite the fact that both the fan and the coil have the same working schedule (Always On Discrete hvac_library) they are not operating both continuously (see pictures 1, 2 and 3 bellow).

Picture 1 picture 1

Picture 2 image description

Picture 3 image description

Then I checked the ZONE THERMOSTAT and the ZONE AIR TEMPERATURE, and I had the same conclusions: Despite the fact that the HVAC system (VRF TERMINAL) is schedule to be ALWAYS ON and its setpoint should be controled by the thermostat (set to be always on 23 degrees, picture 4), the coil is only working on business hours on weekdays (see picture 4 bellow).

Picture 4 image description

As all other zones in my VRF system operate only on business hours, I assume all mission critical thermal zones are copying their schedules, but I could not change it.

Also, I opened the .IDF file to check if Open Studio was reading the wrong schedule and it apper to be all right to me (see picture 5, 6 and 7 bellow).

Picture 5 image description

Picture 6 image description

Picture 7 image description

Adding to that the analisys of the coil operation, I can see it is operating from 7 am to 9:45 pm on a weekday(picture 8), operating from 7:15 am to 4:30 pm on weekends (picture 9) and that the coil is operating everyday (picture 10), contradicting the RunTimeFraction graph, which makes no sense to me.

Picture 8 image description

Picture 9 image description

Picture 10 image description

So my final conclusions are:

  1. Availability Schedule is set to be ALWAYS ON for Terminal Unit, Fan and Coil ( value of 1);
  2. Thermostat Schedule is set to be always on 23 degrees;
  3. VRF Terminal Unit only operates on business hours, contradicting what its designed operation.

Does anyone know how can I fix this?


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Also, I have already tried:

  1. Deleting the VRF terminal and adding it again to the VRF System;
  2. Deliting the Thermostat and adding it again to the Thermal Zone;
  3. I've checked the heating thermostat and it is set to 5 degress and the cooling thermostat is set to 23 degrees, so it has no interaction between them;
  4. Deleting the VRF Terminal and adding a PTAC to replace it with the same Always On Discrete hvac_library (works for all days) but does not represents the system that will be installed.
lrebello gravatar image lrebello  ( 2019-03-13 10:33:06 -0500 )edit

@Irebello please-use-existing-tags!

MatthewSteen gravatar image MatthewSteen  ( 2019-03-13 11:05:13 -0500 )edit

@MatthewSteen I don't know how to use them. How do I use it?

lrebello gravatar image lrebello  ( 2019-03-13 11:25:54 -0500 )edit

testing: VrfTerminal

lrebello gravatar image lrebello  ( 2019-03-13 16:23:27 -0500 )edit

@MatthewSteen I think I just learned how to use it!

lrebello gravatar image lrebello  ( 2019-03-13 16:24:39 -0500 )edit

2 Answers

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answered 2019-03-13 11:01:00 -0500

Interesting runtime fraction plot (Picture 10). The cooling coil total cooling rate and cooling coil runtime fraction would have the same shape if the VRF system available capacity didn't change much. Look at 9/25 around 1 PM where the runtime fraction drops to 0.25 when the cooling coil total cooling rate is not much different than other times when RTF = 1. So what this means to me is that the cooling coil available capacity is severely reduced for some reason, most likely the performance curves.

The scheduling of the VRF TU's looks correct to me. They are always available and the fan operating mode schedule is set to constant fan. What is the availability schedule of the AirConditioner:VariableRefrigerantFlow? If it is off after hours, so will these TU's.

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answered 2019-03-13 11:24:16 -0500

lrebello gravatar image

@rraustad The Condenser has a always on schedule as well but with a different name (see picture 1 and 2).

Picture 1 image description

Picture 2 image description

Also, Design Day Profiles for Summer and Winter have the same schedule (always on - 1 value).

I have already tried to change the availability schedule of the condenser to be exactly the same of the evaporator, but it did not change anything.

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Seems like it should be working on the weekends. I think at this point I would need to see your E+ input file or send you file directly to the help desk at

rraustad gravatar image rraustad  ( 2019-03-13 11:56:13 -0500 )edit

I just sent it to your email.

I will appreciate if you can take a look at it to me!

Thanks a lot

lrebello gravatar image lrebello  ( 2019-03-13 12:46:22 -0500 )edit

For the cooling coil described here (347), the TU with this coil is on a VRF system that has 35 other TUs attached. I am guessing that the VRF model may have a problem when, for example, only 1 or 2% of capacity is needed from the outdoor condenser. Look at 3PAV-ESQ-201KW-VRF-COND-CARRIER-MV5-X72W heat pump part load ratio, it's not 0, in fact it's negative. My suggestion at this point would be to put the TUs that you want to operate 24/7 on their own VRF system to see if that eliminates the problem. Your file takes a long time to run in debug mode so other answers may not come quickly.

rraustad gravatar image rraustad  ( 2019-03-14 12:44:33 -0500 )edit

I am gonna try it! Thanks a lot!

lrebello gravatar image lrebello  ( 2019-03-14 12:47:57 -0500 )edit

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Asked: 2019-03-13 10:17:29 -0500

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Last updated: Mar 13 '19