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Why having unmet hour even though capacity is enough for VRF system?

asked 2018-05-02 00:52:57 -0500

katsuya.obara's avatar

updated 2018-05-02 07:46:46 -0500

I have model which implement VRF system using HVACTemplate:Zone:VRF and HVACTemplate:System:VRF.
I am having problem to reduce unmet hour as below. (Above graph:Indoor Unit Cooling Rate[W]/Below graph:Unmet hour for each zone) Since outdoor unit connected to these zone indoor unit has 35kW capacity, capacity should not be problem.
Does anyone know why and how to solve this problem?

image description

I also tried to change Master Thermostat Priority Control Type, but could not reduce unmet hour below 300 hours.

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When you want to investigate unmet hours, always plot as "Detailed" frequency, not hourly. You'll be able to see if it's just a temporary capacity issue (rapid setpoint temperature changes) or something else.

Julien Marrec's avatar Julien Marrec  ( 2018-05-02 07:48:00 -0500 )edit

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answered 2018-05-02 08:05:01 -0500

updated 2018-05-02 10:48:36 -0500

Occupied unmet hours are usually caused by:

  • insufficient capacity to meet the zone load
  • using thermostat set back and activating set up while starting the HVAC system at the same time as occupancy starts
  • for VRF systems, using many terminal units on a single VRF condenser

The result of insufficient capacity is the loss of zone temperature control. Plot the zone temperature and zone thermostat set point temperature to compare these values. It is also good at this point to plot the number of occupants and occupied unmet hours so that you know when the zone temperature deviates from the set point while there are occupants.

Starting the HVAC system at the time of occupancy usually has little effect on unmet hours IF the zone thermostat is NOT using set back. When set back is used it may take some time before the zone temperature reaches the thermostat set point so it usually helps to start the HVAC system prior to occupancy (which real systems do to avoid occupant discomfort).

For VRF systems, if the VRF condenser is overloaded, the model will limit the terminal unit capacity so that the total operating terminal unit capacity equals the VRF condenser operating capacity. At time when this limit is occurring, these report variables will show that impact.

OutputVariable,*,VRF Heat Pump Maximum Capacity Cooling Rate, Timestep;
OutputVariable,*,VRF Heat Pump Maximum Capacity Heating Rate, Timestep;
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answered 2018-05-03 04:19:16 -0500

katsuya.obara's avatar

When I updated version from 8.6 to 8.8, unmet hour reduces from 621 to 226 hours.
I didn't change any setting.

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I don't see any real change to the VRF model in the changelog.html (root folder of E+) files for V8.7 and V8.8 but it could be due to some other change. I suggest plotting the zone temp, thermostat set point temperature, occupied unmet hours, and occupancy in both versions to get a sense of what might be happening.

rraustad's avatar rraustad  ( 2018-05-03 10:22:42 -0500 )edit

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Asked: 2018-05-02 00:52:57 -0500

Seen: 349 times

Last updated: May 03 '18