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VRF system with huge no. of unmet hours only during heating

asked 2015-04-22 20:59:30 -0500

Daniel Mak's avatar

updated 2015-04-22 22:31:07 -0500

Dear all,

I am in a battle with all these unmet hours in my VRF system and I desperately need some help (OpenStudio).

It is a pretty straight forward system configuration with 1 outdoor unit linking to several indoor units. The zones served by the system have different orientations, internal loads and ventilation requirements. Fresh air is provided by the mixers in the VRF terminals.

I am using dual setpoint for the thermostat, i.e. temperature setpoints are 22C for heating mode and 26C for cooling.

The peculiar thing is that unmet hours happen nearly exclusively during heating mode. There are at least two symptoms that I observed:

  1. It seems that the heating coils have spare capacities but sometimes refuse to heat up the spaces up to the setpoint. As you can see from the attached screenshot the indoor temperature of one of the zones fluctuates at around 21.6C. image description
  2. It takes the system more than 4 hours to heat up the spaces to the setpoint in the morning. I have set a indoor temperature setpoint schedule that gradually ramps up the temperature before occupants come in but it still doesn't seem to work too well. I am also concerned about the fact that in the actual building the spaces are not preheated 4 hours before the spaces are meant to reach 22C.

Any pointers would be highly appreciated.

Thanks in advance, Daniel

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Hi @Daniel Mak, you should re-tag your post with the simulation program you're using (e.g. openstudio, energyplus). Also, take a look at this answer, which may be helpful.

Eric Ringold's avatar Eric Ringold  ( 2015-04-22 21:16:14 -0500 )edit

Thank you Eric,

It is OpenStudio that I am using.

I just tried to use sizing factor 1.5 for autosizing and annihilated a significant portion of the unmet hours. What I don't understand is that autosizing should be based on the design days which are pretty much the worst cases. Why does the system require beefing up if it is sized at near-peak loads?

Furthermore, I am not sure if I am simply suppressing the symptoms rather than dealing with the root of the problem by sizing up the heating capacities.


Daniel Mak's avatar Daniel Mak  ( 2015-04-22 21:49:53 -0500 )edit

Did you change the reporting tolerances? By default, OpenStudio has them set to 0.2 K (0.36 R), which is very tight, and might be causing the number of unmet hours to be over-reported. If you haven't, try changing these values (in Simulation Settings tab, under 'Output Control Reporting Tolerances') to 2.0 R.

Eric Ringold's avatar Eric Ringold  ( 2015-04-22 22:24:46 -0500 )edit

Not sure this applies to your model but you could also try removing people from less frequently occupied spaces such as corridors, restrooms, etc.

MatthewSteen's avatar MatthewSteen  ( 2015-04-22 22:30:18 -0500 )edit

Hi Eric, Yes the tolerances is 0.2 and I know it is very tight but I haven't had trouble meeting the requirement in previous energy models so I am trying to use it as a last resort. This will probably take away some of the hours caused by fluctuating temperature but at this stage I would rather find out if there are control / sizing problems that are causing the temperature to fluctuate.

Daniel Mak's avatar Daniel Mak  ( 2015-04-23 00:12:45 -0500 )edit

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answered 2015-04-29 01:23:10 -0500

Daniel Mak's avatar

Dear all, Thank you very much for all the inputs. It seems that the major reason for all the unmet hours was that the VRF system has fixed fresh air supply and because its availability schedule has been set to "always on", it kept dumping fresh air into the spaces even at night. This caused the indoor temperatures to drop to near outdoor temperature giving the system a hard time during morning start-ups for a given heating capacity. I created a reasonable on/off schedule based on cooling and heating schedules for the VRF system and the unmet hours decreased significantly in most thermal zones. There are still high unmet hours in a few individual zones which I am looking into at the moment but that could be caused by some of the reasons mentioned in other comments in this thread. Regards, Daniel

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answered 2015-04-23 12:47:57 -0500

Archmage's avatar

If the VRF is a heat pump type, then the outdoor unit will completely switch between heating and cooling depending on the dominant requests from the zone units. If one corridor needs heating while all the other zones need cooling, then the corridor doesn't get what it wants and this is the sort of behavior that happens. This is as intended for VRF systems unless they are heat recovery units.

A larger reporting tolerance will help suppress those hours that are just a little unmet and they are going to be more common with VRF.

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Dear Archmage, Thank you for your comment. I have set the "Master Thermostat Priority Control Type" to "Load Priority" so in theory most of the zones should be in the right "mode" i.e. either heating or cooling. As for my case, all zones during winter experience under-heating so the control of VRF might not be the cause. I have set the tolerance to 0.5K to take away the unmet hours due to fluctuation but I am still having problem with morning start-up.

Daniel Mak's avatar Daniel Mak  ( 2015-04-28 04:40:05 -0500 )edit

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Asked: 2015-04-22 20:59:30 -0500

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Last updated: Apr 29 '15