VRF system with huge no. of unmet hours only during heating
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:
- 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.
- 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
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.
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
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.
Not sure this applies to your model but you could also try removing people from less frequently occupied spaces such as corridors, restrooms, etc.
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.