Unmet heating hours with ventilation, VRF System (Openstudio)

asked 2016-09-20 12:54:36 -0600

dpud12
1259 ●1 ●12

I am trying to use a VRF system to condition my residential zones in a multifamily apartment building. However, the model has a high number of unmet hours for heating only (approx 500). The only way that I have been able to eliminate these high unmet hours is to zero out the ventilation and infiltration, which is of course not a realistic solution. I have put each individual zone on its own system, so it is not a simultaneous heating and cooling problem.

I have dug into the outputs and concluded the following based on the attached graphs.

1) The autosizing method is not properly maintaining the thermostat heating setpoint during the design days. As you can see there is mechanical ventilation during the design day and the zone temp trend follows the ventilation ACH trend. However, it is not in the way you would expect, as more ventilation results in a higher temperature in the zone.
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2) During the annual run, the VRF system is not maxing out on capacity in an attempt to meet the heating demand.

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It is as if somewhere in the back end there is a disconnect between the ventilation/infiltration load and the VRF system. Has anyone else experienced this or has an idea what might be causing this?

Thanks in advance for any advice!

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answered 2016-10-07 07:57:57 -0600

dpud12
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updated 2016-10-13 12:43:04 -0600

The unmet hours could be significantly reduced (from 500+ to 160) by making sure that the design day temperatures for the thermostats served by the VRF system were at least 5 degrees C above the operating temps for the run period. @Annie Marston further discovered an additional reduction in unmet hours by exporting the model to E+ and cleaning out the optional arguments in the zone System Sizing and Zone Sizing objects.

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How do you change the design day temps for just the VRF system? Are you changing the Zone Cooling/Heating Supply Air Temps for each zone serviced by the VRF? or you changing the Design Day temps for the project?

dmcgee5 ( 2016-10-11 17:46:22 -0600 )edit

@dmcgee5 Thanks for the clarifying question. I have updated my answer to reflect that it is the thermostat design day temps that need to be changed

dpud12 ( 2016-10-13 12:43:56 -0600 )edit

Thanks for the help. Is there good explanation of why this works as a fix?

dmcgee5 ( 2016-10-13 13:32:35 -0600 )edit

Unfortunately I have not had the time to dig into exactly why this works as a fix. Since the design day weather temps are more extreme than anything the system will see during the annual simulation, it doesn't make sense that you would need to overcompensate with the thermostat profiles for the design days for the sizing to properly work. It is not this way for other HVAC systems that I have tested with the same building model, and so the "error" here seems to be something specific to the VRF system

dpud12 ( 2016-10-14 07:30:02 -0600 )edit

I was modeling a VRF and DOAS, and I ran into an issue of priority between the two systems. Once I worked that out, I was able to significantly reduce the unmet hours. It looks like this specific discussion pertains to a single VRF system using the OA supply within each terminal unit. I look forward to any insights as they come up. The VRF seems to be an increasingly popular choice.

dmcgee5 ( 2016-10-14 09:48:50 -0600 )edit

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answered 2016-09-20 13:21:20 -0600

Annie Marston
4337 ●5 ●32 http://www.resustain.com

I wonder if your zone sizing has a high enough supply temperature. My guess is that this might not be high enough, maybe try raising that, as a rule of thumb I use a supply temperature difference of 11C, which would be 31C in this case, if you already have that maybe try running it with 35C to see if it makes a difference. Also how is your ventilation air coming into the zone, is it from a seperate AHU or through the VRF? If it is through a separate AHU check that the temperature setpoint on the heating coil here is high enough.

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Hey Annie, so I didn't find a way to check the zone sizing supply temps directly in OS with the VRF system, but when I exported the IDF and checked out the temps they are 40C for all zones. Digging around in the IDF a little more I ended up finding that by increasing the heating design day thermostat setpoint to 5C higher than the annual simulation setpoint, I could reduce the unmet hours to 167. This really isn't a satisfying answer though, since the design days have more extreme conditions than the annual simulation, and so really this shouldn't be necessary.

dpud12 ( 2016-09-21 07:34:02 -0600 )edit

@dpud12 I am intrigued what the culprit was. Were you going to answer you own question, I hope so. I always find it easy looking in the idf, but that is what I am used to. Glad you got your problem solved.

Annie Marston ( 2016-09-21 07:38:02 -0600 )edit

That is a bit weird, I am interested now and am happy to take a look through your idf if you can upload it somewhere.

Annie Marston ( 2016-09-21 07:58:44 -0600 )edit

@Annie Marston Thanks I would really appreciate that. I have your email from meeting you at SimBuild so will send it to you.

dpud12 ( 2016-09-21 08:09:55 -0600 )edit

I know this is an old post, but I am working a similar problem right now. I have several dozen zones, each with a VRF terminal. In turn, several VRF (indoor) terminals connect to their own VRF (outdoor) unit, for a total of about a dozen VRF units. In addition, about half of the zones sit on one DOAS and the other half on another DOAS. Initially, I found very high unmet heating and cooling hours. Then I changed the "Account for Dedicated Outdoor Air System" for each zone from "No" to "Yes", and the unmet hours dropped nicely.