Is it correct to associate a Thermal Zone and a Thermostat to a room with no HVAC?

asked 2020-09-15 14:56:39 -0500

rainbow gravatar image

updated 2020-09-15 15:58:05 -0500

I'm working on a building which has 5 rooms and a Variable Refrigerant Flow HVAC consisting of 1 outdoor unit and 3 indoor units.

I'm using OpenStudio Plugin in order to generate the IDF which I'd use as input to a simulation to estimate the electric energy consumed by the HVAC in order to meet the temperature requirements (setpoints).

I divided the building into 5 Thermal Zones (one for each room), and all 3 VRF indoor units are located inside the same Thermal Zone (TZ2, represented in the image attached below).

I associated a Dual Setpoint Thermostat to each Thermal Zone, although only TZ2 contains VRF units; the other 4 Thermal Zones are unconditioned.

This attached PNG image represents the 2D plant of my building on which I also drew the Thermal Zones:

image description

I have some questions regarding my case:

  1. Is it correct to associate a Thermal Zone to a room, although this doesn't contain any VRF units?
  2. Is it correct to associate a DualSetpoint Thermostat even to rooms which don't contain any VRF units ?

Moreover if, in your opinion, the thermal zoning of the building that I did is not so good, you're welcome to propose better ones.

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This post discusses creating an unconditioned thermal zone in OpenStudio and strategies for defining thermal zones. Defining thermal zones depends upon the thermostat settings for heating and cooling and the HVAC equipment that will be used to meet the heating/cooling loads. Your zoning looks fine, simplifying a 5-zone floor plan isn't critical (a 100-zone floor plan would be). Does that answer your questions?

Aaron Boranian gravatar imageAaron Boranian ( 2020-09-15 15:34:04 -0500 )edit

Thanks Aaron!

So (correct me if I'm wrong), it's necessary to associate a heating schedule + a cooling schedule + a heating/cooling system to an OpenStudio Zone to ensure that it will be translated to a conditioned Thermal Zone in EnergyPlus.

I associated a Thermostat to each of the 5 zones, but I associated all VRF units to a single zone (TZ2).

What I obtain in the eplustbl.html seems consistent with that post's answer: I see 4 unconditioned zones and 1 conditioned zone (TZ2).

So, if I remove the thermostats in the 4 unconditioned zones it shouldn't change anything, right?

rainbow gravatar imagerainbow ( 2020-09-16 05:18:18 -0500 )edit

Correct. If you have thermostats in the 4 zones without VRF units, they should report lots of unmet heating and cooling load hours in the html report. If you remove the thermostats in the 4 unconditioned zones, then those unmet heating/cooling hours should go away. That should be the only thing that changes, nothing related to energy consumption should change.

Aaron Boranian gravatar imageAaron Boranian ( 2020-09-16 12:22:47 -0500 )edit

Ok, very clear.

Before I opened this thread, I thought that, in Openstudio, by simply associating a thermostat to a zone with no HVAC, I would ensured that the HVAC systems present in other rooms (if sufficiently powerful) would have worked in order to guarantee that the temperature inside the unconditioned zone also met its thermostat setpoint..

rainbow gravatar imagerainbow ( 2020-09-16 23:09:00 -0500 )edit

I read here that the building load magnitude calculated by using a 1-zone model will NOT be significantly different from the one calculated by using a more accurate model (BTW, this latter has the advantage to make possible to estimate the load distribution, too).

Considering that I'm only interested on the building load magnitude, do you think that (in my case) would it be better to use a 1-zone model (so considering perimeter walls only)?

rainbow gravatar imagerainbow ( 2020-09-16 23:12:55 -0500 )edit