VRF systems unable to simulate inter-zone heat recovery?

[anthony.schneider]

Hi LBT community,

I’ve been reading Honeybee Energy’s documentation for VRF systems and found the following text for both of the VRF submodules in the DOAS subpackage and the HeatCool subpackage:

“All room/zone terminals are connected to the same outdoor unit, meaning that either all rooms must be in cooling or heating mode together.”

It seems that a key efficiency advantage of VRF systems is that they can move heat from one interior zone to another.

If there are equal heating and cooling loads simultaneously in a building, it can move the heat from a zone that needs cooling to a zone that needs heating without needing to run its outdoor units (the compressor still runs to move the refrigerant). When there are both heating and cooling demands in a building simultaneously, but they are imbalanced, the system will turn on some of the outdoor units to cover the discrepancy.

However, the above text from the documentation makes it seem like all zones need to be cooling or heating, and therefore we does this mean we can’t cool some rooms, but heat others? This seems to mean that we can’t make use of the efficiency of moving heat from one internal zone to another by moving the refrigerant in the system, and turning off some of the outdoor units?

I read the following thread where @chris mentions that you can ensure that each zone gets its own outdoor unit by grafting the input to the “HB HeatCool HVAC” component.

How can I creat a split system air conditioner in honeybee (Ladybug Tools 1.4)？ - grasshopper / honeybee - Ladybug Tools | Forum

The following image link is to one of Chris’ screenshots from that thread which shows how this can be done:

https://discourse.ladybug.tools/uploads/default/original/3X/9/8/98300926657d521e82cb5e1121a7afa62b2d2ba6.png

Since the text from the documentation says that all zones are connected to the same outdoor unit, will using this technique allow the system to heat some rooms while cooling other rooms?

And will this technique also allow the system to take advantage of the efficiency of moving heat from one zone to another without running all of the outdoor units?

If not, perhaps there is a percentage based reduction I can make to some components of the end use results of the energy model to account for the efficiencies of turning off some of the outdoor units when some rooms require cooling, and others require heating?

Perhaps I am misinterpreting the wording of the documentation.

Please let me know

Anthony

[anthony.schneider]

Just some extra info:

It seems that a VRF system can simultaneously use some indoor units for heating, and some for cooling, even with only one outdoor unit.

How this works is explained in this video at 10 mins, 24 seconds in:

VRF Systems Explained - Variable refrigerant flow basics HVAC (youtube.com)

This seems to contradict the documentation which states:

“All room/zone terminals are connected to the same outdoor unit, meaning that either all rooms must be in cooling or heating mode together.”

Cheers,
Anthony

[chris]

Hi @anthony.schneider . Sorry for the late response.

@anthony.schneider

It seems that a key efficiency advantage of VRF systems is that they can move heat from one interior zone to another.

My understanding is that not all VRF systems have such sophisticated controls. For example, if you count residential mini-splits as a type of VRF system, those typically only have a single mode operation in my experience.

Maybe I’m also just misunderstanding why I tend to see mechanical engineers grouping similar rooms together to connect them to the same outdoor VRF unit. I thought this was done to help synchronize when the rooms are in cooling/heating modes together but perhaps there’s another reason.

Also, I can see this issue as a case where the industry has probably evolved over time. VRF systems that can only operate in one mode at a time are simpler to design and probably cheaper to produce. But there’s obviously a potential for energy savings if a single VRF outdoor unit can support simultaneous heating and cooling. It may not be compelling value proposition for all building types but the hotel room use-case described in that video is pretty clear. So I would bet that VRF manufacturers have added more support for these types of sophisticated simultaneous heating/cooling controls as the industry continues to create products and systems that use less energy.

In any event, if you want me to take that language out of the docs, I can do so. It might just be good if you could run a test with the particular types of VRFs that we write into the Honeybee-exported IDFs/OSMs to be sure that they handle simultaneous heating/cooling.

[anthony.schneider]

Hi Chris,

No need to apologise! Cheers for getting back to me

@chris

Maybe I’m also just misunderstanding why I tend to see mechanical engineers grouping similar rooms together to connect them to the same outdoor VRF unit. I thought this was done to help synchronize when the rooms are in cooling/heating modes together but perhaps there’s another reason.

I imagine this could also be to minimise the amount of refrigerant piping that needs to be installed?

@chris

VRF systems that can only operate in one mode at a time are simpler to design and probably cheaper to produce

@chris

It may not be compelling value proposition for all building types but the hotel room use-case described in that video is pretty clear.

Totally, agreed - this is probably the main reason. I have been doing some more reading and having conversations about it in the meantime. I’ve heard that it takes a lot of complicated refrigerant piping to make this a reality, and this extra work in piping and controls doesn’t always seem to get repaid by the energy savings of being able to move heat between interior zones (if the individual comfort benefits of simultaneous heating and cooling isn’t important like in that hotel example).

@chris

In any event, if you want me to take that language out of the docs, I can do so. It might just be good if you could run a test with the particular types of VRFs that we write into the Honeybee-exported IDFs/OSMs to be sure that they handle simultaneous heating/cooling

No need to change the language of the documentation at this stage.

I wonder how I could reliably set up this test?

I could set up two side-by-side zones in an energy model, one with low set points (e.g. 5-10 degrees Celsius), and one with high set points (e.g. 30-35 degrees Celcius) and ensure that they both have a single outdoor unit by not grafting the HVAC component into the rooms input on HB Model (as described in the technique from my first post). Therefore, one room will likely need to be cooled while the other needs to be heated.

Is there any way to look at the heating and cooling outputs of each zone using point in time analysis like you would for daylight?

Cheers,
Anthony

[chris]

Thanks, @anthony.schneider

This seems like a good test to me:

@anthony.schneider

I could set up two side-by-side zones in an energy model, one with low set points (e.g. 5-10 degrees Celsius), and one with high set points (e.g. 30-35 degrees Celcius) and ensure that they both have a single outdoor unit by not grafting the HVAC component into the rooms input on HB Model (as described in the technique from my first post). Therefore, one room will likely need to be cooled while the other needs to be heated.

You can look at the Setpoint not Met time from the HB Read Room Comfort Result component to confirm that both the heating and the cooling setpoints are being met at the same time. And just plot the data on an hourly plot to be sure that practically all hours are covered.

[anthony.schneider]

Hi @chris

I’ve had a go at running the test.

There are two zones side by side that have an adiabatic wall between them. I threw some windows on the north side and am using a climate file from Wellington, New Zealand.

One room has set points 30 and 35 degrees Celsius. The other room has set points 10 and 15 degrees Celsius.

I tried using set points 5 and 10 degrees Celsius for the colder room first but there was the following error: “Cooling supply air temperature (calculated) within 2C of zone temperature”.

These are the results for unmet heating:

These are the results for unmet cooling:

And this is the script:

Part 1:

Part 2:

This is showing that both rooms pretty much always have their set points met. And I guess this means that the VRF system must be able to supply heating and cooling simultaneously, right?

Please let me know if you agree, or if you can spot any errors. I’ll attach the Rhino and GH files.

Cheers,
Anthony

VRF Test.3dm (45.1 KB)
VRF Test.gh (85.4 KB)

Also, do you happen to know why my Honeybee rooms have so many vertices and faces? Please let me know if you do

[chris]

Hey @anthony.schneider ,

Yea, I forgot that the VRF template uses a supply air temperature of 12.8 C for cooling the room so you would not be able to get the setpoint lower than that. But we should still be able to use the basic premise of your test with slightly less extreme setpoints like the following:

When I do this, I can see that the heating and cooling hours seem to mutually exclude one another:

… though, after a few days, the switching between heating and cooling seems to get the rooms to roughly be around their setpoints:

VRF Test_CWM.gh (91.1 KB)

But, yea. It looks like these particular VRFs are set up to be in only a single mode of operations and that cooling mode is given preference over heating mode. That’s why you have the heating setpoint not met for the first couple of days of the simulation.

Granted, there’s probably a way to edit the VRFs with either OpenStudio or the IDF Editor to get them to handle simultaneous heating/cooling. But, at that point, you’re getting a little bit beyond the knowledge on this forum and you might have better luck digging through the EnergyPlus Input/Output Reference or asking your question on UnmentHours.

[anthony.schneider]

Hi @chris

Looks like you have settled it - the VRF systems can only be in a single mode of operation. Therefore, the documentation text is completely true and fine!

I guess if individual rooms (such as the hotel rooms example) need really precise individual control then one should use the grafting method to simply ensure that each room has its own outdoor unit. Probably a decent way to emulate that use case of a VRF system, although it won’t necessarily make use of the potential energy savings of moving heat from one interior zone to another.

Thanks for your help and experiments Chris!