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Water-cooled VRF+Boiler do not handle heating load

asked 2021-03-28 04:58:51 -0500

Keigo gravatar image

updated 2021-04-08 06:14:19 -0500

Has anyone been able to handle heating load with a combination of Water-cooled VRF + Boiler?

My project has water-cooled VRF which is connected to cooling tower and boiler. I created the HVAC system with HVACTemplates (Zone:VRF, System:VRF, Plant:Tower, Plant:Boiler and Plant:MixedWaterLoop) in EnergyPlus 9-4-0.

The calculation completes, and cooling in summer works fine, but there is no heating in winter even though there are a very large heating load. No electricity and natural gas are used. Almost all the heating periods are counted as unmet hours.

When I change Condenser Type in HVACTemplate:system:VRF from WaterCooled to AirCooled, VRF handles heating load well. So, I think there is something wrong with MixedWaterLoop, but I don't know what it is. Operation Scheme Type in HVACTemplete:Plant:MixedWaterLoop is Default. I tried various Temperature Design Setpoint settings, but they didn't work. MixedWater is properly selected as Templete Plant Loop Type in HVACTemplete:Plant:Boiler.

I also tried to created the WSHP system with HVACTemplete:Zone:WaterToAirHeatPump instead of VRF, but I faced exactly the same problem with heating in winter.

I'd really appreciate it if someone could tell me why it doesn't go well.

P.S. Here is a screenshot of my svg file.

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@Keigo there has been a similar question posted here, which mentions that "the VRF model does not account for the impact of other zone equipment on the operation of the VRF system". Do you have other HVAC equipment meeting the zone's heating loads?

Also, have you reviewed the SVG diagram of the VRF condenser loop with the cooling tower and boiler to confirm that components are located correctly?

Aaron Boranian gravatar image Aaron Boranian  ( 2021-03-29 10:52:08 -0500 )edit

@Aaron Boranian Thank you for your comment. Yes, there is a previous similar post, and my original model has DOAS, but even if I remove the DOAS, VRF still doesn't handle the heating load. End use energy is 0.

I had not reviewed the SVG diagram. I looked at it and both air-side loop and water-side loop seem to be no problem. One concern is that there is a note saying "Unused Non-Parent Objects: MXLOOP MIXED TEMP MANAGER", but I don't know how to revise my model. FYI, I uploaded a screenshot of the SVG diagram (The number of zones is reduced to simplify the model).

Keigo gravatar image Keigo  ( 2021-03-29 21:51:14 -0500 )edit

@Keigo check out the apartment-highrise prototype model as a reference, it uses WSHPs with a boiler/cooling tower source water loop.

ericmartinpe gravatar image ericmartinpe  ( 2021-03-30 15:01:30 -0500 )edit

@ericmartinpe Currently, I'm using HVACTemplate because my project is still in the very early stage and needs a rough but quick simulation, but the link will be very useful in the future. Thanks!

Keigo gravatar image Keigo  ( 2021-03-31 10:13:18 -0500 )edit

2 Answers

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answered 2021-03-31 20:40:59 -0500

Posted new issue on E+ GitHub site.

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I think the confusion might be that when the system is water-cooled, those inputs apply to the plant water temperature. Maybe the report name should be changed to Min/Max Condenser Inlet Temperature ...

-20, !- Minimum Outdoor Temperature in Heating Mode {C}
16, !- Maximum Outdoor Temperature in Heating Mode {C}

rraustad gravatar image rraustad  ( 2021-04-01 08:56:31 -0500 )edit

@Keigo, this is the warning I get when I run the example file using 16C Max temp in heating mode. Any suggestions for changes?

** Warning ** AirConditioner:VariableRefrigerantFlow "VRF SYS 1 WATER SOURCE VRF HEAT PUMP".
**   ~~~   ** ...InitVRF: VRF Heat Pump Min/Max Operating Temperature in Heating Mode Limits have been exceeded and VRF system is disabled.
**   ~~~   ** ... Outdoor Unit Inlet Water Temperature  = 25.0
**   ~~~   ** ... Heating Minimum Outdoor Unit Inlet Temperature = -19.99
**   ~~~   ** ... Heating Maximum Outdoor Unit Inlet Temperature = 16.0
rraustad gravatar image rraustad  ( 2021-04-01 10:16:27 -0500 )edit

@rraustad, in the warning of my model, Outdoor Unit Inlet Water Temperature = 26.5ºC. This value may depend on weather data and other parameters, but it seems that we need to input higher Maximum Outdoor Temperature in Heating Mode than Outdoor Unit Inlet Water Temperature.

Keigo gravatar image Keigo  ( 2021-04-01 10:34:40 -0500 )edit

Thanks, we are discussing if there are changes needed at I can't think of any significant changes.

rraustad gravatar image rraustad  ( 2021-04-01 11:05:24 -0500 )edit

And yes, you need to enter a value greater than the inlet water temp. The max temp is used to avoid damaging the compressor per the manufacturer. For water-cooled equipment there should be some range of water temps where heating is allowed to operate.

rraustad gravatar image rraustad  ( 2021-04-01 11:07:44 -0500 )edit

answered 2021-03-31 09:52:35 -0500

Keigo gravatar image

updated 2021-03-31 20:04:48 -0500

I figured out the cause by comparing my model and HVACTemplate-5ZoneVRF.idf in ExampleFiles.

The cause was Maximum Outdoor Temperature in Heating Mode in HVACTemplate:System:VRF. The default value is 16ºC. However, if a value that is less than 26.5ºC is input, the WaterCooled VRF doesn't heat zones even when the actual outdoor temperature is much lower than the input value. This might be a bug.

This does not occur when AirCooled is selected as Condenser Type of VRF. The value is set to 30ºC in HVACTemplate-5ZoneVRF.idf (below image), and it runs fine. My model used default 16ºC. When I changed it to 26.5ºC or higher, my model started to heat zones in winter. Conversely, when I changed the value to less than 26.5ºC in HVACTemplate-5ZoneVRF.idf, the model stopped heating.

C:\fakepath\Screencapture_ HVACTemplate-5ZoneVRF.jpg

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Asked: 2021-03-28 04:58:51 -0500

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Last updated: Apr 01