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Modeling Daikin VRF in Equest - Too high heating consumption

asked 2015-04-15 09:31:53 -0500

Yupeng's avatar

updated 2015-04-20 11:04:03 -0500

I used the Daikin guide to simulate VRF in equest. There are two questions.

  1. PSZ was used as the system which only allow one zone for each system. But in my model, each system served several zones. Which system shall be chosen for multiply zones?
  2. Because the question one, I chose PTAC as the system, and changed the curves based on Daikin guide. If, I chose heat pump as the heating source, I got a very high heating consumption in winter. If, I chose electric as the heating source, the heating consumption was almost zero in winter. Who can explain it and which one is correct for VRF heating?
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Given the answers supplied below by Julien and crduggin, should the title be updated to be Daikin specific?

Daniel's avatar Daniel  ( 2015-04-20 10:30:18 -0500 )edit

Yes, definitely. I'll change it

Julien Marrec's avatar Julien Marrec  ( 2015-04-20 11:02:30 -0500 )edit

4 Answers

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answered 2015-04-16 02:14:27 -0500

updated 2015-04-20 10:10:57 -0500

  1. Always use the system recommended by the manufacturer of your VRV. eQuest can't model VRV properly, so manufacturers choose a given eQuest system and create custom curves for THIS system in order to approximate what's going on. That's why they give you a modeling guide and eQuest curves, so use that. It will also help you justify your approach if you're doing this for compliance or an incentive program.

  2. As far as heating going up with heat pump compared to elec, it's likely a modeling mistake and/or lack of analysis of the model's output (unmet hours in particular, maybe outdoor air as well). @Daniel's suggestions are on point and also where I would start to debug.

@crduggin just refreshed my memory. As of July 2014, there was still a mistake in Daikin's curves: eQuest has a limit of 80 characters per line... If you put more it gets truncated.

I emailed some rep twice about it, I don't know if it was corrected or not.

Here's a message I wrote in July 2014 to the eQuest user mailing list:

I've been trying to model a Daikin VRV system and trying to figure out why I was getting 100 times more space heating in the proposed (VRV) model compared to the baseline model (PTHP). After isolating the problem to the VRV, I found a conversation from March 2014 between a few folks (Byron Burns and Bill Bishop mostly - thank you both!), that basically explained why I was getting the issue: the .inp data from Daikin isn't importing correctly.

I just wanted to say that we're in July now, and this still hasn't been fixed (I downloaded the curve from Daikin a few days ago), so if you're having the problem, the fix proposed by Byron and Bill still works. It consists in to replacing the VRV Heating Curve with:

"VRV Heating EIR-F-EWB/ODB AC" = CURVE-FIT      
    TYPE             = BI-QUADRATIC-T
    COEFFICIENTS     = ( -1.85025, 0.0620141, -0.000312822, 0.07145853,
        -0.0003626328, -0.0007776424 )
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At the end of the message, there's an attachment Daikin-VRVperformance-FixedHeatingEIR.inp (direct link here - save an rename as inp). This is the daiking curves after my modification

Julien Marrec's avatar Julien Marrec  ( 2015-04-20 10:12:36 -0500 )edit

Note that the curve Julien is proposing uses identical numbers to the one supplied by Daikin, but different formatting to avoid having a line length longer than 80 characters.

Daniel's avatar Daniel  ( 2015-04-20 10:27:41 -0500 )edit

The 80-character limit is a very old thing: dates from the IBM Punchcard... source

Julien Marrec's avatar Julien Marrec  ( 2015-04-20 11:05:41 -0500 )edit

I was just revisting this thread for adjusting an eQuest model with the old curves imported. I found that Daikin has now updated the files on their website where you replace your BDLLIB.dat with one they have added all their curves to, so you can import them directly to your project from the eQuest library.

crduggin's avatar crduggin  ( 2016-08-17 12:54:50 -0500 )edit

answered 2015-04-15 15:57:42 -0500

updated 2015-04-15 15:58:14 -0500

  1. I would recommend going with PSZ and using the Detailed Edit mode in eQUEST to create a system for every zone. If this is cumbersome, you could also go back to the eQUEST wizard mode and create a PSZ system and assign it to every zone. In the HVAC System Wizard, this is done by choosing System Type: Packaged Single Zone DX and System per Area: System per Zone. The PSZ system type is more flexible than the PTAC system type and allows you to use a wider range of options for heating and cooling. It is also the system type used in the Daikin VRF Guide.
  2. This sounds very odd. One would expect the heating energy to be lower with a heat pump (which typically uses electric heat as a backup when the heat pump can't provide enough heat). I just tried creating a model in the wizard, and switched the heating type from heat pump to electric for a PTAC and the heating energy went up as expected. Can you provide any more information about your model? What is the climate zone? Do you have unmet heating hours? What is the Heating Electric Input Ratio for your heat pumps?
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It also occurs to me that the performance curves could be pushed out of range if the heat pump tries to operate at temperatures where the heating EIR and heating capacity curves give non-sensical results. Have you tried adjusting the Minimum HP Heat Temp under Supp Heat/Defrost? It might be worth evaluating the performance curves from Diakin over a range of temperatures to see what they return. You might also want to create an hourly report which shows the heating energy, heating EIR, heating capacity, and the outdoor air temperature on an hourly basis for one of your heat pump systems.

Daniel's avatar Daniel  ( 2015-04-20 10:04:19 -0500 )edit

answered 2015-04-20 09:06:07 -0500

crduggin's avatar

updated 2015-04-20 09:06:46 -0500

I have tried to use the Daikin curves and modeling guide, but it yielded the same results. I believe there is an error in the VRV Heating EIR curve because if i set it back to the default PVVT curve, the heating energy is more reasonable.

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There's definitely one indeed, at least as of July 2014... I don't know why I couldn't remember that, thanks a lot for refreshing my memory. See my updated answer...

Julien Marrec's avatar Julien Marrec  ( 2015-04-20 10:00:37 -0500 )edit

I was unaware it was a simple truncation issue. Thanks for the info.

crduggin's avatar crduggin  ( 2015-04-20 10:39:11 -0500 )edit

answered 2015-06-03 05:07:57 -0500

shree's avatar

As per Julien and Daniel's suggestion, if we model VRF system considering one system per zone, then how do we input the cooling capacity and supply CFM for each system. Also how much additional savings can we expect considering Daikin curves (versus default DX curves)?

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This isn't an answer, rather a comment, or another question. So you shouldn't use an answer for that.

Anyways, for each unit, you input CFM, fan power (kW/CFM), based on the indoor unit specs. You input cooling EIR, heating EIR, and curves, based on the outdoor unit. Cooling and heating caps is from the indoor unit (though I would reduce it according to the capacity ratio if it's closer to 120%). Read the Daikin modeling guide.

As far as savings compared to DX curves... depends on your building, depends on your climate. try it for yourself.

Julien Marrec's avatar Julien Marrec  ( 2015-06-04 03:27:54 -0500 )edit

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Asked: 2015-04-15 09:31:53 -0500

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Last updated: Apr 20 '15