Performance Curve Selection for WLHP and GSHP

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Hi Everyone,

I've come to the belated realization that I've been using one set of performance curves to describe equipment rated at two very different conditions. I use PVVT systems to model both water-loop heat pumps and ground-source heat pumps, and I've been using the family of performance curves that start with "GSHP/WLHP" for both system types. (These curves are in the "GSHP/WLHP curves" section of the library, and they're the default curves when you specify a PVVT system with a water-cooled condenser.) The name would seem to imply that these curves are valid for both GSHP and WLHP systems, but I had occasion to graph them recently and found that they're normalized to the ISO rating conditions for a ground-source heat pump - an entering water temperature of 32F and an entering air dry bulb of 68F. (I presume the same issue exists for cooling, but for brevity's sake I'll restrict this to heating.) Entering the COP for a heat pump rated at the much higher WLHP rating temperature (68F water) would overpredict performance by a lot.

Does anyone have suggestions for performance curves (eQuest library or otherwise) that are appropriate for a water-loop heat pump? I found some curves labeled "Water-Loop HP" in the eQuest library that seem to fit the bill, since they come to unity at an entering water temperature of 68F and an entering dry bulb of 70. Turns out these are the defaults if you select a Water Loop HP air-side system, which I've never done before because my usual M.O. is to start with PVVT. The library curves in question are:


A small, very nitpicky aside: The ISO ratings actually specify an entering air dry-bulb temperature of 68F, but the eQuest library curves seem to come to unity closer to 70. Far from a big deal in the grand scheme of things, just the scuff on your shoe you keep trying to ignore.

Drew Morrison
Energy Engineer

dmorrison at
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Drew Morrison | Energy Engineer Slipstream 608.210.7151
Drew Morrison's picture
Joined: 2015-11-19
Reputation: 0

Hi Drew,
When you create a PVVT system, and select Water Cooled for the Condenser Type, the WLHP-CATEGORY becomes a required input:
[cid:image002.png at 01D8B7AF.4ABC3DC0]
You then get different default rated entering condenser (water) temperatures for heating and cooling depending on whether you select WATER-LOOP, GROUND-WATER or GROUND-LOOP. You also get different default heating and cooling efficiencies (HEATING-EIR and COOLING-EIR). If you know the heating COP for the particular heat pump you want to model, you convert that to HEATING-EIR and verify that the HT-RATED-ECT corresponds to that COP.
The same set of GSHP/WLHP performance curves is used for all water-cooled heat pumps, but they are normalized (don't ask me how) based on the inputs described above. You can verify this by running the same model, with identical HEATING-EIR and COOLING-EIR but with different rating conditions, and comparing the heating and cooling energy shown in the SS-A reports.
I believe the above guidance also applies any time you model a water-cooled heat pump using the other system types HP, PSZ and PVAVS.

William Bishop, PE, BEMP, BEAP, CEM, LEED AP
Senior Energy Engineer
T: (585) 698-1956 F: (585) 325-6005
bbishop at
134 South Fitzhugh Street
Rochester, NY 14608 [cid:image005.png at 01D8B7C2.3BC0A910]

Bill Bishop's picture
Joined: 2012-02-25
Reputation: 7

This is quite interesting - thanks, Bill!

I'm using eQuest 3.64, and it doesn't seem to support this feature - at least, I tried entering the RATED-ECT and HT-RATED-ECT keywords into an .inp file and got an error while loading. Maybe this is another incentive for our shop to start using the latest model...

I think there is a workaround for 3.64/DOE2.2, and that's to manually perform the calculations that the newest version is doing behind the scenes. The GSHP/WLHP curves seem to be normalized to the ground-source heat pump rating temperatures. If the user enters different temperatures, or selects Water Loop HP (or Groundwater HP) from the list of rating categories, I think the engine just divides the user-input EIR value by the values of the CAPFT and EIRFT functions evaluated at the user-defined temperatures to "convert" the EIR to what it would be when the GSHP/WLHP curves are at unity. It wouldn't be hard to do that in a spreadsheet, then enter the converted EIR into eQuest if you're using 3.64. (I've been experimenting with this to try to more accurately model certain new rating standards related to DOAS systems...)

Drew Morrison | Energy Engineer Slipstream 608.210.7151
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Joined: 2015-11-19
Reputation: 0