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2-pipe fan coil system with dedicated outdoor system for ventilation and a vertical geothermal source

asked 2023-02-14 23:42:25 -0500

raghad's avatar

updated 2023-04-22 08:03:54 -0500


I am trying to model a residential house that has 3 floors above ground and one underground. Each floor is equiped with one 2-pipe fan coil unit connected to a main trunk of fresh air from a dedicated outdoor inlet for ventilation. Also, this dedicated outdoor inlet air goes into an ERV unit with a preheated coil to temper the outdoor air before going to the main supply trunk and then to the fan coil units. The fan coil unit pipe is connected to the geothermal heat pump that is linked to the vertical geothermal heat exchanger.

How do I model this HVAC system in eQuest.

Here is my INPUT file: ( Thank you


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answered 2023-02-20 11:38:32 -0500

Drew Morrison's avatar

updated 2023-02-20 11:39:11 -0500

Hi Raghad,

You've brought in a challenging one! I've found eQuest's options for modeling ground to water heat pumps limited, and I've had to use workarounds in the past. Here's two ideas; I'm assuming you have some experience with eQuest's detailed interface.

The simple way is to use water-to-air heat pumps (like a PTHP system) and add the pumping power from the 2-pipe loop to the ground loop. A possible disadvantage of this is that it might overestimate pumping energy if the 2-pipe loop ever turns off while the ground loop operates continuously, which it might do to provide some freeze protection. Another possible disadvantage is that this will allow the system to address simultaneous heating and cooling loads, which a 2-pipe system can't do. However, if you're doing a small residential building simultaneous loads probably won't be an issue.

If you really want to create a true water-to-water system, I'm afraid it's a bit more complicated:

  • Create a 4-pipe fan coil system in the wizard. Go to the chilled water plant and make sure that you select a water-cooled condenser.
  • Switch over to the detailed interface. Change the loop type of the condenser loop from a condenser loop to a water-loop heat pump loop. Create a ground heat exchanger, assign it to the loop, and delete the cooling tower. One important thing: when you change the loop type to a water-loop heat pump, go to the "Controls" tab and change the "Min Alarm Temp" field (which is in the bottom left) to the lowest temperature you expect to come from the ground heat exchanger.
  • Open up the chilled water loop and change it to a 2-pipe loop. Open up your chiller and change it to a heat pump. DO NOT edit the "CW Loop" field (which will now be blank). eQuest technically only allows the heat pump "chiller" type to connect to a lake/well loop, but for some reason eQuest is "fooled" if you don't try to assign the loop yourself.
  • Delete the hot water loop pump, any boilers attached to the loop, and the hot water loop itself. eQuest will prompt you to reassign some components - assign them to the 2-pipe loop you created in the last step.

Good luck!


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Hello Drew,

Thank you so much for your help.

I followed your steps and I got an error saying " Loop: condenser Water Loop has no attached primary heating equipment"

Do you have an idea how to resolve it?

Thank you,


raghad's avatar raghad  ( 2023-02-25 19:17:35 -0500 )edit


Can you attach the model .inp file you're working on? Also, what version of eQuest are you using? That might prove relevant.

Drew Morrison's avatar Drew Morrison  ( 2023-02-28 16:57:46 -0500 )edit

How do we attach a file here ?

raghad's avatar raghad  ( 2023-02-28 17:04:42 -0500 )edit

Raghad, go back to your original question and edit it - you'll see button to attach a file once you're in the editing window.

Drew Morrison's avatar Drew Morrison  ( 2023-02-28 17:10:43 -0500 )edit

Hello Drew,

I tried does it show up?

I also added a GLHX (vertWell-CM at the place of the water tower), and now I have another type of error telling me that In EL1 Syst 1 (FC) "cooling coil leaving temperature must be at least 6F higher than entering water temp."

raghad's avatar raghad  ( 2023-02-28 17:20:05 -0500 )edit

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Asked: 2023-02-14 23:42:25 -0500

Seen: 377 times

Last updated: Mar 01 '23