On the AHUs page, it notes that:

All of these equipment types follow the same fundamental modeling techniques and subtype from airHandlingEquip.

However, this isn't actually true for the heatPump tag, which seems to be just defined as a marker.

Is there a reason for this? It does cause some problems with validating heat pump models.

I would argue that heat pumps shouldn't be in a list of AHUs at all. That's the other side of the heating/cooling process and not part of and air handling equipment. I guess you could make the argument that a package unit is a heat pump and an air handler, but then why do we not have compressor chillers in this list?

I think that's just a mistake in the docs on AHUs and a holdover from when heatPump used to subtype from AHU.

If you look at the actual equip taxonomy, you can see that heatPump does not subtype from AHU.

I guess the question I have then is how to model something like a water source heat pump (something like https://www.daikinapplied.com/products/water-source-heat-pumps).

To me it seems like it's an air handler, and when I look at the equip taxonomy it's not totally obvious.

Annie -- do you think modeling that as an ahu with a heatPump market tag would be the way you would go?

Here is how I would model a WSHP:

{equip,
 ahu,
 heatPump,
 dxCooling,
 dxHeating,
 condenserWaterRef:@cwPlant,
}

Yes exactly, my understanding is that heatPump is now a descriptive marker tag that can be combined with other equipment tags such as ahu.

In OAP, we have defined a Single Zone Water Source Heat Pump Air Handling Unit which would accurately model that type of Daikin equipment you linked to.

The tags for the equip per OAP would be:

{equip,
 ahu,
 directZone,
 heatPump,
 dxCooling,
 dxHeating,
 waterCooled
}

(Pretty similar to what Leroy posted)