I have clients that have a dedicated hot deck AHU and dedicated cold deck AHU for dual duct VAVs. How would you model airRef on a Dual Duct VAV to have two airRefs, one pointing to the hot deck AHU and the other pointing to the cold deck AHU? Does there need to be a hotAirRef and a coldAirRef? Are there tags on the AHU for output cold air and output hot air?

This is an example of how I wouldn't know where to begin creating new Haystack 4 defs with all the proper tags to model it so it works correctly in the Builder.

Its a good question - I don't think its been discussed exactly that way before.

But it is similar to a the general question which has been raised several times before: what happens when a VAV is supplied by multiple AHUs. The answer to that one is to make airRef a list of refs to all the AHUs. One of the features added to the filter language is the handling of lists so that the == operator will match against anything in the list:

// records
id:@ahu1, dis:"AHU-1", ahu, equip
id:@ahu2, dis:"AHU-2", ahu, equip
id:@vav, airRef:[@ahu1, @ahu2], vav, equip

// all of these filters will match the VAV record
airRef == @ahu1
airRef == @ahu2
airRef->dis == "AHU-1"
airRef->dis == "AHU-2"

So my opinion is that the same technique should be applied to your use case too.