Sentence Correction

I need to understand whether the below sentence is parallel

Sentence:
There are fans but the hall has been quite airy.

I need to understand that are the two parts joined by 'but' completely parallel?

I assume it to be parallel but to my eyesight , they don't have the same structure though.

There are fans (There + helping verb + noun ) (Btw, what part of speech is there? A pronoun?)

The hall has been quite airy (Article +Noun+ has Been(present perfect tense) + adjective).

Please address this issue,
Also throw light on whether simple present/past tense could be parallel to the present/past perfect tense in sentences?

Any insights shall be deeply appreciated!!

Well, but is a coordinating conjunction and in this case, it is connecting two Independent clauses. So, that way it's ok; not that the sentence is perfect, because the meaning is not coming out clearly.

Please note that parallelism does not mean that the entire sentence should be in the same tense.