Hello Ben
There are many mistakes in the writing. I myself is not good at writing, but if I try to correct it, then it will be like:
For air leakage test, according to EN314 of which all the standard our constructed air ducts should follow, only high-pressure class air ducts need to be leakage test. Our ventilation equipment falls within only low and medium classes and therefore, the related ductworks are not required to be leakage tested.
[original]
For air leakage test, according to [1] EN314 of which all the standard should have our constructed air ducts being followed, only high pressure class air duct needed to be leakage tested. Our ventilation equipment fall only on low and medium class and therefore, the related ductworks are not required to be leakage tested.
1. What is the meaning "of which" & "should have"?
3. "being followed" what is the grammatical structure?
If we pick up the relative clause [1] and change it into a normal clause, it will be like: " All the standard of EN314 should have our constructed air ducts being followed", but it makes no sense. This part is like gibberish and it is no use parsing that kind of stuff.
2. "Constructed", is it used in past participle?
It is a past-participle derived adjective.
4. "Needed to be leakage tested" what is the grammatical structure?
It should be "need to be leakage tested".
I myself am an English learner and I am afraid I might have made any mistake. But you can be sure some native speaker comes to correct me in that case.
paco