Did you notify your landlord that the aircon was not working, or just called in the repairman to come and fix it?
In Hong Kong, the landlord is obliged to make sure the aircon is in a working order, but if you expect him to pay for it, you should have notified him beforehand. He may have his own repairman to fix it.
You can't just assume that he will pay it just because your previous landlords have done so. It probably isn't a bad idea to establish at the outset what he would like you to do in the event of something going wrong.
My previous landlord in HK didn't want to be bothered by repairs, so he would ask me to do them and send him the bill, then he would send me a cheque.
When my old washing machine broke down, he told me to buy a new one and deduct the money from the next month's rent. So establish exactly what is to be done in future.
Legally you cannot make him pay for the repair work if he wasn't notified in advance.