In English, the difference between "the tea is heated by them" and "the tea is being heated by them" is that the first construction has a habitual aspect, whereas the second describes what is happening now. You can restate them as "they heat the tea" and "they are heating the tea".
I think that tá an tae téite acu, using the past participle of téamh, is the construction that you are looking for, though the saorbhriathar would probably be preferred in many instances.
Oh, the sublties of the English language. Whenever someone claims it is "the easiest language" I like to point out things like these. English might be fairly easy to grasp when concidering the basics, but being non-native such subtle differences always break my neck. Thanks for the explanation!