L'impianto termico funziona a condizione che sia controllato.

Because impianto begins with a vowel. In Italian, the singular definite article before a vowel is l':

• l'impianto
• l'edificio
• l'acqua

So l'impianto is the normal form. English speakers often expect il because that is the basic masculine article they first learn, but before a vowel, Italian uses l' instead.

Because termico is an adjective agreeing with impianto, and impianto is masculine singular.

So:

• impianto termico = masculine singular
• caldaia termica would be feminine singular
• impianti termici would be masculine plural

Italian adjectives usually change form to match the noun in gender and number.

In everyday Italian, impianto termico usually refers to a building’s heating system or thermal system—for example, the boiler, radiators, pipes, and related equipment.

A learner might be tempted to translate impianto as plant, but in many practical contexts that sounds unnatural in English. In this kind of sentence, heating system is usually the most natural idea.

Funziona is the third-person singular present indicative of funzionare (to work / to function / to operate).

So:

• io funziono
• tu funzioni
• lui/lei funziona

Here it matches l'impianto termico, which is singular: the system works / functions.

Also, the Italian present tense is flexible. Depending on context, it can correspond to English:

• works
• is working
• does work

In this sentence, it expresses a general fact or condition.

A condizione che is a fixed expression meaning provided that, on condition that, or as long as in a fairly formal style.

It introduces a condition that must be met.

A very important grammar point:
a condizione che is followed by the subjunctive.

So you get:

• a condizione che sia...
• not a condizione che è...

Because a condizione che requires the subjunctive mood.

Sia is the present subjunctive of essere:

• che io sia
• che tu sia
• che lui/lei sia
• che noi siamo
• che voi siate
• che loro siano

So:

• a condizione che sia controllato = provided that it be/is checked

Using è would be ungrammatical in standard Italian after a condizione che.

Because Italian often omits subject pronouns when the subject is already clear from the context or the verb form.

Here, the understood subject of sia controllato is still l'impianto termico.

So Italian does not need to say it explicitly. English does, but Italian usually does not.

A more explicit version like a condizione che esso sia controllato is possible in theory, but it sounds heavy and unnatural in normal speech.

Controllato is the past participle of controllare.

In sia controllato, it is used with essere to form a passive or passive-like/resultative structure:

• sia controllato = is checked / be checked / is inspected

It also agrees with l'impianto, which is masculine singular, so the form is controllato.

Compare:

• l'impianto sia controllato — masculine singular
• la macchina sia controllata — feminine singular
• gli impianti siano controllati — masculine plural
• le caldaie siano controllate — feminine plural

Usually not in the English sense of to control meaning to direct or to dominate.

In many Italian contexts, controllare means:

• to check
• to inspect
• to examine
• sometimes to monitor

So sia controllato here most likely means is checked/inspected, not is controlled.

This is a very common false friend for English speakers.

You could, but it is not exactly the same in tone or structure.

• a condizione che sia controllato = provided that it is checked

  • more formal
  • explicitly states a required condition
  • uses the subjunctive

• se è controllato = if it is checked

  • more neutral and simpler
  • not as formal
  • uses a normal if-clause

So se è controllato may be grammatical in some contexts, but a condizione che sia controllato is better when you want to express a formal requirement.

That depends on context, but in a sentence about a heating system, the most natural interpretation is usually checked or inspected.

• checked = general and natural
• inspected = slightly more formal or technical
• monitored = possible, but suggests continuous observation

So if this is about maintenance or safety, checked/inspected is probably the best fit.