Der Fachbetrieb entlüftet den Heizkreis.

This is about case.

• der Fachbetrieb is the subject of the sentence, so it is in the nominative case.
• den Heizkreis is the direct object, so it is in the accusative case.

Both nouns are masculine:

• der Fachbetrieb → nominative masculine singular
• den Fachbetrieb → accusative masculine singular
• der Heizkreis → nominative masculine singular
• den Heizkreis → accusative masculine singular

So the sentence structure is:

• Der Fachbetrieb = the specialist company/business does the action
• entlüftet
• den Heizkreis = the heating circuit receives the action

Fachbetrieb is a compound noun:

• Fach = specialist field, technical area, trade
• Betrieb = business, company, operation

So Fachbetrieb usually means something like:

• specialist company
• qualified contractor
• professional trade business

It often suggests a company with proper expertise, especially in technical or repair contexts.

Because German very often forms compound nouns by joining words together.

Heizkreis is made from:

• Heiz- = heating
• Kreis = circuit / loop

So Heizkreis means heating circuit.

This is completely normal in German. Instead of writing separate words as English often does, German usually combines them into one noun.

Other examples:

• Heizung = heating system
• Heizkörper = radiator
• Wasserkreislauf = water cycle / water circuit

The gender of a compound noun in German is determined by the last part of the compound.

Here, the last part is Kreis, and der Kreis is masculine.

So:

• der Kreis
• therefore der Heizkreis

This is a very useful rule for compound nouns:

• das Haus → das Wohnhaus
• die Anlage → die Heizungsanlage
• der Kreis → der Heizkreis

No, entlüften is not separable.

The prefix ent- here is an inseparable prefix, so it stays attached to the verb:

• entlüften
• Der Fachbetrieb entlüftet den Heizkreis.

Not:

• Der Fachbetrieb lüftet den Heizkreis ent.

That would be wrong.

This also matters in other forms:

• infinitive: entlüften
• present: er entlüftet
• past participle: entlüftet

Notice that the past participle does not add ge-:

• correct: entlüftet
• not: geentlüftet

It is the 3rd person singular present tense of entlüften.

Conjugation:

• ich entlüfte
• du entlüftest
• er/sie/es entlüftet
• wir entlüften
• ihr entlüftet
• sie/Sie entlüften

So in the sentence, der Fachbetrieb is a singular subject, which is why the verb is entlüftet.

Because German main clauses usually follow the V2 rule: the finite verb goes in the second position.

In this sentence:

1. Der Fachbetrieb
2. entlüftet
3. den Heizkreis

That is standard German word order.

You can also move another element to the front, but the verb still stays second:

• Den Heizkreis entlüftet der Fachbetrieb.

That version is grammatically possible, but it sounds more marked or emphasizes den Heizkreis.

In normal German, you usually need the articles here.

So the natural sentence is:

• Der Fachbetrieb entlüftet den Heizkreis.

Without articles:

• Fachbetrieb entlüftet Heizkreis

This sounds unnatural and incomplete in standard German. German uses articles more regularly than English does.

The articles also help show:

• gender
• number
• case
• and therefore the sentence roles

The ü is a front rounded vowel that English does not really have.

A rough way to practice it:

1. Say ee as in see
2. Keep your tongue in that position
3. Round your lips as if saying oo

That gives you something close to ü.

So entlüftet sounds roughly like:

• ent-LUE-fet or ent-LYOO-fet as a very rough English approximation

But the exact sound is more precise than either of those.

A rough pronunciation guide:

• Fachbetrieb ≈ FAKH-beh-treep
• entlüftet ≈ ent-LUEF-tet
• Heizkreis ≈ HITES-kryce

A few details:

• ch in Fach is the throaty German ch, not an English k
• ei in Heiz- sounds like English eye
• z in German is pronounced ts
• ie in -trieb is a long ee
• eu/äu is not in this sentence, but learners often mix it up with ei, so it is good to keep them separate

The passive version is:

• Der Heizkreis wird vom Fachbetrieb entlüftet.

Why?

• The object den Heizkreis becomes the subject der Heizkreis
• German passive uses werden + past participle
• vom Fachbetrieb = by the specialist company

So:

Active:

• Der Fachbetrieb entlüftet den Heizkreis.

Passive:

• Der Heizkreis wird vom Fachbetrieb entlüftet.

A common past form in spoken German is the Perfekt:

• Der Fachbetrieb hat den Heizkreis entlüftet.

This uses:

• hat = auxiliary verb
• entlüftet = past participle

Again, because ent- is inseparable, the participle is just entlüftet, not geentlüftet.

A simple past form is also possible:

• Der Fachbetrieb entlüftete den Heizkreis.

But in everyday spoken German, hat ... entlüftet is often more common.