Sobald der Heizkreis entlüftet ist, steigt die Rücklauftemperatur.

Because sobald introduces a subordinate clause. In German, the conjugated verb usually goes to the end of a subordinate clause.

So:

• Sobald = as soon as
• der Heizkreis = the heating circuit
• entlüftet ist = is vented / has been bled

This is the normal pattern:

• Sobald + subject + ... + verb

Compare:

• Der Heizkreis ist entlüftet. = The heating circuit is vented.
• Sobald der Heizkreis entlüftet ist, ... = As soon as the heating circuit is vented, ...

Because the sentence starts with a subordinate clause:

• Sobald der Heizkreis entlüftet ist, ...

When a subordinate clause comes first, it takes up position 1 in the main clause structure. In a German main clause, the finite verb must come in position 2. So the next thing after the comma is the verb:

• Sobald der Heizkreis entlüftet ist, steigt die Rücklauftemperatur.

This is the same rule you see in sentences like:

• Heute geht er nach Hause.
• Wenn ich Zeit habe, komme ich.

So steigt comes before die Rücklauftemperatur because German is following the verb-second rule.

Sobald means as soon as. It emphasizes that one thing happens immediately after another.

• Sobald der Heizkreis entlüftet ist, steigt die Rücklauftemperatur. = As soon as the heating circuit is vented, the return temperature rises.

Wenn can mean if or when, depending on context, and is often less immediate.

• Wenn der Heizkreis entlüftet ist, steigt die Rücklauftemperatur. = When / if the heating circuit is vented, the return temperature rises.

So:

• sobald = as soon as
• wenn = when / if

In technical instructions, sobald often suggests a direct consequence.

It is originally the past participle of the verb entlüften.

• entlüften = to vent / to bleed air from something
• entlüftet = vented / bled

In this sentence, it is used with sein:

• der Heizkreis ist entlüftet

That functions very much like an adjective in meaning: the heating circuit is vented.

So grammatically, it comes from a participle, but in this sentence it behaves like a predicate adjective / participial adjective with sein.

Those are just the grammatical genders of the nouns:

• der Heizkreis = masculine
• die Rücklauftemperatur = feminine

So the articles must match:

• der for masculine nominative singular
• die for feminine nominative singular

In this sentence, both nouns are subjects in their own clauses:

• der Heizkreis is the subject of ist
• die Rücklauftemperatur is the subject of steigt

You can tell from the verbs and the sentence structure.

In the subordinate clause:

• der Heizkreis entlüftet ist

The thing that is vented is der Heizkreis, so that is the subject.

In the main clause:

• steigt die Rücklauftemperatur

The thing that rises is die Rücklauftemperatur, so that is the subject.

Both are in the nominative case, which is the usual case for subjects.

Because in German, a subordinate clause is normally separated from the main clause by a comma.

Here the subordinate clause comes first:

• Sobald der Heizkreis entlüftet ist, ← subordinate clause
• steigt die Rücklauftemperatur. ← main clause

This comma is required in standard German.

Yes, it is one word, and that is very normal in German. German often forms compound nouns by joining smaller words together.

Here:

• Rücklauf = return flow
• Temperatur = temperature
• Rücklauftemperatur = return-flow temperature / return temperature

Likewise:

• Heizkreis
  • heizen / Heiz- = heating
  • Kreis = circuit
  • Heizkreis = heating circuit

German compounds are extremely common, especially in technical language.

German often uses the present tense to express:

• general truths
• technical processes
• future results that follow naturally from a condition

So:

• Sobald der Heizkreis entlüftet ist, steigt die Rücklauftemperatur.

uses present tense because it describes a general process or regular consequence.

English might also use the present here:

• As soon as the heating circuit is vented, the return temperature rises.

In some contexts, English might prefer will rise, but German often still keeps the present tense in this kind of statement.

Yes, that is possible, but it is slightly different in nuance.

• Sobald = as soon as → stresses immediacy
• Wenn = when / if → more neutral

So both are grammatical, but sobald is better if the idea is:

Immediately after venting, the return temperature goes up.

It helps to divide it into two parts:

1. Subordinate clause

   • Sobald der Heizkreis entlüftet ist
   • conjunction + subject + predicate + finite verb at the end

2. Main clause

   • steigt die Rücklauftemperatur
   • finite verb first here because the subordinate clause already occupies position 1

A useful pattern to remember is:

• Subordinate clause, verb + subject + ...

So this sentence follows a very common German structure:

• Sobald ..., steigt ...
• Wenn ..., kommt ...
• Obwohl ..., bleibt ...

Yes. You can also put the main clause first:

• Die Rücklauftemperatur steigt, sobald der Heizkreis entlüftet ist.

That version is also correct. The difference is mainly in emphasis:

• Starting with Sobald ... highlights the condition or trigger first.
• Starting with Die Rücklauftemperatur steigt ... highlights the result first.

When the main clause comes first, its normal word order returns:

• Die Rücklauftemperatur steigt
  not
• Steigt die Rücklauftemperatur

because now nothing else is occupying position 1 before the verb.