Kompressoren starter når varmebehovet øker.

Because -en is the definite article attached to the noun.
So:

• en kompressor = a compressor
• kompressoren = the compressor

Norwegian usually adds the definite article to the end of the noun instead of putting a separate word before it, as English does.

Norwegian often makes compound nouns by joining words together.

Here, varmebehovet is built from:

• varme = heat
• behov = need / demand
• -et = the definite ending

So varmebehovet literally means the heat need, but in natural English that is usually the heating demand or the need for heat.

This kind of compounding is very common in Norwegian.

It is the definite ending for a neuter noun.

The base noun is:

• et varmebehov = a heating demand / a need for heat

In definite form:

• varmebehovet = the heating demand / the need for heat

So just like -en in kompressoren, -et marks definiteness, but for a different gender.

Yes. Starter is the present tense of å starte.

So:

• å starte = to start
• starter = starts / is starting / does start

In Norwegian, the present tense is often used for general facts, routines, and even some future meaning depending on context.
Here, Kompressoren starter når varmebehovet øker means something like a general rule: the compressor starts when the heating demand increases.

Here, når means when.

A useful basic distinction is:

• når = when, for something general, repeated, or future-related
• da = when, for a specific event in the past

So in this sentence, når is used because it describes a general condition or repeated situation:

• Kompressoren starter når varmebehovet øker.
  = The compressor starts when the heating demand increases.

If you were talking about one specific past event, Norwegian would more likely use da.

Because after når, you get a subordinate clause, and in subordinate clauses the normal order is usually:

subject + verb

So:

• varmebehovet = subject
• øker = verb

That gives:

• når varmebehovet øker = when the heating demand increases

This is different from the main-clause word order rules in Norwegian, where the verb often has to stay in second position.

Yes, absolutely. That is a very natural alternative.

Both mean the same thing:

• Kompressoren starter når varmebehovet øker.
• Når varmebehovet øker, starter kompressoren.

The difference is mainly emphasis and sentence flow.
If the når clause comes first, the main clause follows Norwegian V2 word order, so you get:

• Når varmebehovet øker, starter kompressoren.

Notice that it is starter kompressoren, not kompressoren starter, because the verb must come second in the main clause after the introductory subordinate clause.

Øker means increases or rises.

It comes from the verb å øke = to increase.

In this sentence it is used intransitively, meaning the heating demand itself goes up:

• Varmebehovet øker. = The heating demand increases.

Norwegian å øke can also sometimes be used transitively in other contexts, depending on style and usage.

Because Norwegian does not use do/does the way English does in ordinary affirmative sentences.

English:

• The compressor starts
• The compressor does start only for emphasis

Norwegian normally just uses the main verb:

• Kompressoren starter

So there is no extra helper verb needed here.

In a technical sentence like this, it usually means something like starts, switches on, or begins operating.

The exact best English translation depends on context:

• for machinery: starts or turns on
• for a process: begins

So Kompressoren starter most naturally means the compressor starts or the compressor turns on.

A rough pronunciation guide is:

• Kompressoren ≈ kom-pre-SOR-en
• starter ≈ STAR-ter
• når ≈ nor / nohr
• varmebehovet ≈ VAR-me-be-HO-vet
• øker ≈ UR-ker or EUH-ker, depending on how exact you want to be

The letter ø is a vowel that does not exist in standard English. It is somewhat similar to the vowel in French deux or German schön.

So:

• øker has a front rounded vowel, not a normal English o

Even if your ø is not perfect, learners are usually understood if the rest of the word is clear.