Nem azért légtelenítjük, mert új, hanem azért, mert kezdődik a fűtésszezon.

This is a very common Hungarian pattern for contrasting reasons:

• nem azért ..., mert ... = not because ...
• hanem azért, mert ... = but because ... / rather because ...

So the structure means:

• Not for that reason ..., because ...
• but for that reason ..., because ...

In natural English, you usually just say not because X, but because Y. Hungarian often spells out the reason word with azért.

Here azért means something like for that reason or that’s why.

In this sentence, it does not stand alone; it works together with mert:

• azért ..., mert ... = for the reason that ... / because ...

So:

• Nem azért légtelenítjük, mert új
  = We’re not bleeding/venting it because it’s new
• hanem azért, mert kezdődik a fűtésszezon
  = but because the heating season is starting

Hungarian often uses this kind of paired structure more explicitly than English does.

Hanem is used after a negative statement when you correct or replace what was just denied.

Compare:

• de = but
• hanem = but rather / instead

In this sentence, the first reason is rejected:

• Nem azért ... = Not because of that ...

So Hungarian naturally uses hanem to introduce the real reason:

• nem ..., hanem ... = not ..., but rather ...

Using de here would sound wrong or at least much less natural in standard Hungarian.

Légtelenítjük comes from the verb légtelenít, which means:

• to bleed
• to vent
• to remove air from

This is often used for:

• radiators
• heating systems
• pipes

The form légtelenítjük breaks down like this:

• légtelenít- = verb stem
• -jük = we do it with a definite object

So légtelenítjük means we bleed it / we vent it / we remove the air from it.

Hungarian verbs have two main conjugations:

• indefinite
• definite

The definite conjugation is used when the object is specific or already known.

Here, légtelenítjük has the definite ending -jük, which tells you that the thing being vented is definite:

• we vent it
• we bleed the system/radiator

Even though the object is not said out loud, it is understood from context. Hungarian often leaves out an obvious object if everyone knows what is being talked about.

If the object were indefinite, you would expect a different form.

Because Hungarian often omits things that are clear from context.

In English, you might say:

• We’re not bleeding the radiator because it’s new...
• We’re not venting it because it’s new...

In Hungarian, if the listener already knows what is being discussed, you can simply say:

• Nem azért légtelenítjük...

The verb ending already tells you there is a definite object, so the sentence still feels complete.

Because Hungarian often leaves out both the subject and the verb to be in the present tense.

So:

• mert új literally looks like because new
• but it means because it is new

What is omitted?

• the subject: it
• the present-tense is

This is very normal in Hungarian. Predicate adjectives often stand alone in the present tense:

• A ház nagy. = The house is big.
• Ez új. = This is new.

So mert új is perfectly natural Hungarian for because it’s new.

Kezdődik means starts / begins.

This verb is the intransitive form: the thing itself begins.

• A fűtésszezon kezdődik. = The heating season is starting.

By contrast, kezd is often used when someone starts something:

• Elkezdem a munkát. = I start the work.

So in this sentence, the subject is a fűtésszezon and it is the season itself that is beginning, which is why kezdődik fits well.

Hungarian uses the definite article a/az much like English the.

So:

• a fűtésszezon = the heating season

It sounds like a specific, known season, not just any heating season in general. That is why the article is natural here.

Also, fűtésszezon is a compound noun:

• fűtés = heating
• szezon = season

Together: fűtésszezon = heating season

It can very naturally refer to the near future.

Hungarian often uses the present tense for:

• scheduled events
• expected near-future events
• seasonal changes

So kezdődik a fűtésszezon can mean:

• the heating season is starting
• the heating season starts now / is about to start

This is similar to English sentences like:

• School starts next week.
• The movie begins at eight.

Hungarian word order is flexible, but it is strongly connected to focus and emphasis.

Here the sentence foregrounds the reason contrast:

• Nem azért ... hanem azért ...

So the structure is organized around the two competing reasons:

1. the rejected reason
2. the real reason

That is why azért comes early. It sets up the contrast very clearly.

A more literal breakdown is:

• Nem azért légtelenítjük
  = It is not for that reason that we bleed it
• mert új
  = because it is new
• hanem azért, mert kezdődik a fűtésszezon
  = but rather because the heating season is starting

The word order is doing discourse work, not just grammar.

Because Hungarian normally separates these clause boundaries with commas.

You have:

• Nem azért légtelenítjük, mert új, hanem azért, mert kezdődik a fűtésszezon.

The commas help mark:

1. the mert clause
2. the contrast introduced by hanem
3. the second mert clause

Hungarian punctuation is often quite consistent about putting commas before conjunctions that introduce subordinate clauses, such as mert.

Sometimes in speech, Hungarian can be a bit looser, but in this sentence the repeated azért is very natural and stylistically neat.

• nem azért ..., hanem azért, mert ...

This full pattern clearly balances the two parts:

• rejected reason
• accepted reason

If you removed the second azért, the sentence might still be understandable in some contexts, but the original version is the most standard and elegant here.

It is basically neutral standard Hungarian.

Nothing in it is especially slangy or especially formal. It would fit well in:

• everyday conversation
• practical explanations
• household or maintenance contexts
• written instructions with a natural tone

The only potentially specialized word is légtelenítjük, because it belongs to the vocabulary of heating/radiators/pipes, but grammatically the sentence is very standard.