Varmeveksleren varmer luftstrømmen.

Because -en is the usual definite singular ending for many common-gender nouns in Norwegian.

So:

• varmeveksler = a heat exchanger
• varmeveksleren = the heat exchanger

and

• luftstrøm = an airflow / air current
• luftstrømmen = the airflow / the air current

Unlike English, Norwegian often puts the definite article at the end of the noun instead of using a separate word like the.

In this sentence, Norwegian uses the attached definite article.

So instead of:

• English: the heat exchanger
• Norwegian: varmeveksleren

And instead of:

• English: the airflow
• Norwegian: luftstrømmen

This is one of the most important basic patterns in Norwegian.
A separate article is used for the indefinite form:

• en varmeveksler = a heat exchanger
• en luftstrøm = an airflow

But for the definite form, the article is usually added to the noun:

• varmeveksleren
• luftstrømmen

Yes. varmer is the present tense of the verb å varme, meaning to heat or to warm.

So:

• å varme = to heat / to warm
• varmer = heats / warms / is heating

In Norwegian, the present tense often covers both:

• heats
• is heating

depending on context.

They are related, but they are doing different jobs.

• In varmer, it is a verb: heats
• In varmeveksler, varme is part of a compound noun

The noun varmeveksler is made up of:

• varme = heat
• veksler = exchanger / switcher

So varmeveksler literally means something like heat exchanger.

This is very common in Norwegian: words that are related in meaning can appear both as parts of nouns and as verbs.

Both main nouns are compound nouns, which are very common in Norwegian.

1. varmeveksleren

This can be broken down as:

• varme = heat
• veksler = exchanger
• -en = the

So: the heat exchanger

2. luftstrømmen

This can be broken down as:

• luft = air
• strøm = flow / current
• -men = definite ending based on strømmen

So: the airflow / the air stream

A useful rule: in Norwegian compounds, the last part is usually the main word, and the earlier part modifies it.

The word order is:

• Subject: Varmeveksleren
• Verb: varmer
• Object: luftstrømmen

So it follows a straightforward subject–verb–object pattern:

• Varmeveksleren = the subject
• varmer = the action
• luftstrømmen = what is being heated

This is the same basic order as in English:

• The heat exchanger heats the airflow

Mostly from the word order.

In a normal Norwegian main clause, the noun before the verb is usually the subject, and the noun after the verb is usually the object.

So:

• Varmeveksleren comes before varmer → subject
• luftstrømmen comes after varmer → object

Norwegian does not usually mark the object with a special case ending the way some other languages do, so word order is very important.

Both varmeveksler and luftstrøm are normally treated as common-gender nouns, which is why their definite singular form ends in -en.

So:

• en varmeveksler → varmeveksleren
• en luftstrøm → luftstrømmen

In Norwegian Bokmål, common gender nouns typically use:

• en in the indefinite singular
• -en in the definite singular

Yes. Norwegian present tense often does the work of both the English simple present and present progressive.

So varmer can mean:

• heats
• is heating

Which one sounds best in English depends on context.

It would be:

En varmeveksler varmer en luftstrøm.

That means:

• A heat exchanger heats an airflow

Here the nouns are indefinite, so they use the separate article en instead of the attached definite ending.

Compare:

• En varmeveksler varmer en luftstrøm. = A heat exchanger heats an airflow.
• Varmeveksleren varmer luftstrømmen. = The heat exchanger heats the airflow.

A rough English-friendly guide would be:

VAR-me-vek-sler-en

But there are a few things to note:

• r in Norwegian is usually not like a strong English r
• veksler may sound more compact than an English speaker expects
• the stress is mainly near the beginning: VAR-meveksleren

A rough guide for the whole sentence:

VAR-me-vek-sler-en VAR-mer LOOFT-strøm-men

That said, the exact vowel sounds are important, especially in luftstrømmen, where ø does not exist in English.

The letter ø represents a vowel sound that English does not really have.

In strøm, the ø is a rounded front vowel. A rough approximation is something like the vowel in some pronunciations of bird or burn, but with rounded lips.

So strøm is not pronounced like:

• strom
• stream
• strum

It has its own vowel quality, and it is worth practicing separately.

Yes. All of those can work depending on context.

Luftstrøm can mean:

• airflow
• air stream
• air current

In a technical sentence like this, airflow is often the most natural choice.

Yes. A passive version could be:

Luftstrømmen varmes av varmeveksleren.

That means:

The airflow is heated by the heat exchanger.

Here:

• Luftstrømmen becomes the grammatical subject
• varmes is the passive form
• av varmeveksleren means by the heat exchanger

The original sentence is active and usually more direct:

Varmeveksleren varmer luftstrømmen.