Det kommer an på om varmebehovet er lavt.

It is a fixed expression: å komme an på, which means to depend on.

A useful thing to notice is that it is split in the sentence:

• Det kommer an på ...

That happens because kommer is the finite verb, and in a Norwegian main clause the finite verb normally comes in position 2. So the full expression gets separated.

Compare:

• Det kommer an på om ...
• Alt kan komme an på dette.
• Å komme an på noe

So you should learn komme an på as one unit, even though it may appear split in real sentences.

Here det is a dummy subject, just like it in English it depends.

Norwegian main clauses normally need a subject, so det fills that role. It does not refer to a specific thing here.

So:

• Det kommer an på ... = It depends on ...

This is very common in Norwegian with weather, time, and general statements:

• Det regner. = It is raining.
• Det er sent. = It is late.
• Det kommer an på ... = It depends on ...

Om introduces an indirect yes/no clause, so here it means whether or sometimes if.

In this sentence:

• Det kommer an på om varmebehovet er lavt.

the structure is basically:

• It depends on whether the heating need is low.

This om is not the same as English about.

It is also different from hvis, which usually means if in a conditional sense:

• Om varmebehovet er lavt = whether the heating need is low
• Hvis varmebehovet er lavt, ... = if the heating need is low, ...

After expressions like det kommer an på, om is the natural choice.

Because Norwegian usually writes compound nouns as one word.

So varmebehovet is built like this:

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

Together:

• varmebehovet = the heating need / the heat demand

This is very normal in Norwegian. English often uses several words where Norwegian prefers one compound word.

Other examples:

• bil dør in English would be car door, but in Norwegian: bildør
• energy use = energibruk
• heat pump = varmepumpe

Because the base noun behov is a neuter noun:

• et behov = a need

The definite singular form is:

• behovet = the need

When behov is the last part of a compound noun, it keeps its gender, so:

• et varmebehov = a heating need
• varmebehovet = the heating need

In Norwegian compounds, the last part usually determines the gender.

Because lavt agrees with the noun it describes, and varmebehovet is singular neuter.

The adjective lav changes like this:

• lav for common gender singular
• lavt for neuter singular
• lave for plural

So:

• en temperatur er lav
• et behov er lavt
• prisene er lave

Here the noun is:

• varmebehovet → from et varmebehov

So the correct form is:

• varmebehovet er lavt

Even though varmebehovet is definite, the predicate adjective still follows the usual gender/number agreement pattern.

The dictionary form is å komme an på.

In this sentence you see the present tense:

• kommer an på = depends

Some useful forms are:

• å komme an på = to depend on
• kommer an på = depends / depends on
• kom an på = depended on
• har kommet an på = has depended on

So if you want to learn it properly, learn the whole phrase:

• å komme an på noe
• å komme an på om ...

The sentence follows normal Norwegian word order.

Main clause:

• Det kommer an på ...

Here the finite verb kommer is in second position, which is the normal V2 pattern in Norwegian.

Then comes the clause introduced by om:

• om varmebehovet er lavt

Inside that subordinate clause, the order is straightforward:

• varmebehovet = subject
• er = verb
• lavt = predicate adjective

A useful thing to notice is what happens with negation in a subordinate clause:

• om varmebehovet ikke er lavt

So after om, words like ikke come before the finite verb.

Yes. A common alternative is:

• Det avhenger av om varmebehovet er lavt.

This also means it depends on whether the heating need is low.

The difference is mostly style:

• det kommer an på = very common, everyday, idiomatic
• det avhenger av = also common, slightly more formal or more direct in tone

Both are correct, but det kommer an på is an expression worth learning because Norwegians use it a lot.

It can definitely mean a low level, not just something physically short or low.

In this sentence, lavt refers to the level of the heating need, so it means something like:

• low
• small
• limited

Norwegian uses lav / lavt / lave in many abstract contexts:

• lav temperatur = low temperature
• lav pris = low price
• lavt energiforbruk = low energy use
• lavt varmebehov = low heating need

So this is a very natural use of lavt.