Die Rücklauftemperatur ist niedrig.

Die is the definite article for the noun Rücklauftemperatur.

In this sentence, Rücklauftemperatur is a feminine singular noun, so in the nominative case it takes die:

• die Rücklauftemperatur = the return temperature

This die does not mean they here. German die can mean different things depending on context, but in this sentence it is clearly the article the.

It is in the nominative case because it is the subject of the sentence.

The structure is:

• Die Rücklauftemperatur = subject
• ist = verb
• niedrig = predicate adjective

So this is basically the same kind of structure as:

• Das Wasser ist kalt.
• Der Druck ist hoch.

The thing being described is in the nominative.

Because niedrig comes after the verb ist and works as a predicate adjective.

In German, adjectives usually take endings only when they come before a noun:

• die niedrige Temperatur = the low temperature

But after sein, werden, or bleiben, the adjective normally stays in its base form:

• Die Rücklauftemperatur ist niedrig.
• Die Temperatur wird niedrig.
• Die Temperatur bleibt niedrig.

So niedrig has no ending here because it is not directly modifying the noun inside a noun phrase.

Ist is the 3rd person singular form of sein = to be.

It matches the singular subject die Rücklauftemperatur:

• ich bin
• du bist
• er / sie / es ist
• wir sind
• ihr seid
• sie / Sie sind

So ist means is here.

Yes. German very often combines several ideas into one compound noun.

Rücklauftemperatur is made up of:

• Rücklauf = return flow / return line / return
• Temperatur = temperature

Together, the whole word means the temperature of the return flow/return line, depending on context.

This is very normal in German, especially in technical language. English often uses several words where German uses one long noun.

In German, nouns are capitalized, but in a compound noun only the first letter of the entire compound is capitalized:

• der Rücklauf
• die Temperatur
• die Rücklauftemperatur

You do not write RücklaufTemperatur. German compounds are usually written as one word, with normal capitalization only at the start.

A helpful rough guide:

• Rücklauftemperatur ≈ RUEK-lowf-tem-pe-ra-TOOR
• niedrig ≈ NEE-drikh or in some regions closer to NEE-drig

A few pronunciation points:

• ü in Rück- does not exist in standard English. It is like saying ee with rounded lips.
• au in -lauf- sounds like ow in cow.
• Final -ig is often pronounced something like -ich in standard German, especially in careful speech: niedrich.
• Stress in Rücklauftemperatur falls mainly on the last part: ...tur.

If you are learning standard pronunciation, niedrich is a very useful pronunciation to know.

Because German main clauses normally follow the V2 rule: the finite verb comes in the second position.

Here the first element is the subject:

• Die Rücklauftemperatur | ist | niedrig

That is perfectly normal German word order.

You can also move another element to the front, but the verb still stays second:

• Heute ist die Rücklauftemperatur niedrig.

So the important rule is not subject first, but finite verb second in a main clause.

Usually niedrig is the natural choice for a value such as temperature when you simply mean low.

• niedrig = low
• tiefer = lower/deeper
• klein = small

Why not the others?

• tiefer is comparative, so it means lower, not just low.
• klein usually means small, not low in the sense of a measured value.

So for a simple statement about a low temperature, niedrig is the normal word.

Normally, no. In standard German, a singular countable noun like this usually needs an article or some other determiner.

So you normally say:

• Die Rücklauftemperatur ist niedrig.

Not usually:

• Rücklauftemperatur ist niedrig.

Omitting the article might happen in special technical writing, note-taking, headings, or telegram-like style, but it is not the normal full sentence form.

The grammatical gender of the compound is determined by its last element, which is called the head of the compound.

Here the last part is:

• die Temperatur — feminine

So the whole compound is also feminine:

• die Rücklauftemperatur

This is a very useful rule in German compound nouns. The final noun usually determines:

• the gender
• the plural
• the general category of the word

The plural is:

• die Rücklauftemperaturen

That is because the head noun is Temperatur, whose plural is:

• die Temperatur
• die Temperaturen

So the compound follows the same pattern:

• die Rücklauftemperatur
• die Rücklauftemperaturen

Example:

• Die Rücklauftemperaturen sind niedrig.

It sounds technical or at least specialized, because Rücklauftemperatur is not an everyday household word for most learners.

The grammar itself is completely ordinary and simple, but the vocabulary is typical of areas like:

• heating systems
• engineering
• energy systems
• plumbing/HVAC
• industrial processes

So a learner might think: the sentence structure is basic, but the noun is domain-specific.