Die Vorlauftemperatur ist höher als die Rücklauftemperatur.

Because both Vorlauftemperatur and Rücklauftemperatur are feminine singular nouns.

A very useful rule in German is that in compound nouns, the last part usually determines the gender. Here, the last part is Temperatur, and die Temperatur is feminine. So:

• die Vorlauftemperatur
• die Rücklauftemperatur

In this sentence, both are also in the nominative case, because they are being compared in a sentence with sein (ist).

Because German very often forms compound nouns by joining words together into a single word.

So:

• Vorlauf
  • Temperatur = Vorlauftemperatur
• Rücklauf
  • Temperatur = Rücklauftemperatur

This is completely normal in German. English often uses separate words, but German prefers to build one noun.

A good way to understand German compounds is to look at the last element first, because that tells you the basic category of the word. Both of these are types of Temperatur.

Because German main clauses usually follow the verb-second rule, often called V2.

In this sentence:

• Die Vorlauftemperatur = first element
• ist = second element
• höher als die Rücklauftemperatur = the rest

So the structure is normal for a German statement.

If you moved another element to the front, the verb would still stay in second position. For example:

• Heute ist die Vorlauftemperatur höher als die Rücklauftemperatur.

Here Heute is first, and ist is still second.

Because German usually forms the comparative of adjectives by changing the adjective itself, not by using a separate word like English more.

The basic adjective is hoch = high.

Its comparative is:

• hoch → höher = higher

This form is a little irregular because the vowel changes:

• o → ö

So you just need to learn that:

• hoch = high
• höher = higher
• am höchsten = highest

Because als is the normal word for than in comparisons of inequality.

So:

• höher als = higher than
• größer als = bigger than
• schneller als = faster than

German uses als when two things are not equal.

By contrast, German uses wie for equality:

• so hoch wie = as high as

So:

• höher als = higher than
• so hoch wie = as high as

In this sentence, it is nominative.

A helpful way to think about it is that als die Rücklauftemperatur is understood as a shortened comparison. The full idea is something like:

• Die Vorlauftemperatur ist höher, als die Rücklauftemperatur ist.

Because die Rücklauftemperatur would be nominative in that full clause, it stays nominative here.

This is especially noticeable with pronouns in more advanced examples, but for this sentence, the important point is simply:

• die Rücklauftemperatur is nominative singular feminine.

Because höher is being used as a predicate adjective, not an adjective directly before a noun.

Compare:

• die höhere Temperatur = the higher temperature
  → here the adjective comes before the noun, so it takes an ending

• Die Temperatur ist höher.
  → here the adjective comes after ist, so it does not take an ending

In your sentence, höher comes after ist, so no adjective ending is added.

Because in German, all nouns are capitalized.

So in this sentence:

• Vorlauftemperatur
• Rücklauftemperatur

are capitalized because they are nouns.

This is one of the most noticeable spelling differences between English and German.

höher is pronounced roughly like HUR-her, but with a German ö sound that does not exist naturally in standard English.

A rough breakdown:

• hö = a rounded vowel, something like saying eh while rounding your lips
• her = not like English her, but with a lighter German r

Also notice that the h in the middle shows that the vowels belong to separate syllables:

• hö-her

So it is two syllables, not one.

It is grammatically completely normal, but the vocabulary is technical or specialized.

Words like Vorlauftemperatur and Rücklauftemperatur are common in contexts such as:

• heating systems
• plumbing
• engineering
• energy technology

So the grammar is basic and standard, but the nouns belong to a more technical register.

Yes, if the comparison is already clear from context.

For example, if everyone already knows you are comparing it with the return temperature, German could simply say:

• Die Vorlauftemperatur ist höher.

But if you want to state the comparison explicitly, you use the full version:

• Die Vorlauftemperatur ist höher als die Rücklauftemperatur.

So the full sentence is clearer and more precise.

The sentence has this structure:

• Die Vorlauftemperatur = subject
• ist = verb
• höher = predicate adjective/comparative
• als die Rücklauftemperatur = comparison phrase

So you can think of it as:

subject + verb + comparative adjective + als + thing being compared

This pattern is very common in German, for example:

• Das Haus ist größer als die Garage.
• Mein Auto ist schneller als deins.
• Heute ist es wärmer als gestern.