Wir senken die Vorlauftemperatur.

Because Vorlauftemperatur is a feminine noun in German.

A very useful rule here is that in German compound nouns, the last part determines the grammatical gender.
The last part of Vorlauftemperatur is Temperatur, and die Temperatur is feminine.

So:

• die Temperatur
• die Vorlauftemperatur

Also, in this sentence the noun is the direct object of senken, so it is in the accusative case. But for feminine nouns, the article stays die in both nominative and accusative:

• nominative: die Vorlauftemperatur
• accusative: die Vorlauftemperatur

So the form does not change.

This is a very common and important question.

• sinken = to sink / to go down / to fall on its own
• senken = to lower something / to reduce something

So:

• Die Temperatur sinkt. = The temperature is dropping.
  
  • Here, the temperature itself is going down.
• Wir senken die Temperatur. = We lower the temperature.
  
  • Here, we actively cause it to go down.

So in Wir senken die Vorlauftemperatur, the subject wir is doing something to the temperature, which is why senken is the correct verb.

Because in the present tense, the wir form is identical to the infinitive for most German verbs.

For senken:

• ich senke
• du senkst
• er/sie/es senkt
• wir senken
• ihr senkt
• sie/Sie senken

So wir senken is completely normal.

This happens very often in German:

• wir machen
• wir lernen
• wir wohnen
• wir senken

It is a compound noun, which German uses all the time.

Vorlauftemperatur = Vorlauf + Temperatur

A simplified breakdown:

• Vorlauf = flow going out first / supply flow / forward flow
• Temperatur = temperature

In heating or plumbing contexts, Vorlauftemperatur usually refers to the supply temperature or flow temperature of the water going from the heating system into the circuit.

The important grammar point is this:

• German often combines several words into one long noun
• the final element is the main noun
• that final element decides the gender and basic meaning

So Vorlauftemperatur is a kind of Temperatur.

Usually, yes.

In normal German, singular countable nouns are very often used with an article or another determiner. So:

• Wir senken die Vorlauftemperatur. = We are lowering the flow temperature.

Without the article, Wir senken Vorlauftemperatur, the sentence sounds wrong in standard German.

You could omit the article only in certain special styles, such as headlines, technical labels, or note-like writing, but not in normal full sentences.

Because this is a normal main clause in German, and German main clauses usually put the conjugated verb in second position.

Here the structure is:

• Wir = subject
• senken = finite verb
• die Vorlauftemperatur = object

So:

• Wir | senken | die Vorlauftemperatur

This is similar to standard English word order: We lower the flow temperature.

German can move other elements to the front, but the verb still stays in second position:

• Die Vorlauftemperatur senken wir.

That version is more marked and gives extra emphasis to die Vorlauftemperatur.

It is in the accusative case because it is the direct object of senken.

Ask yourself: What are we lowering?
Answer: die Vorlauftemperatur

That makes it the direct object.

Because the noun is feminine, the accusative article is still die:

• nominative: die Vorlauftemperatur
• accusative: die Vorlauftemperatur

If it were masculine, you would see a clearer change:

• nominative: der Druck
• accusative: den Druck

Example:

• Der Druck sinkt.
• Wir senken den Druck.

A helpful rough English-style approximation is:

FOR-lowf-tem-peh-ra-TOOR

A few pronunciation notes:

• Vor- sounds roughly like for
• lauf rhymes approximately with English louse without the final s
• Temperatur is pronounced something like tem-peh-ra-TOOR
• the final -tur is not like English ture in temperature; it is closer to toor

A more German-like breakdown would be:

VOR-lauf-tem-pe-ra-TUR

The main stress is usually on the last part of Temperatur: -tur.

Yes, very often.

German regularly forms new nouns by combining existing nouns or noun-like elements into a single written word. This is one of the most noticeable differences from English.

So where English might write:

• flow temperature
• supply temperature
• heating system temperature

German often writes one compound noun:

• Vorlauftemperatur

This is completely normal and not unusual or overly formal. German technical language especially uses many such compounds.

Yes, absolutely.

Both are possible, but there is a slight difference in feel:

• senken = to lower
• reduzieren = to reduce

In many contexts, they mean almost the same thing.
However, senken often sounds especially natural when talking about making a level, value, or temperature go down.

So:

• Wir senken die Vorlauftemperatur.
• Wir reduzieren die Vorlauftemperatur.

Both are correct, but senken is very idiomatic here.

Yes.

The German present tense can often correspond to both:

• we lower
• we are lowering

So Wir senken die Vorlauftemperatur can mean either, depending on context.

German does not require a separate continuous form like English are lowering.

For example, depending on the situation, this sentence could mean:

• a general action: We lower the flow temperature
• something happening now: We are lowering the flow temperature

Context tells you which meaning is intended.

They are capitalized for two different reasons:

• Wir is capitalized because it is the first word of the sentence
• Vorlauftemperatur is capitalized because all nouns are capitalized in German

So even in the middle of a sentence, Vorlauftemperatur would still be capitalized:

• Heute senken wir die Vorlauftemperatur.

This is one of the most consistent spelling rules in German.

Yes.

In a yes/no question, German typically puts the verb first:

• Senken wir die Vorlauftemperatur?
  = Are we lowering the flow temperature? / Do we lower the flow temperature?

If you use a question word, that question word comes first and the verb stays in second position:

• Warum senken wir die Vorlauftemperatur?
  = Why are we lowering the flow temperature?

So the statement:

• Wir senken die Vorlauftemperatur.

becomes the yes/no question:

• Senken wir die Vorlauftemperatur?