Gulvvarmen er tændt.

Because Danish very often forms compound nouns by writing them as a single word.

• gulv = floor
• varme = heat/heating
• gulvvarme = underfloor heating / floor heating
• gulvvarmen = the underfloor heating

This is very normal in Danish. Where English might use two words or a hyphen, Danish often prefers one compound word.

The ending -en is the definite article attached to the noun. In Danish, instead of putting the before the noun, you usually add it to the end.

So:

• gulvvarme = underfloor heating
• gulvvarmen = the underfloor heating

This is one of the big differences from English.

In Danish, the definite article is usually a suffix, not a separate word.

Compare:

• English: the heating
• Danish: varmen

For common-gender nouns, the definite singular ending is often -en.
For neuter nouns, it is often -et.

So gulvvarmen follows a very standard Danish pattern.

In this sentence, tændt is the past participle of tænde (to turn on / light / switch on), but it is being used like an adjective after er.

So:

• tænde = to switch on
• har tændt = has switched on
• er tændt = is switched on / is on

This is very common in Danish:

• Lyset er tændt = The light is on
• Computeren er tændt = The computer is on
• Gulvvarmen er tændt = The underfloor heating is on

Both ideas can exist, but er tændt is especially common for things that have been switched on, such as lights, heating, appliances, and electronics.

So Gulvvarmen er tændt sounds very natural.

You may also hear:

• Gulvvarmen er slået til = The underfloor heating has been switched on
• Gulvvarmen er på = The underfloor heating is on

But er tændt is a very standard and idiomatic choice.

Yes. Gulvvarme is the standard word for underfloor heating.

Literally it is floor-heating, but in natural English the usual meaning is underfloor heating. So even if the meaning has already been shown to you, it is useful to know that this is a completely ordinary everyday Danish word.

The structure is:

• Gulvvarmen = subject
• er = verb
• tændt = complement/predicative adjective-like participle

So the sentence follows a simple pattern:

subject + verb + complement

This is the normal word order for a basic Danish statement.

You add ikke after the verb:

• Gulvvarmen er ikke tændt. = The underfloor heating is not on.

This is a very useful pattern in Danish:

• Lyset er ikke tændt.
• Ovnen er ikke tændt.
• Computeren er ikke tændt.

Yes, but the meaning changes.

• Gulvvarmen er tændt. = The underfloor heating is on.
• Gulvvarme er dyrt. = Underfloor heating is expensive.
• Vi har gulvvarme. = We have underfloor heating.

So gulvvarmen refers to a specific heating system, while gulvvarme is the general noun.

A rough learner-friendly guide is something like GULV-var-men, but spoken Danish often softens or reduces sounds more than that spelling suggests.

A few helpful points:

• gulv is not pronounced exactly like it looks to an English speaker
• the v sounds can be quite soft
• the ending -en is usually a light unstressed ending

You do not need perfect pronunciation at first; the important thing is to recognize it as one compound noun: gulvvarmen.

No. Although learners often first meet it with lights, it is used much more broadly for things that are switched on.

For example:

• Lyset er tændt. = The light is on.
• Radioen er tændt. = The radio is on.
• Motoren er tændt. = The engine is on/started.
• Gulvvarmen er tændt. = The underfloor heating is on.

So tændt is not limited to candles or lamps; it is a general word for something activated/switched on.

No. Unlike German, Danish does not capitalize all nouns.

So the sentence is correctly written:

Gulvvarmen er tændt.

Only the first word of the sentence is capitalized here because it starts the sentence, not because it is a noun.