Σήμερα το καλοριφέρ δεν δουλεύει καλά στο σαλόνι.

Greek word order is fairly flexible. Putting Σήμερα first is very natural because it sets the time frame right away: today.

You could move it:

• Το καλοριφέρ δεν δουλεύει καλά σήμερα στο σαλόνι.
• Στο σαλόνι το καλοριφέρ δεν δουλεύει καλά σήμερα.

These versions are possible, but the original sounds very natural and gives today some emphasis.

Greek often leaves out subject pronouns when they are not needed. In this sentence, the subject is already stated as το καλοριφέρ, so there is no need to add a separate word for it.

Also, the verb ending -ει already shows that the verb is 3rd person singular: it works / it is working.

Καλοριφέρ is a neuter noun, so it takes the article το.

So:

• το καλοριφέρ = the radiator / the heating unit

This is a loanword, and learners often just need to memorize its gender together with the article:

• το καλοριφέρ

Δεν is the basic word for not in Greek.

It normally goes right before the verb:

• δεν δουλεύει = does not work / isn’t working

That placement is very standard in Greek. So if you want to negate a verb, you usually put δεν directly in front of it.

No. Δουλεύω can be used both for:

• a person working
• a machine or device functioning

So το καλοριφέρ δουλεύει means the radiator/heating unit works / is functioning.

In this sentence, δουλεύει is:

• present tense
• 3rd person singular
• from the verb δουλεύω

A slightly more formal alternative would be λειτουργεί, but δουλεύει is extremely common and natural in everyday Greek.

Because καλά is an adverb, and it modifies the verb δουλεύει.

• καλά = well
• καλό = good (neuter singular adjective)

Here you need well, not good, because the sentence is describing how the radiator works.

So:

• δουλεύει καλά = works well

This is a very common pattern in Greek.

Στο is a contraction of:

• σε + το = στο

Here, σε means something like in / at / to, and το is the article the.

So:

• στο σαλόνι = in the living room

This contraction is extremely common in Greek:

• στο = σε το
• στη = σε τη(ν)
• στον = σε τον

They are both forms of the article the, but they are used in different structures.

• το καλοριφέρ: here το is the article with the subject noun
• στο σαλόνι: here το is part of σε + το

Also, σαλόνι is neuter, so after σε it still appears with το:

• σε το σαλόνι → στο σαλόνι

For neuter singular nouns, the form of the article is often the same in nominative and accusative, which is why both phrases contain το.

Yes. Greek allows a good deal of movement of adverbial phrases like place expressions.

For example:

• Σήμερα στο σαλόνι το καλοριφέρ δεν δουλεύει καλά.
• Το καλοριφέρ δεν δουλεύει καλά στο σαλόνι σήμερα.

The original sentence is very natural because it leaves the location until the end:

• Σήμερα το καλοριφέρ δεν δουλεύει καλά στο σαλόνι.

That final position often sounds smooth and conversational.

Yes, very normal.

Greek often uses:

• δουλεύει καλά = works well
• δεν δουλεύει καλά = doesn’t work well / isn’t working properly

So this is a completely natural everyday expression for appliances, heating, electronics, and similar things. It is not strange or overly literal.