Lämmitys on päällä yöllä.

Lämmitys means heating or the heating system.

In this sentence, it refers to the heating being active, not to warmth in general.

A few useful contrasts:

• lämmitys = heating, heating system
• lämpö = heat, warmth
• lämmitin = heater, heating device

So Lämmitys on päällä is more like The heating is on than The heat is on.

Yes, literally päällä often means on, on top, or even wearing in some contexts. But olla päällä is also a very common expression meaning:

• to be on
• to be switched on
• to be running / active

So here:

• on = is
• päällä = part of the expression on päällä

Together, on päällä means is on.

You can use this with many things:

• Valot ovat päällä. = The lights are on.
• Televisio on päällä. = The TV is on.
• Lämmitys on päällä. = The heating is on.

Because Finnish usually puts time expressions into a case form instead of leaving them as a plain dictionary form.

Here, yöllä means at night or during the night. It is a common time expression in the adessive case, ending in -lla / -llä.

Compare:

• aamulla = in the morning
• päivällä = during the day / in the daytime
• illalla = in the evening
• yöllä = at night

So yö is just the basic form night, while yöllä is the form you use for at night.

Finnish does not have articles like a, an, and the.

So lämmitys can mean:

• heating
• the heating

The exact meaning comes from context. In English, we usually say the heating is on, but Finnish does not need a separate word for the.

On is the third person singular form of the verb olla, which means to be.

So:

• minä olen = I am
• sinä olet = you are
• hän on = he/she is
• lämmitys on = the heating is

Since lämmitys is singular, Finnish uses on.

The given sentence has a very natural word order:

Lämmitys on päällä yöllä.

That is basically:

• subject: lämmitys
• verb phrase: on päällä
• time expression: yöllä

But Finnish word order is fairly flexible. You could also say:

• Yöllä lämmitys on päällä.

This gives more emphasis to at night.

So the original sentence is natural and neutral, but other word orders are possible depending on emphasis.

Not exactly, though they can be similar in some contexts.

• yöllä = at night / during the night
• öisin = at night, nights, on nights, usually with a more habitual or repeated sense

So:

• Lämmitys on päällä yöllä. = The heating is on at night.
• Lämmitys on päällä öisin. = The heating is on at night / on nights, often suggesting a regular habit or repeated pattern.

If you are describing a general routine, öisin can sound very natural. The original yöllä is still completely correct.

You can say:

Lämmitys ei ole päällä yöllä.

Here:

• ei = not
• ole = the negative form used with olla

So Finnish does not negate it by adding something like English not after is. Instead, it uses the negative verb ei:

• on = is
• ei ole = is not

You may also hear:

• Lämmitys on pois päältä yöllä. = The heating is off at night.

Yes, it is a normal and natural way to say it.

Finnish often uses olla päällä for systems, machines, lights, and devices that are active or switched on. So for heating, it sounds very natural.

Depending on context, a speaker might also say something more specific, such as:

• Lämmitys on kytketty päälle. = The heating has been switched on.
• Patterit ovat päällä. = The radiators are on.
• Lämmitys toimii yöllä. = The heating runs at night.

But Lämmitys on päällä yöllä is a straightforward, idiomatic sentence.