Lämpöpumppu on päällä siksi, että ulkona on kylmä.

Here päällä means on, as in a device being switched on.

• Lämpöpumppu on päällä = The heat pump is on
• Literally, päällä can also mean on top / on, depending on context.

This is a very common Finnish expression:

• valo on päällä = the light is on
• televisio on päällä = the TV is on

So in this sentence, päällä does not mean physically on top of something. It means the heat pump is operating or switched on.

Because there are two clauses, and both need their own verb.

• Lämpöpumppu on päällä = The heat pump is on
• ulkona on kylmä = outside it is cold / it is cold outside

In both clauses, on is the 3rd person singular present form of olla (to be).

So Finnish does the same basic thing English does here: each clause has its own is.

Siksi, että means because or more literally for the reason that.

You can think of it like this:

• siksi = for that reason / therefore
• että = that

Together:

• siksi, että ulkona on kylmä = because it is cold outside

This structure is very natural in Finnish, though learners also often meet koska for because:

• Lämpöpumppu on päällä, koska ulkona on kylmä.

That version means basically the same thing.

In standard Finnish writing, a subordinate clause is normally separated by a comma.

So:

• Lämpöpumppu on päällä, siksi että ulkona on kylmä.
• or as given: Lämpöpumppu on päällä siksi, että ulkona on kylmä.

The important learner point is that Finnish often uses commas between clauses where English may or may not, depending on style.

Also, siksi, että is often written with a comma after siksi in careful written Finnish, because että introduces the explanatory clause.

Ulkona means outside / outdoors.

It is the form used when talking about location:

• ulkona = outside
• sisällä = inside

Examples:

• Lapset ovat ulkona. = The children are outside.
• Ulkona sataa. = It’s raining outside.

So in your sentence:

• ulkona on kylmä = it is cold outside

You do not use the basic dictionary form ulko here, because that is not the normal way to say outside in a sentence like this.

Because Finnish does not need a dummy subject like English it.

In English, we say:

• It is cold outside

But the it does not refer to anything real. It is just required by English grammar.

Finnish does not need that kind of subject, so it simply says:

• ulkona on kylmä = outside is cold → natural English: it is cold outside

So this is a very important pattern:

• on kylmä = it is cold
• on kuuma = it is hot
• on pimeää = it is dark

In this sentence, kylmä is an adjective used in a basic descriptive structure:

• ulkona on kylmä = it is cold outside

Finnish weather and condition expressions can be tricky because both adjective and partitive-type expressions exist in different patterns.

Here, kylmä is the normal form for this kind of statement.

Compare:

• Vesi on kylmää. = The water is cold
  Here kylmää is used because the structure is different.
• Ulkona on kylmä. = It is cold outside

So the short answer is: this sentence uses the standard weather-expression pattern, where kylmä is correct.

Because Finnish does not have articles like a or the.

So:

• lämpöpumppu can mean a heat pump or the heat pump
• the exact meaning comes from context

In this sentence, English naturally translates it as the heat pump, because the speaker probably has a specific heat pump in mind. But Finnish does not need a separate word for the.

This is true very broadly in Finnish:

• koira = a dog / the dog
• talo = a house / the house

It is a compound noun:

• lämpö = heat
• pumppu = pump

So:

• lämpöpumppu = heat pump

Finnish makes a huge number of words by joining nouns together. This is very normal and very productive.

Other examples:

• kahvikuppi = coffee cup
• kirjahylly = book shelf / bookshelf
• pesukone = washing machine

So if you are learning Finnish, it is very useful to get used to spotting compounds.

Yes. That is very common.

For example:

• Lämpöpumppu on päällä, koska ulkona on kylmä.

This also means The heat pump is on because it is cold outside.

The difference is mostly about style and structure:

• koska is the most direct everyday word for because
• siksi, että can feel a little more like for the reason that

In many contexts, they are interchangeable.

Yes. Finnish word order is fairly flexible.

For example:

• Koska ulkona on kylmä, lämpöpumppu on päällä.
• Ulkona on kylmä, siksi lämpöpumppu on päällä.

These all express roughly the same basic idea, but the emphasis changes a little.

The original sentence starts with the result:

• The heat pump is on and then gives the reason:
• because it is cold outside

That is a very natural way to say it.

No. It is common, but there are other possibilities depending on nuance.

For example:

• on päällä = is on / switched on
• on käynnissä = is running / operating

So:

• Lämpöpumppu on päällä focuses on the state on
• Lämpöpumppu on käynnissä focuses more on the machine actively running

In many everyday contexts, päällä is the most straightforward choice.

Formally, päällä is the adessive form of pää (head), and its literal meaning is something like on top / on.

This is one of those Finnish forms whose original spatial meaning has expanded into many common expressions.

So:

• literal: pöydällä = on the table
• similarly: päällä can mean on, including for clothing and devices

Examples:

• Takki on päällä. = The coat is on / someone is wearing the coat
• Radio on päällä. = The radio is on

So even though the form comes from a location-type case, here it works as a fixed everyday expression.

A helpful breakdown is:

• Lämpöpumppu = subject
• on päällä = predicate, is on
• siksi, että = because
• ulkona = outside
• on kylmä = is cold

So the structure is:

[main clause] + [reason clause]

• Lämpöpumppu on päällä
• siksi, että ulkona on kylmä

That is why the sentence feels very logical and transparent once you know the pieces.