Cuando hace frío, la caldera no funciona bien.

In Spanish, weather expressions often use hacer:

• hace frío = it is cold
• hace calor = it is hot
• hace sol = it is sunny
• hace viento = it is windy

This is just the normal Spanish pattern. The hace here does not mean makes in a literal sense.

• es frío usually describes something as being cold by nature or characteristic, for example El clima aquí es frío.
• hay frío is not the normal way to talk about the weather.

So in this sentence, Cuando hace frío is the natural way to say when it’s cold.

There is no real subject in the English sense. This is an impersonal weather expression.

In hace frío, the verb is always third person singular:

• Hace frío
• Hace calor

You do not normally ask what makes cold? The phrase works as a fixed structure.

Because this sentence is talking about a general repeated situation: whenever it is cold, the boiler does not work well.

With cuando, Spanish uses:

• indicative for habitual, general, or past/factual situations
• subjunctive for future or not-yet-realized situations

So:

• Cuando hace frío, la caldera no funciona bien.
  = a general fact / habit

But:

• Cuando haga frío, la caldera no funcionará bien.
  = when it gets cold / whenever it is cold in the future

That is a very common contrast in Spanish.

Because Spanish often uses the present tense for general truths, habits, and regularly occurring situations.

This sentence means something like:

• whenever it is cold, this usually happens

So the present tense is exactly what you would expect:

• hace = it is cold
• no funciona = does not work

It does not have to mean only right now.

In Spanish, articles are used more often than in English. Here, la caldera refers to the boiler in a house or building, the one the speaker is talking about.

Spanish normally includes the definite article with a specific noun like this:

• la caldera
• el coche
• la puerta

Leaving out the article here would sound incomplete or unnatural.

In Spain, caldera commonly means the boiler in a home or building, especially for heating and/or hot water.

Depending on context, English might translate it as:

• boiler
• sometimes heating boiler
• occasionally something close to heater, though boiler is usually more exact

A learner might think of cauldron, because that is another meaning of caldera, but in everyday home-related Spanish in Spain, caldera very often means the household boiler.

In Spanish, no normally goes directly before the conjugated verb:

• no funciona
• no trabaja
• no sirve

Then bien comes after the verb:

• no funciona bien

So the structure is:

• no
  • verb + adverb

This is the standard word order.
You would not normally say funciona no bien.

Yes, often you could, but there is a slight difference in tone.

• no funciona bien = it does not work well
• funciona mal = it works badly / it works poorly

Both are natural.
However, no funciona bien can sometimes sound a bit softer or less direct than funciona mal.

So:

• La caldera no funciona bien = maybe it is unreliable or underperforming
• La caldera funciona mal = more directly, it works badly

In many everyday situations, they are very close.

The comma is very common and helpful here because the sentence begins with a subordinate clause:

• Cuando hace frío, la caldera no funciona bien.

It separates the when-clause from the main clause.

In informal writing, some people might omit it, but using the comma is the safest and clearest choice, especially in standard writing.

Yes. You can also say:

• La caldera no funciona bien cuando hace frío.

That means the same thing.

The difference is mostly one of emphasis:

• Cuando hace frío, la caldera no funciona bien.
  emphasizes the condition first
• La caldera no funciona bien cuando hace frío.
  starts with the main point, then adds the condition

Both are correct and natural.

Frío has a written accent because of Spanish stress rules and vowel separation.

It is pronounced in two syllables:

• frí-o

The accent mark shows that the i is stressed and does not combine into a diphthong with the o.

Without the accent, the pronunciation rules would suggest something different. So the accent is necessary.

A natural pronunciation would be approximately:

• KWAHN-doh AH-seh FREE-oh, lah kah-DEH-rah noh foon-see-OH-nah byehn

A few useful notes:

• cu in cuando sounds like kw
• h in hace is silent
• c in hace is pronounced th in most of Spain: AH-theh
• frío has two clear syllables: FRÍ-O
• ll in caldera does not appear here, but d in caldera is usually softer than English d
• bien is one syllable

In much of Spain, a more local approximation would be:

• KWAHN-doh AH-theh FREE-oh, lah kah-DEH-rah noh foon-thee-OH-nah byehn