Apague la calefacción cuando el radiador esté demasiado caliente, por favor.

Because this sentence is addressing one person formally: usted.

In Spanish commands, the form changes depending on who you are talking to:

• apaga = informal singular command (tú)
• apague = formal singular command (usted)
• apagad = informal plural in Spain (vosotros)
• apaguen = formal plural / Latin American plural (ustedes)

So Apague la calefacción... means something like Please turn off the heating... said politely or formally to one person.

For usted commands, Spanish uses the present subjunctive form.

So grammatically, apague is the same form as the present subjunctive of apagar, but in this sentence it is functioning as a command.

That is very normal in Spanish:

• Hable más despacio. = Speak more slowly.
• Pase por aquí. = Come this way.
• Apague la calefacción. = Turn off the heating.

So the short answer is: it is a command, and the form used for that command is the subjunctive form.

Because cuando is referring to a situation that has not happened yet.

In Spanish, when cuando means when in the future, it is usually followed by the subjunctive:

• cuando llegue = when he/she arrives
• cuando sea posible = when it is possible
• cuando el radiador esté demasiado caliente = when the radiator is too hot

The idea is not that the radiator is hot right now as a known fact, but that this is the condition under which the person should turn the heating off.

Yes, but it would mean something slightly different.

• cuando el radiador esté demasiado caliente
  = when the radiator gets/is too hot in the future, as a condition or expected event

• cuando el radiador está demasiado caliente
  = when the radiator is too hot, more like a general/habitual statement describing what happens

In instructions like this, Spanish usually prefers the subjunctive because it is talking about a future condition: turn it off when it becomes too hot.

Spanish uses definite articles more often than English.

Here:

• la calefacción = the heating / the heating system
• el radiador = the radiator

Even when English might say turn off heating, Spanish normally says apagar la calefacción.

This is very common. Spanish often includes el / la / los / las where English leaves the article out.

Not exactly.

• la calefacción = the heating system, central heating, or heating in general
• el radiador = the radiator itself

So the sentence means:

• the radiator is the thing getting too hot
• the person should turn off the heating system

That distinction is natural in Spain, where calefacción often refers to the whole heating system in a home or building.

Because apagar is the normal verb for turning off things like lights, heating, electrical devices, engines, etc.

Examples:

• apagar la luz = turn off the light
• apagar la televisión = turn off the TV
• apagar la calefacción = turn off the heating

So this is standard Spanish usage.

There is an important difference:

• muy caliente = very hot
• demasiado caliente = too hot

So demasiado implies more than is acceptable or desirable.

That matters here, because the sentence is giving a reason to act:

• if it is very hot, that is just a description
• if it is too hot, that suggests you should do something about it

Caliente is the normal adjective meaning hot.

It does not change between masculine and feminine in this case because adjectives ending in -e often have the same form for both genders:

• el radiador caliente
• la sopa caliente

It only changes for number:

• el radiador caliente
• los radiadores calientes

So caliente is exactly the correct adjective here.

Yes, por favor is flexible.

All of these are possible:

• Por favor, apague la calefacción cuando el radiador esté demasiado caliente.
• Apague, por favor, la calefacción cuando el radiador esté demasiado caliente.
• Apague la calefacción cuando el radiador esté demasiado caliente, por favor.

Putting por favor at the end is very common and sounds natural. It softens the command and makes it more polite.

Not with exactly the same meaning.

• cuando el radiador esté demasiado caliente = when the radiator is too hot
• si el radiador está/estuviera demasiado caliente = if the radiator is too hot

Cuando suggests that this is the moment at which the action should happen.
Si would make it sound more like a condition that may or may not be met.

In an instruction like this, cuando is perfectly natural if the idea is turn it off once it gets too hot.

Because calefacción is stressed on the last syllable: -ción.

Words ending in -n, -s, or a vowel are normally stressed on the second-to-last syllable unless there is a written accent. The accent mark shows that the stress is different:

• ca-le-fac-CIÓN

This -ción ending is very common in Spanish:

• información
• situación
• atención
• calefacción

You would usually say:

Apaga la calefacción cuando el radiador esté demasiado caliente, por favor.

That uses the tú command apaga instead of the formal usted command apague.

If you were speaking to several people informally in Spain, you would use:

Apagad la calefacción cuando el radiador esté demasiado caliente, por favor.

So the rest of the sentence stays the same; only the command changes depending on who is being addressed.