Стояк проверят управляющая компания и аварийная служба.

Because Russian word order is flexible. The first noun is not automatically the subject.

In this sentence:

• стояк = the direct object, what will be checked
• управляющая компания и аварийная служба = the subject, who will check it

Two clues help:

1. проверят is plural, so it matches the plural meaning of управляющая компания и аварийная служба taken together.
2. стояк is singular, so it cannot match проверят as the subject here.

So the sentence means:

• The management company and the emergency repair service will inspect the riser.

Стояк is in the accusative singular, because it is the direct object of проверят.

But for many inanimate masculine nouns, the accusative singular looks exactly like the nominative singular.

So:

• nominative: стояк
• accusative: стояк

That is completely normal in Russian.

Because together they form a compound subject:

• управляющая компания и аварийная служба

In English, this works the same way:

• The company and the service are...
• not The company and the service is...

So Russian uses the 3rd person plural verb:

• проверят = they will check / inspect

Проверят is the simple future of the perfective verb проверить.

That means the sentence presents the action as a single completed event in the future:

• they will inspect / they will check

Russian has two common ways to talk about the future:

1. Perfective future: проверят
   
   • one completed action
2. Imperfective future: будут проверять
   
   • process, repeated action, or less focus on completion

Here, проверят suggests a straightforward future result: they will come and check it.

Yes. That is also correct, and for many learners it feels more natural because it is closer to normal English word order.

Compare:

• Управляющая компания и аварийная служба проверят стояк.
  More neutral: The management company and the emergency service will inspect the riser.

• Стояк проверят управляющая компания и аварийная служба.
  Puts стояк first for emphasis or as the topic: As for the riser, the management company and the emergency service will inspect it.

So the original sentence is not strange; it just has a different information focus.

Russian often puts the known topic first and the new or important information later.

So if people are already discussing the riser, starting with стояк makes sense:

• Стояк проверят... = The riser will be inspected...

Then the end of the sentence tells you by whom:

• ...управляющая компания и аварийная служба

This kind of structure is very common in Russian.

In housing and maintenance contexts, стояк usually means a vertical pipe/riser in a building, such as part of the:

• water system
• heating system
• sewage/plumbing system

So it is not just any pipe. It is specifically the main vertical line running through the building.

The stress is:

• стоя́к

Yes, управляющая компания is a standard expression meaning management company or property management company.

Управляющая agrees with компания, which is a feminine singular noun. That is why the form is feminine singular:

• управляющая компания

If the noun were masculine, the modifier would change accordingly.

You can think of управляющая here as functioning like an adjective: managing / management.

In this context, аварийная служба usually means an emergency repair service, especially for building problems like:

• leaks
• plumbing issues
• heating failures
• electrical problems

So here it is not likely to mean police, fire, or ambulance. In housing/building Russian, it usually refers to the repair team that handles urgent breakdowns.

Because Russian has no articles.

So Russian does not have direct equivalents of:

• a
• an
• the

Whether something is definite or indefinite is understood from context.

That is why:

• стояк can mean a riser or the riser
• управляющая компания can mean a management company or the management company

In this sentence, the context would usually make it clear that a specific riser and specific services are meant.